Vertical Drains
Content
1.4 PREFABRICATED VERTICAL (WICK) DRAINS INTRODUCTION The largest change in the last ten years of the use of prefabricated vertical (wick drains has been the change in ter!inology and the co!!on acceptance of their use as a ground i!prove!ent tool throughout the United "tates# $erein the ter! %& drains will be used in lieu of the ter! wick drains 'any organi(ations have already changed to this ter!inology) so the reader is to assu!e that whenever the ter! prefabricated vertical drains or %& drains is used) it is referring as to what was previously known as wick drains# The reason for the change in the ter!inology is that %& drains do not perfor! any wicking process) but rather are prefabricated drains# The re!ainder section will focus on changes in the last ten years rather than atte!pt to be a co!plete !anual of usage# D*"CRI%TION The pri!ary use of %& drains is to accelerate consolidation to greatly decrease the settle!ent ti!e of e!bank!ents over soft soils# +y doing so) %& drains also accelerate the rate of strength gain of the in situ soft soils %& drains are band shaped (rectangular cross,section products consisting of a geote-tile filter !aterial surrounding a plastic core# .hile there are so!e variations) the si(e of a %& drain is typically /0 c! (1 in wide by 2 to 3 !! (/45 to 245 in in thickness# The !aterial consists of a plastic core for!ed to create channels which are wrapped in a geote-tile filter# %& drains are only one general type of a vertical drain syste! with the others being sand drains and sand wicks or fabric encased sand drains# The general principles that govern all vertical drain installations are si!ilar to all types of drains# $owever) the advantages of%& drains over other vertical drain syste!s has resulted in their al!ost e-clusive use) e-cept for unusual circu!stances# +RI*6 $I"TORIC78 O&*R&I*. The develop!ent of %& drains closely parallels the develop!ent of the vertical sand drain concept# 7 U#" patent for a sand drain syste! was granted in /39:# The California Division of $ighways) 'aterials and Research Depart!ent conducted laboratory and field tests on vertical sand drain perfor!ance as early as /322# Until a little over 90 years ago) sand drains were used al!ost e-clusively for preload pro;ects across the United "tates that re<uired a vertical drain solution# %& drains) on the other hand) were developed in the early /320=s and were used e-tensively in "weden after /323# 8ater these drains were used in >apan and other countries# $owever) even in *urope) the drawbacks to the original design) which was known as the cardboard wick) prevented !uch e-pansion of their use until the early /3?0=s when new designs facilitated their use# %& drains did not co!e into use in the United "tates until the !id to late /3?0=s# prior to this) the use of a ;etted and augered non,displace!ent sand drain had proved to be an effective) <uality vertical drain syste!# 7nother contributing factor was that all of the practical e-perience had been attained in foreign areas) !ostly in *urope and >apan# .ith environ!ental and econo!ic factors slowly eli!inating the use of ;etted and augered sand drains) a new design %& drain !aterial known as 7lidrain was introduced in the United "tates 7cceptance) while initially slow due to the lack of prior e-perience and design procedures) grew <uite rapidly# "ince the develop!ent of the first cardboard wick) there have been over @0 different %& drains used worldwide and at least /0 in the United "tates# Today there are as !any as 900 pro;ects co!pleted yearly In fact) %& Drains are now used al!ost e-clusively where a vertical drain solution is re<uired# C$7NA*" IN T$* 87"T T*N B*7R" 7ND %OT*NTI78 6UTUR* C$7NA*" In the last ten years) %& drains have been used in a !uch greater variety of pro;ects than previously# The initial applications were !ore in the transportation field) especially highway e!bank!ents leading up to bridge abut!ents# .hile the transportation field still accounts for a relatively large portion of the work) !any other types of pro;ects such as shopping centers) tank far!s) housing develop!ents) warehouses) and industrial plants) account for !ore than 10C of the work In addition) %& drains are now used in ha(ardous waste re!ediation and stabili(ation of very deep !ine tailing ponds# 7 significant recent study is the use of %& drains to reduce the li<uefaction potential in fine grain soils# 6OCU" 7ND "CO%* The purpose of this report is to give a brief review of %& drains) but) !ore i!portantly) to point out the changes in the use of%& drains in the last ten years# This is not intended to be an intricate design !anual# Therefore) the reader should consult the references listed at the end of this article 'uch of the infor!ation contained herein is taken fro! practical e-perience and the following listed referencesD
E#%refabricated &ertical Drains) U#" Depart!ent of Transportation) 6ederal $ighway 7d!inistration) Research) Develop!ent) and Technology) &ol# I# *ngineering Auidelines) Report No# 6$. 74RD,5:, /:5 E"hared *-perience in Aeotechnical *ngineeringD .ick Drains) Transportation Research Circular) No#203) "ept# /35:) I""N 003?,5@/@ EIn "itu "oil I!prove!ent Techni<ues) 77"$TO,7AC,7RT+7 >oint Co!!ittee) "ubco!!ittee on New $ighway 'aterials) Task 6orce 9? Report #ED#T# +ergado) 8#R# 7nderson) N# 'iura) 7#"# +alasubra!ania!# (/33: # "oft Around I!prove!ent in 8owland and Other *nviron!ents) 7"C* %ress) 110 pp# #E$ansbo) "# (/3?3 FConsolidation of Clay by +and,"haped %refabricated Drains#F Around *ngineering) &ol# /9) No#@) pp# /:,9@# #E$olt() R#D# (/35? # F%reloading with %refabricated &ertical "trip Drains#F Aeote-tile sand Aeo!e!branes) pp# /03,/2/# #Technical "u!!aries for Around I!prove!ent Technologies) .ick Drains# 6$. 7 De!o %ro;ect //:) (publication e-pected in /33? # This report will concentrate on the !a;or use of %& drains for consolidation of soft soils in con;unction with preloading and4or surcharging# Other %& drain applications include# pressure relief wells to reduce pore pressures due to seepageG lowering perched water table conditionsG and reducing li<uefaction potential in fine grained soils# .hen used in con;unction with surcharging or preloading) the principal benefits of using %& drains are# #ETo decrease the settle!ent ti!e re<uired such that final construction can be co!pleted in a reasonable ti!e with !ini!al post construction settle!ent ETo decrease the a!ount of surcharge or preload !aterial re<uired to achieve a settle!ent in the given ti!e# ETo increase the rate of strength gain due to consolidation of soft soils when stability is of concern# 7nyone of these benefits !ay be the sole reason for use on a particular pro;ect) or any co!bination of benefits !ay be the desired result# D*"IAN CON"ID*R7TION" ,7%%8IC7TION" In practice) %& drains are !ost co!!only used in consolidation situations where the soil to be treated is a !oderate to highly co!pressible soil with low per!eability and fully saturated in its natural state# "uch soils are typically described as silts) clays) organic silts) organic clays) !uck) peat) swa!ps) !uskeg and sludges# .hen considering the use of %& drains for increasing consolidation rates) the soil should be fully saturated and either nor!ally or slightly overconsolidated prior to loading# The loading should e-ceed !a-i!u! past consolidation pressure for the %& drains to be beneficial# Ordinarily) %& drains are not used in highly organic !aterials) or where secondary consolidation will result in significant postconstruction settle!ent# $owever) additional surcharging !ay be used with %& drain solutions to !ini!i(e the effect of secondary consolidation# Table /#1,/ illustrates !any of the co!!on uses but is not to be considered all inclusive# Table /#1,/# Uses of%&Drains $ighways 'arine .alls Roadways 'arginal 7reas "tructure 7pproach 6ills +uildings 7lternative to a "tructure Depart!ent "tores 7irfields .arehouses Runways $ousing Tracts Ta-iways H Cargo 7prons 7part!ent +uildings Cargo +uildings $otels and 'otels *arth Da!s "ite Develop!ent 6oundations "hopping Centers *!bank!ents H 8evees Industrial %arks Cellular Cofferda!s Industrial %lants "tabili(e 6ill in Cells .astewater Treat!ent %lants "torage Tanks "ettle!ent H "ludge +asins %ile 6oundations Digesters Reduce INegativeJ "kin 6riction %lant +uildings *-cavations Rapid Transit "teepen 7llowable "lopes Inter!odal Transfer "tations $a(ardous .aste "tabili(ation
6UTUR* 7%%8IC7TION One of the potential use of %& drains which has not yet been fully e-plored is that of installation of %& drain to reduce the potential for li<uefaction# There has been at least a few pro;ects co!pleted in the United "tates where this has been the sole purpose# One was a tank storage pro;ect overlying silty sands with nu!erous intervening silt layers inhibiting drainage# %& drains provide two potential benefits which will reduce the possibility of li<uefaction in susceptible soils# drainage and reinforce!ent# 6or both of these benefits) there has been little to no research to deter!ine the e-tent of potential li<uefaction reduction $owever) because %& drains offer a pore pressure relief !echanis! and have so!e tensile strength) the result will be so!e degree of reduction of li<uefaction potential# %resently new designs are being developed to take advantage of such potential# It is e-pected that the ne-t ten years will provide nu!erous e-a!ples of the use of %& drains for the reduction of li<uefaction potential# The other !a;or area of potential growth of %& drains is in the stabili(ation of ha(ardous waste# 7lready pro;ects have been perfor!ed where %& drains were used to stabili(e water tables and to help in providing a vapor e-traction syste!# "ince the %& drains have the potential to drain and in;ect fluids) they can be used to stabili(e very ha(ardous che!ical sites# 6*7"I+I8ITB *&78U7TION" .hen %& drains are used to accelerate settle!ent) each of the following subsoil criteria !ust be !etF E#'oderate to highly co!pressible E#8ow per!eability E#6ully saturated E#6inal e!bank!ent loads !ust e-ceed !a-i!u! past pressure E#"econdary consolidations is not a !a;or concern or can be overco!e by e-tra surcharge %rovided the soils !eet the above criteria) the pro;ect !ust be evaluated for the possible effects of environ!ental and site conditions# *N&IRON'*NT78 CON"ID*R7 TION" If the in,situ soils are conta!inated with any kind of ha(ardous waste or !aterials) then it is possible that the e-cess pore water draining through the %& drain will need to be collected and treated# In such situations) care !ust be e-ercised to prevent the %& drain fro! penetrating into a highly per!eable layer below the co!pressible stratu! "IT* CONDITION" "ite topography and in situ soil conditions can have a considerable effect on the econo!ics of a %& drain solution# "o!e of the specific site and soil conditions which affect the econo!ics or feasibility include# E#Uneven working surface E#8i!ited head roo!# E#Obstructions above the co!pressible layer E#Unstable working surface# E#*-tre!e depth of %& Drains E#"tiff to very stiff co!pressible layers# E#*-tre!ely soft layer for anchoring #%oor site accessibility# E#Overhead or subsurface utility interference# Uneven workin !"r#$%e# %& drains cannot be installed econo!ically on steep slopes# Therefore) the area will have to be benched with widths sufficient to allow for the e<uip!ent# Aenerally) a !ini!u! bench width of : ! (90 ft is re<uired 6or shallow depths) %& drains can be installed on slopes as steep as five on one# Deeper drains will re<uire an al!ost level working surface 7 constant !inor slope is !ore preferable than an up and down surface and it also facilitates the construction of a !ore effective drainage blanket# Li&i'e( )e$( roo& 7 rule of thu!b is that the depth of a %& drain needs to be 2 ! ( /0 ft shorter than the a!ount of head roo! in order to be econo!ically installed# 8i!ited head roo! situations occur !ost often when installing under an e-isting bridge# %& drains can be installed vertically in seg!ents with li!ited head roo!) but the resultant cost would !ost# I be as high as five ti!es the nor!al unit installation price#
*+!'r"%'ion! $+ove ')e %o&,re!!i+-e -$.er# .here obstructions !ust be penetrated above the co!pressible layer) considerable e-tra costs could be involved# 7 stiff or dense upper layer can be penetrat d without predrilling and will only slightly add to the cost# $owever) obstructions suc as concrete) rock) rubble) slag) brick) wood) riprap) stone) debris) rubbish or trash can result in very e-pensive pre drilling costs# Un!'$+-e workin !"r#$%e# I general) !ost unstable working surfaces can be !ade stable prior to the installatio of %& drains with the use of geote-tiles and granular soil for the drainage layer 0 to / !) or 9 to 2 ft thick The installation e<uip!ent will usually penetrates !aterials without difficulty# .here the ground cannot be stabili(ed) speciali(ed lightweight e<uip!ent is available at a substantial increase in the unit cost# E/'re&e (e,')# %& drains have been installed to depths of :0 ! (900 ft with the use of speciali(ed e<uip!ent# 7 rule of thu!b is that %& drains over 9/ ! (?0 ft in depth will re<uire a crane for instal/atioK Depths over 2: ! (/90 ft re<uire a very large crane and speciali(ed installation !asts S'i## 'o ver. !'i## %o&,re!!i+-e -$.er# If the layer which is considered co!pressible is <uite stiff) the entire length of %& drains !ay need to be preaugered or predrilled# In such cases) it is not nor!ally advisable to use %& drains The preaugering or drilling will create a certain a!ount of disturbance and there would be a large void area around the drain If the void could be filled with sand) a %& drain is not necessary to begin with# E/'re&e-. !o#' -$.er #or $n%)orin # In so!e cases) designers have elected a depth which does not fully penetrate the co!pressible layer based on econo!ics# $owever) if this soil has a very low shear strength) it !ay beco!e very difficult to anchor the drain at that depth and either additional depth will be necessary or special e<uip!ent procedures will be re<uired# This situation slows production and adversely i!pacts cost# Si'e $%%e!!i+i-i'.# .hile the e<uip!ent for %& drain installation is relatively easy to transport) there are so!e situations where site accessibility !ay add to the cost# In !any of these cases) costly access roads !ay be necessary to transport the e<uip!ent down steep slopes or across unstable areas# *-tra !obili(ations !ay be necessary to reach isolated parts of the pro;ect# *ver)e$( or !"+!"r#$%e "'i-i'. o+!'r"%'ion! Usually underground utilities can be located prior to %& drain installation and drains can be installed around the! to avoid any proble!s# $owever) large sewer pipes inter!i-ed with several other utilities !ay create a situation where drains cannot be installed for a significant width Overhead wires can possibly present !ore of a logistic proble! If the wires cannot be deenergi(ed) significant widths of treat!ent !ight need to be eli!inated or the use of angle drains specified# "hould any of the above site conditions be encountered) it would be advisable to contact specialty contractors e-perienced in the installation of %& drains) in order to deter!ine the !agnitude of difficulty# 7ll of the above cases have been encountered and drains installed successfully# $owever) the additional costs can be very significant# 8I'IT7TION" It is i!portant to re!e!ber that a %& drain serves no structural function (e-cept perhaps in li<uefaction reduction applications # +y providing a shorter drainage path) it provides a faster release of e-cess pore water) thereby resulting in faster settle!ent and a <uicker strength gain by consolidation 6or sites with a stability proble!) the soil will initially have the sa!e strength with or without %& drains installed Therefore) in situations where stability is of any concern) the rate of increase of load should be carefully controlled and !onitored# %& drains can accelerate only pri!ary consolidation Therefore) it is i!portant to esti!ate the !agnitude and ti!e rate of secondary consolidation In these cases) !eans of !ini!i(ing the a!ount of secondary settle!ent by e-cess surcharge and4or e-tending waiting periods prior to final construction) should be i!ple!ented# "oils with significant organic content should be carefully evaluated for secondary consolidation Other li!itations on the use of %& drains should be considered# 7lthough they have been installed up to :0 ! (900 ft in depth) the use of %& drains below 1@ ! (/@0 ft should be evaluated by a specialist# In !ost situations the flow properties of a good <uality %& drain will not inhibit consolidation ti!es# $owever) for e-tre!ely deep %& drains) co!bined with heavy loading and a relatively high in situ soil per!eability) the flow capacity of the syste! could be a li!iting design consideration# In these rare cases) well resistance in the drain occurs and consolidation ti!e will be deter!ined !ore by discharge capacity of the drain rather than the hori(ontal per!eability of the in situ soils# $. 7 (/35: and Des!ond (/331 contain specific technical guidance for this condition#
It is not reco!!ended to install %& drains where the entire length or lower length re<uires predrilling# In these cases) sand drain would be considered a good alternative# 78T*RN7T* "O8UTION" 7lternate solutions can be functionally divided into three different categories as follows E#7ccept ti!e constraints without the use of any vertical drain syste!# E#+y,pass the co!pressible soil) using deep structural foundations# E#I!prove the co!pressibility of the in situ soil# .here there is sufficient ti!e for settle!ent to occur under the final load conditions) %& drains obviously are not needed In so!e cases an additional surcharge without the use of any %& drains !ay be all that is necessary to obtain consolidation within the allowable ti!e constraints# The cost of the e-cess surcharge should be co!pared against the cost of using %& drains 7nother !ethod would be to design for e-cessive post construction settle!ent and accept the anticipated cost of repairs or corrections to the ground or structure) as a long ter! !aintenance responsibility# Use of a deep foundation is an effective) although e-pensive !eans of by,passing co!pressible soils# This can be done by e-tending the bridge) or in the case of a structure) using a deep foundation "uch solutions are usually !uch !ore e-pensive and !ay re<uire significant future !aintenance# They also li!it the fle-ibility of future uses of the site# Decreasing the co!pressibility of in situ soils offers the greatest variety of solutions# .hile usually !uch !ore e-pensive than %& drains) the following are alternative !ethods of i!proving the soil# S'one %o-"&n!# This !ethod is used in very soft subsurface soils to both accelerate settle!ent and provide sufficient strength increase to !ini!i(e settle!ent and preclude deep seated shear failure# D.n$&i% %o&,$%'ion# Used !ostly for consolidation of industrial fills and other near surface soft deposits above the water table# Not considered very effective for soft co!pressible soils below the water table# 0ro"'in # Used to fill voids and other speciali(ed situations# Dee, !oi- &i/in # Used to change the in situ co!pression characteristics of soils E/%$v$'ion and replace!ent of co!pressible soil# Li )'wei )' #i--!# Used to reduce the e!bank!ent load# 7ll of these !ethods will strengthen the soil and reduce its co!pressibility) e-cept lightweight fills .here stability is a significant proble!) co!bined solutions !ay be warranted in order to achieve the desired result In this situation) the !ost unstable areas can be treated with stone colu!ns) grouting) or deep soil !i-ing) while the re!aining areas are treated with %& drains# *-cavation of weak in situ soils and replace!ent with granular !aterials creating a shear key in the !ost critical areas can also be used with %& drains in the re!aining soft soil area# The use of lightweight fills to !ini!i(e the total a!ount of settle!ent has been used !ore fre<uently in recent years .hile lightweight fills do not result in soil i!prove!ent they reduce settle!ent and stability proble!s by reducing i!posed loads "everal e-a!ples of lightweight fill !aterial used are) wood chips) lightweight slag !aterial and synthetic !aterials such as geofoa! and blocks *ach of these !aterials re<uire specific design considerations) cost evaluations and should be considered) if their use can result in a significant reduction in total settle!ent# CON"TRUCTION *KUI%'*NT There are !any different ways of installing %& drains) but !ost e!ploy the sa!e principles# Table /1,9lists the various !ethods of installation# .ith few e-ceptions) all !ethods e!ploy a steel covering !andrel which protects the %& drain !aterial as it is installed# 7ll !ethods e!ploy so!e for! of anchoring syste! to hold the drain in place while the !andrel is withdrawn# 7nother co!!on feature is that the %& drain !aterial co!es in rolls and is threaded through the !andrel in a variety of ways# The !a;or difference between the listed !ethods is in the techni<ue used to insert the !andrel into the ground
Co!!only used !ethods e!ploy an installation !ast which contains the !aterial reels) !andrel and !ethod of installation force# 7dded to this is a carrier) which is a crawler e-cavator or crawler crane) depending so!ewhat on the depth of installation Usually for drains up to depths of9/ ! (?0 ft the !ast can be !ounted on a crawler e-cavator# Drains re<uiring greater than 9/ ! (?0 ft !ost often re<uire an installation !ast !ounted to a crane in order to provide stability# In so!e situations the installation !ast has been set up on !arsh buggies and secured with guy wires# "uch e<uip!ent has been used for deep installations in very soft ground# Table /#1,9# 'ethods of %& drain installation# a# "tatic /# Chain DrivenE 9# Cable %ulldownE 2# $eavy .eight 1# $ydraulic %iston %ush b &ibratory /# Offset $a!!er ,full supported !andrelE 9# Direct $a!!er ,offset !aterialE 2# Inside 'andrel with *nlarged "hoe c# >etting /# Covered 'andrel with Outside >ets 9# >et %robe with No Covering 'andrel d# "yste! Co!bination /# "tatic with &ibratoryE 9# "tatic with >ettingE 2# &ibratory with >etting 1# 'ultiple Drains ,7ll "yste!s e# %redrilling full depth E%resently used in the United "tates Kuite often specifications have concentrated al!ost entirely on the %& drain !aterial and very little on the !ethod of installation# The !ost i!portant criteria for !ethod of installation is the si(e of the installing !andrel# The !andrel should be kept to a !ini!u! si(e) usually no greater than 50 s<uare centi!eters (/9 s<uare in unless the depth of installation would re<uire so!ething larger# 7 typical specification usually describes the acceptable !ethod of installation as static) vibratory) or static,vibratory# Other ter!s that have been used in lieu of static are constant rate of advance!ent or constant load# 7nother i!portant point concerning the installation is the rate of !andrel advance# %revious papers) including the Ten Bear Update (.elsh /35? ) listed a rate of !andrel advance# This rate is actually detri!ental rather than a help The faster the !andrel is advanced) the less pull or dragdown occurs and therefore) it is i!portant not to specify the rate of advance!ent# "%*CI78IL*D *KUI%'*NT .hile not co!!only used) there are situations which will re<uire very speciali(ed e<uip!ent to install the %& drains# "ituations where this !ight occur are# #Unstable working surfaces #"loped surfaces #*-tre!ely s!all <uantities co!bined with widely spaced drains #"ubsoils which are very difficult to penetrate #*-tre!ely deep drains "o!e e-a!ples of unusual e<uip!ent were !entioned previously in the discussion of nor!al installation syste!s) such as the need for special carrier pieces to do e-tre!ely deep drains) or the use of !arsh buggies on unstable working surface situations# One e-a!ple is when a special e-ternal ;etting tool was !ounted on alight weight skid platfor! and cable wenched into place on steep slopes# 7nother is the use of test boring e<uip!ent to install pipe drains with %& drain !aterials as relief wells in e-isting da!s# In one very unusual situation involving sludge deposits) the %& drains were installed fro! a row boat using a long plastic insertion rod .hile these speciali(ed !ethods are usually very costly on a per !eter (ft basis) they can be applicable) especially where <uantities are s!all *-a!ples of layers that !ight re<uire special drilling techni<ues are fills containing large a!ounts of rubble) concrete) old slabs or footings) buried riprap or large boulders and any ce!ented layers# Nor!ally) if the soil can be augered for preloosening) its cost should be included in the unit for %& drain installation $owever) if it is anticipated that obstruction drilling as described above will be necessary) a special obstruction predrilling pay ite! should be established# CON"TRUCTION '7T*RI78"
7s !entioned in the beginning of the report) %& drains are relatively flat and appro-i!ately /0 c! (1 in wide by 2 to 3 !! (/45 to 245 in in thickness# The !aterial generally consists of a plastic core for!ed to !ake channels and a cover of a geote-tile filter# These two co!ponents are e<ually i!portant in the function of a %& drain Over the last ten years) there have been new brands of %& drain !aterials as well as the eli!ination of so!e of the older ones# Typical brand na!es available in the United "tates today are the following# /# 7lidrain @# 7!erdrain (Type 1/? 9# 7lidrain " : 'ebra,Drain (?10? 2 7liwick ?# 'ebra,Drain ('D 55 1# 7!erdrain (Type 10? 5# Colbonddrain CM /000 DR7IN7A* 87B*R The purpose of the drainage layer is to provide a clear drainage path for the pore water to the at!osphere) without creating any significant head loss# Until recently) the typical drainage blanket consisted of sand or gravel# If sand was used it was usually 0# : to / ! (9 to 2 ft in thickness) but a gravel layer could be as little as /@ c! (: in if protected by a filter fabric Typically the filter fabric) if used) was placed below the sand or gravel prior to the installation of %& drains Then after installation) a second layer was so!eti!es placed above the gravel The past ten years have seen the increased use of synthetic drains in lieu of a drainage layer Typically the !aterial) usually called strip drains) is si!ilar to highway edge drains and co!es in a thickness of appro-i!ately 9@ c! (/ in and widths varying fro! /@ to 1@ c! (: in to /5 in Two uses of strip drains are illustrated in 6igure /1,/
6igure /1,/ "trip drains Consideration !ust be given to stability of the working surface Often the thickness of the granular blanket !ust be increased to allow for support of the %& drain installation e<uip!ent 7nother alternative is to reinforce the drainage blanket with gcote-tiles and4or geogrids This !ay have a twofold effect# to provide a stable working surface and to !ini!i(e the necessary thickness of the drainage layer due to conta!ination fro! the soils below 7 co!bination of a working platfor! and drainage layer is often cost effective# If %& drains are installed on uneven surfaces such as on the sides of an e-isting e!bank!ent) the drainage layer effectiveness and stability !ust be considered# The working surface !ay have to be altered to allow for the installation of %& drains) such as benching procedure which !ay disrupt the continuity of the drainage blanket To ensure proper functioning) the drainage layer !ust be outletted# %& DR7IN D*"IAN CONC*%T" 7s previously !entioned under 6ocus and "cope) this paper is not intended to be a design !anual# $owever) it should be re!e!bered that the first ob;ective of any design is to define the reason for the use of%& drains Once
this has been deter!ined) the a!ount of detail of the design can be outlined# 6 or detailed design procedures the designer should consult the previously cited references for design reference and other papers in the reference section## The proper design for a %& drain installation for consolidation purposes re<uires knowledge of the type and e-tent of the foundation soils and their pertinent engineering properties# *ngineering analyses !ust include) a!ong other ite!s) predictions of the a!ount and rate of settle!ent) both during and after construction and the e!bank!ent stability during all phases of construction# 6or consolidation analyses) the subsurface investigation progra! !ust define the e-tent and depth of the co!pressible strata and secure high <uality undisturbed sa!ples to deter!ine past !a-i!u! pressures) coefficients of co!pressibility and coefficients of consolidation) both in a vertical and hori(ontal direction# In addition) "tandard %enetration Test blow counts (N are also significant in deter!ining the installation costs and potential for the need for predrilling prior to place!ent of the %& drains# The theory of consolidation has been well docu!ented and e-plained in !any references# 6 or the purpose of discussion of significant ite!s concerning the design) the ideal case of radial drainage when the effects of soil disturbance is ignored are e-plained in the following for!ula to deter!ine the ti!e for consolidation
t=
D9 / F ( n ln 5c h / −U h
whereD tN ti!e re<uired to achieve desired degree of consolidation Uh Uh N average degree of consolidation due to hori(ontal drainage D N dia!eter of the cylinder of influence of the drain (drain influence (one ch N coefficient of consolidation for hori(ontal drainage 6(n N drain spacing factorf(n N InK# ,0#?@ (si!plified d N dia!eter of a circular drain The above basic relationships can be !odified to consider disturbance and well resistance) although these effects are often ignored for typical pro;ects# 6or detailed designs and4or disturbance considerations) the reader should consult 6$. 7 (/35: ) in order to fully evaluate and design for those effects# .ell resistance is rarely significant) e-cept for e-tre!ely deep drains or where a co!bination of high loads and very per!eable soils are present# 6or guidance in these situations) the applicable reference is $ansbo (/3?3 # In the last ten years there has been !ore significant work on the effects of disturbance of the in situ soils during installation# .hile %& drains are often referred to as !ini!al displace!ent drains) there is so!e effect) especially in very sensitive soils# There has been !ore significant work acco!plished in this area in the overseas areas and therefore) the reader is advised to refer to +ergado et al (/33: The other ite! also discussed in +ergado et al (/33: is that of !icro folding# It has been the writer=s e-perience that !icro folding is rarely) if ever) a significant factor in the perfor!ance of %& drains# *-cavation of !any sites has revealed the drains to be !ore in a rounded to sine curve nature than in accordion for!# $owever) in tests perfor!ed by the New Bork "tate D#O#T#) it was shown that the sa!e a!ount of reduction in flow capacity occurred in studded and grooved drains when sub;ected to a 30F bend# .hile this reduction in flow characteristics was in the range of @0C) !ost of the drains had a flow capacity well above !a-i!u! flows) even with a @0C reduction# 6urther study on the well reduction fro! !icro folding would certainly be welco!e# 6*7"I+I8ITB D*"IAN .hile not necessarily applicable in every case) the designer can acco!plish a si!ple preli!inary deter!ination of spacing a %& drain solution with !ini!al soil data) such as li<uid li!its and pro;ect geo!etry# The definition of pro;ect geo!etry reflects the area of loading) depth of soft co!pressible strata and whether e-cess surcharge will be necessary# 7n appro-i!ate coefficient of consolidation can be esti!ated fro! the li<uid li!it using the correlation graphs fro! the N7&67C 'anual (Navy /359 # &arious publications indicate the no!inal effective drain dia!eter of %& drains is appro-i!ately : to ?#@ c! (9,/49 to 2 in # 7ssu!ing that the desired percentage of pri!ary consolidation has been established or is in the 30 to 3@ percent range) the designer can then use either so!e of the co!puter progra!s available for vertical drain design or the no!ograph fro! the N7&67C 'anual CON"TRUCTION "%*CI6IC7TION" There are !any specifications in the suggested references# 'ost of the specifications are general in nature and should be !odified for each individual pro;ect One a!using note includes a phrase stating that there should be
/@ to 20 c! (: to /9 in of !aterial protruding above the surface (or other si!ilar wording # The reason for this specification re<uire!ent is that the first pro;ect resulted in the %& drains being installed prior to the drainage blanket On that pro;ect) the e-cess length was necessary to assure continuity of drainage into the sand blanket# *ver since then) this re<uire!ent has been placed in al!ost all specifications# $owever) !ore than :0C of the pro;ects already have the sand blanket in place and therefore) no e-cess !aterial would be re<uired# 'ONITORINA 7ND CONTRO8 7typical %& drain installation for a highway e!bank!ent is illustrated in 6igure /#1,9# In this diagra! can be seen !ost all of the possible !onitoring devices used on a very co!plicated %& drain design pro;ect Not all of these ite!s are necessary on each pro;ect and !any would not warrant their cost 6ield instru!entation such as pie(o!eters) settle!ent platfor! and gauges) and inclino!eters are used to !onitor perfor!ance of the %& drains and possibly control the rate of construction of e!bank!ent and4or surcharge# It is i!portant that both the designer and the instru!entation personnel have a full appreciation of the particular instru!entation being installed# Aenerally settle!ent !easuring devices) whether platfor!s) deep settle!ent points or hori(ontal deflection devices) are used to !easure only the rate and total a!ount of consolidation# 7n inclino!eter is used !ostly to !easure hori(ontal deflection with depth and as a warning against potential failure# The pore pressure devices (pie(o!eters are used for both calculation of achieved consolidation rate and e-cessive build,up of pore pressure which are an indication of potential failure# One caution concerning the use of pore pressure devices is that there have been a significant nu!ber of pro;ects where the rate of settle!ent has not agreed with the rate of pore pressure dissipation# In such situations) settle!ent data should be given priority as indicators of the rate of consolidation# The proper selection of instru!entation devices and the fre<uency of !onitoring during a pro;ect are i!portant# 6or si!ple pro;ects where stability is of no concern) and ti!e is not the critical factor) only surface settle!ent platfor!s) which are relatively easy to install) are needed# In situations where stability is critical) pore pressure !easure!ents and !easure!ents of hori(ontal defor!ations are also necessary# .here stability is of concern) daily readings !ay be necessary both during loading and for the first few weeks after loading# CO"T *&78U7TION Often when reporting the cost of %& drains) only the actual cost of the installed %& drain is considered rather than the solution as a whole Table /#1,2 below details other factors effecting the total cost of the %& drain solution Note that the unit cost of installation) whether including the !obili(ation or not) is only one of !any factors that effect the total cost of a %& drain solution#
Deep settle!ent points
surcharge
6ir! "oil pie(o!eters NOT TO "C78*
6igure /#1,9# Typical %& drain installation for a highway e!bank!ent# Table /#1,2# 6actors effecting the total cost of a %& drain solution#
E#%ro;ect si(e) topography #Obstructions) dense soils E#7llowable Construction and Consolidation Ti!e E#7llowable %ostconstruction "ettle!ent E#%reload and "urcharge# ,Type and 'aterial 7vailable ,Reuse of 'aterial ,7!ount of "urcharge E#"and +lanket or $ori(ontal Drainage %ath E#Design) Instru!entation and 'onitoring E#Unit Cost of Installation There are detailed e-planations of these factors in the references# $erein the concentration is on the unit cost of installation $owever) it is very i!portant to note the effect of these other factors when evaluating the total cost of a %& drain solution Typical pro;ect unit costs for %& drains for pro;ects where the soils do not present !a;or difficulty in penetration) re<uire special e<uip!ent) or at unusually difficult sites are su!!ari(ed belowD "i(e Category 8inear Di!ension Unit %rice Range "!all 'ediu! 8arge 2)000 to 3)000 ! /0)000 to 20)000 ft 3)000 to 1@) 000 ! 20)000 to /@0)000 ft 1@)000 ! H larger /@0)000 ft H larger O0#90 to O0#@0 per 8' O0#:0 to O/#@0 per 86 O0#/1 to O0#9? per 8' O0#1@ to O0#30 per 86 O0#03 to O0#/5 per 8' O0#20 to O0:0 per 86
In addition) there is usually a !obili(ation charge varying fro! O?)000#00 to O/@)000#00# .here there are severe conditions in the area of weather) labor conditions) site conditions) and4or difficulties in penetration) the unit cost could be significantly higher# %ro;ect si(es have been as s!all as :/0 lineal ! (8' P9)000 lineal ft(86 Q and larger than 2)0@0)000 8' (/0)000)000 86 # %roduction rates per installation unit have been reported to be as high as /9)900 8' (10)000 86 per day) but this is very unusual# "cheduling esti!ators of typical pro;ects with no !a;or penetration or other proble!s should anticipate production rates of 2)0@0 to 1)@?@ 8'(/0)000 to /@)000 86 per installation unit per day# $owever) every schedule should allow so!e ti!e for set,up) trial drain procedures) and dis!antling of e<uip!ent CONC8U"ION" 7ND R*CO''*ND7TION" Twenty years ago prefabricated vertical (wick drains were anew and innovative techni<ue used to solve stability and settle!ent proble!s with very soft co!pressible soils# Today they are accepted as a co!!on solution to such an e-tent that by the ti!e this is published) they will have been used in every state in the United "tates and !any of its territories# Ten years ago appro-i!ately @0 pro;ects were acco!plished yearly while today there are well over /OO %& drain pro;ects per year# .hile co!!only accepted) there are still !any new and innovative usages for %& drains# %articular areas showing potential pro!ise are the stabili(ation of ha(ardous waste and the reduction of li<uefaction potential# Results of past pro;ects have shown that %& drains have been very effective# This is !ainly because they have been conservatively designed# 7s designs approach their li!its) !ore care !ust be e-ercised in both the design and construction facets of %& drain solutions# +I+8IOAR7%$B In addition to those publications referenced in the preceeding sections) the following publications on wick or prefabricated vertical drains are reco!!endedD +arron) R#7# (/315 # FConsolidation of 6ine,Arained "oils by Drain .ells#F Trans# 7!# "oc# Civ# *ngrs# &ol# //2) %aper No# 921:# Charles) R# D# (/351 F%erfor!ance of &ertical .ick Drains in "oft "oils#F %ractical 7pplication of Drainage in Aeotechnical *ngineering) %roceedings of the /@th) Ohio River &alley "oils "e!inar) Nove!ber 9# $annon) I# +# and .alsh) T# I# (/359 # F.ick Drains) 'e!brane Reinforce!ent and 8ight .eight 6ill for *!bank!ent Construction at Du!bartonF) Transportation Research Record 53?) pp# 2?,19#
Roerner) Robert '# (/35: # Designing .ith Aeosynthetics# %rentice $all Inc# Ryfor)L# A#) 'asi) I# I#) and Ae!!e) R# 8## (/35: # F%erfor!ance of a %refabricated &ertical Drain Installation +eneath an *!bank!entF) %ro;ect $iawatha +lvd# /,5/ Interchange# New Bork "tate) Depart!ent of Transportation) "oil 'echanics +ureau) 7ugust /35:# 'orrison) 7# (/359 # FThe +oo!ing +usiness in .ick Drains#F Civil *ngineering)&ol# @2) No#2) pp# 1?,@/# "y!posiu! in %rint of &ertical Drains# (/35/ # Aeotechni<ue# &ol#2l)No#/# $aley H 7ldrich) Inc# (/35: # F%refabricated &ertical Drains &ol# 9) "u!!ary of Research *ffort#F 6$.74RD, 5:4I:3# Runesson) R#) $ansbo) "#) and .iberg) N# *# (/35@ FThe *fficiency of %artially %enetrating &ertical Drains#F Aeotechni<ue) &ol# 2@) No#1) pp# @//,@/:# 6$.7 De!o %ro;ect //:) Technical "u!!aries for Around I!prove!ent Technologies) .ick Drains# (Due for publication soon# R*6*R*NC*" 7nony!ous# (/35: # F"hared *-perience in Aeotechnical *ngineeringD .ick Drains#FTransportation Research Circular# Nu!ber 203# "epte!ber +ergado) +#T# 7nderson) 8#R# 'iura) N# +alasubra!ania) 7#"# (/33: # "oft Around I!prove!ent in 8owland and Other *nviron!ents) 7"C* %ress) 110 pp# Des!ond) C# (/331 # FCalhoun County Rehab Uses .icks to Drain) "peed "ettle!en8F 'ichigan Contractor H +uilder) &ol 55) No 2) 'arch 9: 6$.7# (/35: # %refabricated &ertical Drains) U#"# Depart!ent of Transportation) 6ederal $ighway 7d!inistration) Research) Develop!ent) and Technology) &ol I# *ngineering Auidelines) Report No 6$. 74RD,5:4/:5# $ansbo) "# (/3?3 FConsolidation of Clay by +and,"haped %refabricated DrainsF Around*ngineering) &ol/9) No#@) pp# /:,9@# $olt() R#D# (/35? # F%reloading with %refabricated &ertical "trip DrainsF Aeote-tiles and Aeo!e!branes) pp /03,/2/# FIn "itu "oil I!prove!ent Techni<uesF) 77"$TO,7AC,7RT+7 >oint Co!!ittee) "ubco!!ittee on New $ighway 'aterials) Task 6orce 9? Report Navy# (/359 # "oil 'echanics) N767C Design 'anual ?#/# Naval 6acilities *ngineering Co!!and) Depart!ent of the Navy) 'ay# .elsh) >#%#) *ditor (/35? # "oil I!prove!ent ,7 Ten,Bear Update) 7"C* Aeotechnical "pecial %ublication No#/9##
E#%refabricated &ertical Drains) U#" Depart!ent of Transportation) 6ederal $ighway 7d!inistration) Research) Develop!ent) and Technology) &ol# I# *ngineering Auidelines) Report No# 6$. 74RD,5:, /:5 E"hared *-perience in Aeotechnical *ngineeringD .ick Drains) Transportation Research Circular) No#203) "ept# /35:) I""N 003?,5@/@ EIn "itu "oil I!prove!ent Techni<ues) 77"$TO,7AC,7RT+7 >oint Co!!ittee) "ubco!!ittee on New $ighway 'aterials) Task 6orce 9? Report #ED#T# +ergado) 8#R# 7nderson) N# 'iura) 7#"# +alasubra!ania!# (/33: # "oft Around I!prove!ent in 8owland and Other *nviron!ents) 7"C* %ress) 110 pp# #E$ansbo) "# (/3?3 FConsolidation of Clay by +and,"haped %refabricated Drains#F Around *ngineering) &ol# /9) No#@) pp# /:,9@# #E$olt() R#D# (/35? # F%reloading with %refabricated &ertical "trip Drains#F Aeote-tile sand Aeo!e!branes) pp# /03,/2/# #Technical "u!!aries for Around I!prove!ent Technologies) .ick Drains# 6$. 7 De!o %ro;ect //:) (publication e-pected in /33? # This report will concentrate on the !a;or use of %& drains for consolidation of soft soils in con;unction with preloading and4or surcharging# Other %& drain applications include# pressure relief wells to reduce pore pressures due to seepageG lowering perched water table conditionsG and reducing li<uefaction potential in fine grained soils# .hen used in con;unction with surcharging or preloading) the principal benefits of using %& drains are# #ETo decrease the settle!ent ti!e re<uired such that final construction can be co!pleted in a reasonable ti!e with !ini!al post construction settle!ent ETo decrease the a!ount of surcharge or preload !aterial re<uired to achieve a settle!ent in the given ti!e# ETo increase the rate of strength gain due to consolidation of soft soils when stability is of concern# 7nyone of these benefits !ay be the sole reason for use on a particular pro;ect) or any co!bination of benefits !ay be the desired result# D*"IAN CON"ID*R7TION" ,7%%8IC7TION" In practice) %& drains are !ost co!!only used in consolidation situations where the soil to be treated is a !oderate to highly co!pressible soil with low per!eability and fully saturated in its natural state# "uch soils are typically described as silts) clays) organic silts) organic clays) !uck) peat) swa!ps) !uskeg and sludges# .hen considering the use of %& drains for increasing consolidation rates) the soil should be fully saturated and either nor!ally or slightly overconsolidated prior to loading# The loading should e-ceed !a-i!u! past consolidation pressure for the %& drains to be beneficial# Ordinarily) %& drains are not used in highly organic !aterials) or where secondary consolidation will result in significant postconstruction settle!ent# $owever) additional surcharging !ay be used with %& drain solutions to !ini!i(e the effect of secondary consolidation# Table /#1,/ illustrates !any of the co!!on uses but is not to be considered all inclusive# Table /#1,/# Uses of%&Drains $ighways 'arine .alls Roadways 'arginal 7reas "tructure 7pproach 6ills +uildings 7lternative to a "tructure Depart!ent "tores 7irfields .arehouses Runways $ousing Tracts Ta-iways H Cargo 7prons 7part!ent +uildings Cargo +uildings $otels and 'otels *arth Da!s "ite Develop!ent 6oundations "hopping Centers *!bank!ents H 8evees Industrial %arks Cellular Cofferda!s Industrial %lants "tabili(e 6ill in Cells .astewater Treat!ent %lants "torage Tanks "ettle!ent H "ludge +asins %ile 6oundations Digesters Reduce INegativeJ "kin 6riction %lant +uildings *-cavations Rapid Transit "teepen 7llowable "lopes Inter!odal Transfer "tations $a(ardous .aste "tabili(ation
6UTUR* 7%%8IC7TION One of the potential use of %& drains which has not yet been fully e-plored is that of installation of %& drain to reduce the potential for li<uefaction# There has been at least a few pro;ects co!pleted in the United "tates where this has been the sole purpose# One was a tank storage pro;ect overlying silty sands with nu!erous intervening silt layers inhibiting drainage# %& drains provide two potential benefits which will reduce the possibility of li<uefaction in susceptible soils# drainage and reinforce!ent# 6or both of these benefits) there has been little to no research to deter!ine the e-tent of potential li<uefaction reduction $owever) because %& drains offer a pore pressure relief !echanis! and have so!e tensile strength) the result will be so!e degree of reduction of li<uefaction potential# %resently new designs are being developed to take advantage of such potential# It is e-pected that the ne-t ten years will provide nu!erous e-a!ples of the use of %& drains for the reduction of li<uefaction potential# The other !a;or area of potential growth of %& drains is in the stabili(ation of ha(ardous waste# 7lready pro;ects have been perfor!ed where %& drains were used to stabili(e water tables and to help in providing a vapor e-traction syste!# "ince the %& drains have the potential to drain and in;ect fluids) they can be used to stabili(e very ha(ardous che!ical sites# 6*7"I+I8ITB *&78U7TION" .hen %& drains are used to accelerate settle!ent) each of the following subsoil criteria !ust be !etF E#'oderate to highly co!pressible E#8ow per!eability E#6ully saturated E#6inal e!bank!ent loads !ust e-ceed !a-i!u! past pressure E#"econdary consolidations is not a !a;or concern or can be overco!e by e-tra surcharge %rovided the soils !eet the above criteria) the pro;ect !ust be evaluated for the possible effects of environ!ental and site conditions# *N&IRON'*NT78 CON"ID*R7 TION" If the in,situ soils are conta!inated with any kind of ha(ardous waste or !aterials) then it is possible that the e-cess pore water draining through the %& drain will need to be collected and treated# In such situations) care !ust be e-ercised to prevent the %& drain fro! penetrating into a highly per!eable layer below the co!pressible stratu! "IT* CONDITION" "ite topography and in situ soil conditions can have a considerable effect on the econo!ics of a %& drain solution# "o!e of the specific site and soil conditions which affect the econo!ics or feasibility include# E#Uneven working surface E#8i!ited head roo!# E#Obstructions above the co!pressible layer E#Unstable working surface# E#*-tre!e depth of %& Drains E#"tiff to very stiff co!pressible layers# E#*-tre!ely soft layer for anchoring #%oor site accessibility# E#Overhead or subsurface utility interference# Uneven workin !"r#$%e# %& drains cannot be installed econo!ically on steep slopes# Therefore) the area will have to be benched with widths sufficient to allow for the e<uip!ent# Aenerally) a !ini!u! bench width of : ! (90 ft is re<uired 6or shallow depths) %& drains can be installed on slopes as steep as five on one# Deeper drains will re<uire an al!ost level working surface 7 constant !inor slope is !ore preferable than an up and down surface and it also facilitates the construction of a !ore effective drainage blanket# Li&i'e( )e$( roo& 7 rule of thu!b is that the depth of a %& drain needs to be 2 ! ( /0 ft shorter than the a!ount of head roo! in order to be econo!ically installed# 8i!ited head roo! situations occur !ost often when installing under an e-isting bridge# %& drains can be installed vertically in seg!ents with li!ited head roo!) but the resultant cost would !ost# I be as high as five ti!es the nor!al unit installation price#
*+!'r"%'ion! $+ove ')e %o&,re!!i+-e -$.er# .here obstructions !ust be penetrated above the co!pressible layer) considerable e-tra costs could be involved# 7 stiff or dense upper layer can be penetrat d without predrilling and will only slightly add to the cost# $owever) obstructions suc as concrete) rock) rubble) slag) brick) wood) riprap) stone) debris) rubbish or trash can result in very e-pensive pre drilling costs# Un!'$+-e workin !"r#$%e# I general) !ost unstable working surfaces can be !ade stable prior to the installatio of %& drains with the use of geote-tiles and granular soil for the drainage layer 0 to / !) or 9 to 2 ft thick The installation e<uip!ent will usually penetrates !aterials without difficulty# .here the ground cannot be stabili(ed) speciali(ed lightweight e<uip!ent is available at a substantial increase in the unit cost# E/'re&e (e,')# %& drains have been installed to depths of :0 ! (900 ft with the use of speciali(ed e<uip!ent# 7 rule of thu!b is that %& drains over 9/ ! (?0 ft in depth will re<uire a crane for instal/atioK Depths over 2: ! (/90 ft re<uire a very large crane and speciali(ed installation !asts S'i## 'o ver. !'i## %o&,re!!i+-e -$.er# If the layer which is considered co!pressible is <uite stiff) the entire length of %& drains !ay need to be preaugered or predrilled# In such cases) it is not nor!ally advisable to use %& drains The preaugering or drilling will create a certain a!ount of disturbance and there would be a large void area around the drain If the void could be filled with sand) a %& drain is not necessary to begin with# E/'re&e-. !o#' -$.er #or $n%)orin # In so!e cases) designers have elected a depth which does not fully penetrate the co!pressible layer based on econo!ics# $owever) if this soil has a very low shear strength) it !ay beco!e very difficult to anchor the drain at that depth and either additional depth will be necessary or special e<uip!ent procedures will be re<uired# This situation slows production and adversely i!pacts cost# Si'e $%%e!!i+i-i'.# .hile the e<uip!ent for %& drain installation is relatively easy to transport) there are so!e situations where site accessibility !ay add to the cost# In !any of these cases) costly access roads !ay be necessary to transport the e<uip!ent down steep slopes or across unstable areas# *-tra !obili(ations !ay be necessary to reach isolated parts of the pro;ect# *ver)e$( or !"+!"r#$%e "'i-i'. o+!'r"%'ion! Usually underground utilities can be located prior to %& drain installation and drains can be installed around the! to avoid any proble!s# $owever) large sewer pipes inter!i-ed with several other utilities !ay create a situation where drains cannot be installed for a significant width Overhead wires can possibly present !ore of a logistic proble! If the wires cannot be deenergi(ed) significant widths of treat!ent !ight need to be eli!inated or the use of angle drains specified# "hould any of the above site conditions be encountered) it would be advisable to contact specialty contractors e-perienced in the installation of %& drains) in order to deter!ine the !agnitude of difficulty# 7ll of the above cases have been encountered and drains installed successfully# $owever) the additional costs can be very significant# 8I'IT7TION" It is i!portant to re!e!ber that a %& drain serves no structural function (e-cept perhaps in li<uefaction reduction applications # +y providing a shorter drainage path) it provides a faster release of e-cess pore water) thereby resulting in faster settle!ent and a <uicker strength gain by consolidation 6or sites with a stability proble!) the soil will initially have the sa!e strength with or without %& drains installed Therefore) in situations where stability is of any concern) the rate of increase of load should be carefully controlled and !onitored# %& drains can accelerate only pri!ary consolidation Therefore) it is i!portant to esti!ate the !agnitude and ti!e rate of secondary consolidation In these cases) !eans of !ini!i(ing the a!ount of secondary settle!ent by e-cess surcharge and4or e-tending waiting periods prior to final construction) should be i!ple!ented# "oils with significant organic content should be carefully evaluated for secondary consolidation Other li!itations on the use of %& drains should be considered# 7lthough they have been installed up to :0 ! (900 ft in depth) the use of %& drains below 1@ ! (/@0 ft should be evaluated by a specialist# In !ost situations the flow properties of a good <uality %& drain will not inhibit consolidation ti!es# $owever) for e-tre!ely deep %& drains) co!bined with heavy loading and a relatively high in situ soil per!eability) the flow capacity of the syste! could be a li!iting design consideration# In these rare cases) well resistance in the drain occurs and consolidation ti!e will be deter!ined !ore by discharge capacity of the drain rather than the hori(ontal per!eability of the in situ soils# $. 7 (/35: and Des!ond (/331 contain specific technical guidance for this condition#
It is not reco!!ended to install %& drains where the entire length or lower length re<uires predrilling# In these cases) sand drain would be considered a good alternative# 78T*RN7T* "O8UTION" 7lternate solutions can be functionally divided into three different categories as follows E#7ccept ti!e constraints without the use of any vertical drain syste!# E#+y,pass the co!pressible soil) using deep structural foundations# E#I!prove the co!pressibility of the in situ soil# .here there is sufficient ti!e for settle!ent to occur under the final load conditions) %& drains obviously are not needed In so!e cases an additional surcharge without the use of any %& drains !ay be all that is necessary to obtain consolidation within the allowable ti!e constraints# The cost of the e-cess surcharge should be co!pared against the cost of using %& drains 7nother !ethod would be to design for e-cessive post construction settle!ent and accept the anticipated cost of repairs or corrections to the ground or structure) as a long ter! !aintenance responsibility# Use of a deep foundation is an effective) although e-pensive !eans of by,passing co!pressible soils# This can be done by e-tending the bridge) or in the case of a structure) using a deep foundation "uch solutions are usually !uch !ore e-pensive and !ay re<uire significant future !aintenance# They also li!it the fle-ibility of future uses of the site# Decreasing the co!pressibility of in situ soils offers the greatest variety of solutions# .hile usually !uch !ore e-pensive than %& drains) the following are alternative !ethods of i!proving the soil# S'one %o-"&n!# This !ethod is used in very soft subsurface soils to both accelerate settle!ent and provide sufficient strength increase to !ini!i(e settle!ent and preclude deep seated shear failure# D.n$&i% %o&,$%'ion# Used !ostly for consolidation of industrial fills and other near surface soft deposits above the water table# Not considered very effective for soft co!pressible soils below the water table# 0ro"'in # Used to fill voids and other speciali(ed situations# Dee, !oi- &i/in # Used to change the in situ co!pression characteristics of soils E/%$v$'ion and replace!ent of co!pressible soil# Li )'wei )' #i--!# Used to reduce the e!bank!ent load# 7ll of these !ethods will strengthen the soil and reduce its co!pressibility) e-cept lightweight fills .here stability is a significant proble!) co!bined solutions !ay be warranted in order to achieve the desired result In this situation) the !ost unstable areas can be treated with stone colu!ns) grouting) or deep soil !i-ing) while the re!aining areas are treated with %& drains# *-cavation of weak in situ soils and replace!ent with granular !aterials creating a shear key in the !ost critical areas can also be used with %& drains in the re!aining soft soil area# The use of lightweight fills to !ini!i(e the total a!ount of settle!ent has been used !ore fre<uently in recent years .hile lightweight fills do not result in soil i!prove!ent they reduce settle!ent and stability proble!s by reducing i!posed loads "everal e-a!ples of lightweight fill !aterial used are) wood chips) lightweight slag !aterial and synthetic !aterials such as geofoa! and blocks *ach of these !aterials re<uire specific design considerations) cost evaluations and should be considered) if their use can result in a significant reduction in total settle!ent# CON"TRUCTION *KUI%'*NT There are !any different ways of installing %& drains) but !ost e!ploy the sa!e principles# Table /1,9lists the various !ethods of installation# .ith few e-ceptions) all !ethods e!ploy a steel covering !andrel which protects the %& drain !aterial as it is installed# 7ll !ethods e!ploy so!e for! of anchoring syste! to hold the drain in place while the !andrel is withdrawn# 7nother co!!on feature is that the %& drain !aterial co!es in rolls and is threaded through the !andrel in a variety of ways# The !a;or difference between the listed !ethods is in the techni<ue used to insert the !andrel into the ground
Co!!only used !ethods e!ploy an installation !ast which contains the !aterial reels) !andrel and !ethod of installation force# 7dded to this is a carrier) which is a crawler e-cavator or crawler crane) depending so!ewhat on the depth of installation Usually for drains up to depths of9/ ! (?0 ft the !ast can be !ounted on a crawler e-cavator# Drains re<uiring greater than 9/ ! (?0 ft !ost often re<uire an installation !ast !ounted to a crane in order to provide stability# In so!e situations the installation !ast has been set up on !arsh buggies and secured with guy wires# "uch e<uip!ent has been used for deep installations in very soft ground# Table /#1,9# 'ethods of %& drain installation# a# "tatic /# Chain DrivenE 9# Cable %ulldownE 2# $eavy .eight 1# $ydraulic %iston %ush b &ibratory /# Offset $a!!er ,full supported !andrelE 9# Direct $a!!er ,offset !aterialE 2# Inside 'andrel with *nlarged "hoe c# >etting /# Covered 'andrel with Outside >ets 9# >et %robe with No Covering 'andrel d# "yste! Co!bination /# "tatic with &ibratoryE 9# "tatic with >ettingE 2# &ibratory with >etting 1# 'ultiple Drains ,7ll "yste!s e# %redrilling full depth E%resently used in the United "tates Kuite often specifications have concentrated al!ost entirely on the %& drain !aterial and very little on the !ethod of installation# The !ost i!portant criteria for !ethod of installation is the si(e of the installing !andrel# The !andrel should be kept to a !ini!u! si(e) usually no greater than 50 s<uare centi!eters (/9 s<uare in unless the depth of installation would re<uire so!ething larger# 7 typical specification usually describes the acceptable !ethod of installation as static) vibratory) or static,vibratory# Other ter!s that have been used in lieu of static are constant rate of advance!ent or constant load# 7nother i!portant point concerning the installation is the rate of !andrel advance# %revious papers) including the Ten Bear Update (.elsh /35? ) listed a rate of !andrel advance# This rate is actually detri!ental rather than a help The faster the !andrel is advanced) the less pull or dragdown occurs and therefore) it is i!portant not to specify the rate of advance!ent# "%*CI78IL*D *KUI%'*NT .hile not co!!only used) there are situations which will re<uire very speciali(ed e<uip!ent to install the %& drains# "ituations where this !ight occur are# #Unstable working surfaces #"loped surfaces #*-tre!ely s!all <uantities co!bined with widely spaced drains #"ubsoils which are very difficult to penetrate #*-tre!ely deep drains "o!e e-a!ples of unusual e<uip!ent were !entioned previously in the discussion of nor!al installation syste!s) such as the need for special carrier pieces to do e-tre!ely deep drains) or the use of !arsh buggies on unstable working surface situations# One e-a!ple is when a special e-ternal ;etting tool was !ounted on alight weight skid platfor! and cable wenched into place on steep slopes# 7nother is the use of test boring e<uip!ent to install pipe drains with %& drain !aterials as relief wells in e-isting da!s# In one very unusual situation involving sludge deposits) the %& drains were installed fro! a row boat using a long plastic insertion rod .hile these speciali(ed !ethods are usually very costly on a per !eter (ft basis) they can be applicable) especially where <uantities are s!all *-a!ples of layers that !ight re<uire special drilling techni<ues are fills containing large a!ounts of rubble) concrete) old slabs or footings) buried riprap or large boulders and any ce!ented layers# Nor!ally) if the soil can be augered for preloosening) its cost should be included in the unit for %& drain installation $owever) if it is anticipated that obstruction drilling as described above will be necessary) a special obstruction predrilling pay ite! should be established# CON"TRUCTION '7T*RI78"
7s !entioned in the beginning of the report) %& drains are relatively flat and appro-i!ately /0 c! (1 in wide by 2 to 3 !! (/45 to 245 in in thickness# The !aterial generally consists of a plastic core for!ed to !ake channels and a cover of a geote-tile filter# These two co!ponents are e<ually i!portant in the function of a %& drain Over the last ten years) there have been new brands of %& drain !aterials as well as the eli!ination of so!e of the older ones# Typical brand na!es available in the United "tates today are the following# /# 7lidrain @# 7!erdrain (Type 1/? 9# 7lidrain " : 'ebra,Drain (?10? 2 7liwick ?# 'ebra,Drain ('D 55 1# 7!erdrain (Type 10? 5# Colbonddrain CM /000 DR7IN7A* 87B*R The purpose of the drainage layer is to provide a clear drainage path for the pore water to the at!osphere) without creating any significant head loss# Until recently) the typical drainage blanket consisted of sand or gravel# If sand was used it was usually 0# : to / ! (9 to 2 ft in thickness) but a gravel layer could be as little as /@ c! (: in if protected by a filter fabric Typically the filter fabric) if used) was placed below the sand or gravel prior to the installation of %& drains Then after installation) a second layer was so!eti!es placed above the gravel The past ten years have seen the increased use of synthetic drains in lieu of a drainage layer Typically the !aterial) usually called strip drains) is si!ilar to highway edge drains and co!es in a thickness of appro-i!ately 9@ c! (/ in and widths varying fro! /@ to 1@ c! (: in to /5 in Two uses of strip drains are illustrated in 6igure /1,/
6igure /1,/ "trip drains Consideration !ust be given to stability of the working surface Often the thickness of the granular blanket !ust be increased to allow for support of the %& drain installation e<uip!ent 7nother alternative is to reinforce the drainage blanket with gcote-tiles and4or geogrids This !ay have a twofold effect# to provide a stable working surface and to !ini!i(e the necessary thickness of the drainage layer due to conta!ination fro! the soils below 7 co!bination of a working platfor! and drainage layer is often cost effective# If %& drains are installed on uneven surfaces such as on the sides of an e-isting e!bank!ent) the drainage layer effectiveness and stability !ust be considered# The working surface !ay have to be altered to allow for the installation of %& drains) such as benching procedure which !ay disrupt the continuity of the drainage blanket To ensure proper functioning) the drainage layer !ust be outletted# %& DR7IN D*"IAN CONC*%T" 7s previously !entioned under 6ocus and "cope) this paper is not intended to be a design !anual# $owever) it should be re!e!bered that the first ob;ective of any design is to define the reason for the use of%& drains Once
this has been deter!ined) the a!ount of detail of the design can be outlined# 6 or detailed design procedures the designer should consult the previously cited references for design reference and other papers in the reference section## The proper design for a %& drain installation for consolidation purposes re<uires knowledge of the type and e-tent of the foundation soils and their pertinent engineering properties# *ngineering analyses !ust include) a!ong other ite!s) predictions of the a!ount and rate of settle!ent) both during and after construction and the e!bank!ent stability during all phases of construction# 6or consolidation analyses) the subsurface investigation progra! !ust define the e-tent and depth of the co!pressible strata and secure high <uality undisturbed sa!ples to deter!ine past !a-i!u! pressures) coefficients of co!pressibility and coefficients of consolidation) both in a vertical and hori(ontal direction# In addition) "tandard %enetration Test blow counts (N are also significant in deter!ining the installation costs and potential for the need for predrilling prior to place!ent of the %& drains# The theory of consolidation has been well docu!ented and e-plained in !any references# 6 or the purpose of discussion of significant ite!s concerning the design) the ideal case of radial drainage when the effects of soil disturbance is ignored are e-plained in the following for!ula to deter!ine the ti!e for consolidation
t=
D9 / F ( n ln 5c h / −U h
whereD tN ti!e re<uired to achieve desired degree of consolidation Uh Uh N average degree of consolidation due to hori(ontal drainage D N dia!eter of the cylinder of influence of the drain (drain influence (one ch N coefficient of consolidation for hori(ontal drainage 6(n N drain spacing factorf(n N InK# ,0#?@ (si!plified d N dia!eter of a circular drain The above basic relationships can be !odified to consider disturbance and well resistance) although these effects are often ignored for typical pro;ects# 6or detailed designs and4or disturbance considerations) the reader should consult 6$. 7 (/35: ) in order to fully evaluate and design for those effects# .ell resistance is rarely significant) e-cept for e-tre!ely deep drains or where a co!bination of high loads and very per!eable soils are present# 6or guidance in these situations) the applicable reference is $ansbo (/3?3 # In the last ten years there has been !ore significant work on the effects of disturbance of the in situ soils during installation# .hile %& drains are often referred to as !ini!al displace!ent drains) there is so!e effect) especially in very sensitive soils# There has been !ore significant work acco!plished in this area in the overseas areas and therefore) the reader is advised to refer to +ergado et al (/33: The other ite! also discussed in +ergado et al (/33: is that of !icro folding# It has been the writer=s e-perience that !icro folding is rarely) if ever) a significant factor in the perfor!ance of %& drains# *-cavation of !any sites has revealed the drains to be !ore in a rounded to sine curve nature than in accordion for!# $owever) in tests perfor!ed by the New Bork "tate D#O#T#) it was shown that the sa!e a!ount of reduction in flow capacity occurred in studded and grooved drains when sub;ected to a 30F bend# .hile this reduction in flow characteristics was in the range of @0C) !ost of the drains had a flow capacity well above !a-i!u! flows) even with a @0C reduction# 6urther study on the well reduction fro! !icro folding would certainly be welco!e# 6*7"I+I8ITB D*"IAN .hile not necessarily applicable in every case) the designer can acco!plish a si!ple preli!inary deter!ination of spacing a %& drain solution with !ini!al soil data) such as li<uid li!its and pro;ect geo!etry# The definition of pro;ect geo!etry reflects the area of loading) depth of soft co!pressible strata and whether e-cess surcharge will be necessary# 7n appro-i!ate coefficient of consolidation can be esti!ated fro! the li<uid li!it using the correlation graphs fro! the N7&67C 'anual (Navy /359 # &arious publications indicate the no!inal effective drain dia!eter of %& drains is appro-i!ately : to ?#@ c! (9,/49 to 2 in # 7ssu!ing that the desired percentage of pri!ary consolidation has been established or is in the 30 to 3@ percent range) the designer can then use either so!e of the co!puter progra!s available for vertical drain design or the no!ograph fro! the N7&67C 'anual CON"TRUCTION "%*CI6IC7TION" There are !any specifications in the suggested references# 'ost of the specifications are general in nature and should be !odified for each individual pro;ect One a!using note includes a phrase stating that there should be
/@ to 20 c! (: to /9 in of !aterial protruding above the surface (or other si!ilar wording # The reason for this specification re<uire!ent is that the first pro;ect resulted in the %& drains being installed prior to the drainage blanket On that pro;ect) the e-cess length was necessary to assure continuity of drainage into the sand blanket# *ver since then) this re<uire!ent has been placed in al!ost all specifications# $owever) !ore than :0C of the pro;ects already have the sand blanket in place and therefore) no e-cess !aterial would be re<uired# 'ONITORINA 7ND CONTRO8 7typical %& drain installation for a highway e!bank!ent is illustrated in 6igure /#1,9# In this diagra! can be seen !ost all of the possible !onitoring devices used on a very co!plicated %& drain design pro;ect Not all of these ite!s are necessary on each pro;ect and !any would not warrant their cost 6ield instru!entation such as pie(o!eters) settle!ent platfor! and gauges) and inclino!eters are used to !onitor perfor!ance of the %& drains and possibly control the rate of construction of e!bank!ent and4or surcharge# It is i!portant that both the designer and the instru!entation personnel have a full appreciation of the particular instru!entation being installed# Aenerally settle!ent !easuring devices) whether platfor!s) deep settle!ent points or hori(ontal deflection devices) are used to !easure only the rate and total a!ount of consolidation# 7n inclino!eter is used !ostly to !easure hori(ontal deflection with depth and as a warning against potential failure# The pore pressure devices (pie(o!eters are used for both calculation of achieved consolidation rate and e-cessive build,up of pore pressure which are an indication of potential failure# One caution concerning the use of pore pressure devices is that there have been a significant nu!ber of pro;ects where the rate of settle!ent has not agreed with the rate of pore pressure dissipation# In such situations) settle!ent data should be given priority as indicators of the rate of consolidation# The proper selection of instru!entation devices and the fre<uency of !onitoring during a pro;ect are i!portant# 6or si!ple pro;ects where stability is of no concern) and ti!e is not the critical factor) only surface settle!ent platfor!s) which are relatively easy to install) are needed# In situations where stability is critical) pore pressure !easure!ents and !easure!ents of hori(ontal defor!ations are also necessary# .here stability is of concern) daily readings !ay be necessary both during loading and for the first few weeks after loading# CO"T *&78U7TION Often when reporting the cost of %& drains) only the actual cost of the installed %& drain is considered rather than the solution as a whole Table /#1,2 below details other factors effecting the total cost of the %& drain solution Note that the unit cost of installation) whether including the !obili(ation or not) is only one of !any factors that effect the total cost of a %& drain solution#
Deep settle!ent points
surcharge
6ir! "oil pie(o!eters NOT TO "C78*
6igure /#1,9# Typical %& drain installation for a highway e!bank!ent# Table /#1,2# 6actors effecting the total cost of a %& drain solution#
E#%ro;ect si(e) topography #Obstructions) dense soils E#7llowable Construction and Consolidation Ti!e E#7llowable %ostconstruction "ettle!ent E#%reload and "urcharge# ,Type and 'aterial 7vailable ,Reuse of 'aterial ,7!ount of "urcharge E#"and +lanket or $ori(ontal Drainage %ath E#Design) Instru!entation and 'onitoring E#Unit Cost of Installation There are detailed e-planations of these factors in the references# $erein the concentration is on the unit cost of installation $owever) it is very i!portant to note the effect of these other factors when evaluating the total cost of a %& drain solution Typical pro;ect unit costs for %& drains for pro;ects where the soils do not present !a;or difficulty in penetration) re<uire special e<uip!ent) or at unusually difficult sites are su!!ari(ed belowD "i(e Category 8inear Di!ension Unit %rice Range "!all 'ediu! 8arge 2)000 to 3)000 ! /0)000 to 20)000 ft 3)000 to 1@) 000 ! 20)000 to /@0)000 ft 1@)000 ! H larger /@0)000 ft H larger O0#90 to O0#@0 per 8' O0#:0 to O/#@0 per 86 O0#/1 to O0#9? per 8' O0#1@ to O0#30 per 86 O0#03 to O0#/5 per 8' O0#20 to O0:0 per 86
In addition) there is usually a !obili(ation charge varying fro! O?)000#00 to O/@)000#00# .here there are severe conditions in the area of weather) labor conditions) site conditions) and4or difficulties in penetration) the unit cost could be significantly higher# %ro;ect si(es have been as s!all as :/0 lineal ! (8' P9)000 lineal ft(86 Q and larger than 2)0@0)000 8' (/0)000)000 86 # %roduction rates per installation unit have been reported to be as high as /9)900 8' (10)000 86 per day) but this is very unusual# "cheduling esti!ators of typical pro;ects with no !a;or penetration or other proble!s should anticipate production rates of 2)0@0 to 1)@?@ 8'(/0)000 to /@)000 86 per installation unit per day# $owever) every schedule should allow so!e ti!e for set,up) trial drain procedures) and dis!antling of e<uip!ent CONC8U"ION" 7ND R*CO''*ND7TION" Twenty years ago prefabricated vertical (wick drains were anew and innovative techni<ue used to solve stability and settle!ent proble!s with very soft co!pressible soils# Today they are accepted as a co!!on solution to such an e-tent that by the ti!e this is published) they will have been used in every state in the United "tates and !any of its territories# Ten years ago appro-i!ately @0 pro;ects were acco!plished yearly while today there are well over /OO %& drain pro;ects per year# .hile co!!only accepted) there are still !any new and innovative usages for %& drains# %articular areas showing potential pro!ise are the stabili(ation of ha(ardous waste and the reduction of li<uefaction potential# Results of past pro;ects have shown that %& drains have been very effective# This is !ainly because they have been conservatively designed# 7s designs approach their li!its) !ore care !ust be e-ercised in both the design and construction facets of %& drain solutions# +I+8IOAR7%$B In addition to those publications referenced in the preceeding sections) the following publications on wick or prefabricated vertical drains are reco!!endedD +arron) R#7# (/315 # FConsolidation of 6ine,Arained "oils by Drain .ells#F Trans# 7!# "oc# Civ# *ngrs# &ol# //2) %aper No# 921:# Charles) R# D# (/351 F%erfor!ance of &ertical .ick Drains in "oft "oils#F %ractical 7pplication of Drainage in Aeotechnical *ngineering) %roceedings of the /@th) Ohio River &alley "oils "e!inar) Nove!ber 9# $annon) I# +# and .alsh) T# I# (/359 # F.ick Drains) 'e!brane Reinforce!ent and 8ight .eight 6ill for *!bank!ent Construction at Du!bartonF) Transportation Research Record 53?) pp# 2?,19#
Roerner) Robert '# (/35: # Designing .ith Aeosynthetics# %rentice $all Inc# Ryfor)L# A#) 'asi) I# I#) and Ae!!e) R# 8## (/35: # F%erfor!ance of a %refabricated &ertical Drain Installation +eneath an *!bank!entF) %ro;ect $iawatha +lvd# /,5/ Interchange# New Bork "tate) Depart!ent of Transportation) "oil 'echanics +ureau) 7ugust /35:# 'orrison) 7# (/359 # FThe +oo!ing +usiness in .ick Drains#F Civil *ngineering)&ol# @2) No#2) pp# 1?,@/# "y!posiu! in %rint of &ertical Drains# (/35/ # Aeotechni<ue# &ol#2l)No#/# $aley H 7ldrich) Inc# (/35: # F%refabricated &ertical Drains &ol# 9) "u!!ary of Research *ffort#F 6$.74RD, 5:4I:3# Runesson) R#) $ansbo) "#) and .iberg) N# *# (/35@ FThe *fficiency of %artially %enetrating &ertical Drains#F Aeotechni<ue) &ol# 2@) No#1) pp# @//,@/:# 6$.7 De!o %ro;ect //:) Technical "u!!aries for Around I!prove!ent Technologies) .ick Drains# (Due for publication soon# R*6*R*NC*" 7nony!ous# (/35: # F"hared *-perience in Aeotechnical *ngineeringD .ick Drains#FTransportation Research Circular# Nu!ber 203# "epte!ber +ergado) +#T# 7nderson) 8#R# 'iura) N# +alasubra!ania) 7#"# (/33: # "oft Around I!prove!ent in 8owland and Other *nviron!ents) 7"C* %ress) 110 pp# Des!ond) C# (/331 # FCalhoun County Rehab Uses .icks to Drain) "peed "ettle!en8F 'ichigan Contractor H +uilder) &ol 55) No 2) 'arch 9: 6$.7# (/35: # %refabricated &ertical Drains) U#"# Depart!ent of Transportation) 6ederal $ighway 7d!inistration) Research) Develop!ent) and Technology) &ol I# *ngineering Auidelines) Report No 6$. 74RD,5:4/:5# $ansbo) "# (/3?3 FConsolidation of Clay by +and,"haped %refabricated DrainsF Around*ngineering) &ol/9) No#@) pp# /:,9@# $olt() R#D# (/35? # F%reloading with %refabricated &ertical "trip DrainsF Aeote-tiles and Aeo!e!branes) pp /03,/2/# FIn "itu "oil I!prove!ent Techni<uesF) 77"$TO,7AC,7RT+7 >oint Co!!ittee) "ubco!!ittee on New $ighway 'aterials) Task 6orce 9? Report Navy# (/359 # "oil 'echanics) N767C Design 'anual ?#/# Naval 6acilities *ngineering Co!!and) Depart!ent of the Navy) 'ay# .elsh) >#%#) *ditor (/35? # "oil I!prove!ent ,7 Ten,Bear Update) 7"C* Aeotechnical "pecial %ublication No#/9##
