Field Investigations on Performance of T-ShapedDeep Mixed Soil Cement Column–Supported

Embankments over Soft GroundSong-Yu Liu, M.ASCE1; Yan-Jun Du2; Yao-Lin Yi3; and Anand J. Puppala, M.ASCE4

Abstract: The soil cement deep mixing method has been used to improve soft clayey soils under embankment loading conditions. Acompacted granular fill layer or geosynthetic reinforcement layer is placed over the top of soil cement deep mixed (DM) columns to reducedifferential settlement between DM soil and the surrounding untreated soil, which, in turn, increases embankment stability. Typically, inconventional deep mixing methodology, the soil cement columns are closely spaced, indicating large area replacement ratios in the con-struction projects. Such practice could increase construction costs substantially. In this research, a new type of DM column, called a T-shapedDM (TDM) column, was designed and used as an alternative to the large-area-replacement-ratio DM columns employed in the field. Unlike inthe conventional column, the cross section of the new column varies along the installation depth. Large amounts of cement slurry are injectedand thoroughly mixed with the native shallow soil using specially designed mixing blades. At greater depths, deep mixing methodology isapplied only to smaller-diameter columns, resulting in large-diameter columns near the surface and smaller-diameter columns deeper. Fieldtrials were conducted to investigate the performance of TDM column–supported soft ground under embankment loading. For comparison,performance of conventional DM column–supported soft ground under similar embankment loading is presented. Differences in qualitycontrol studies and in situ plate loading tests on TDM and conventional DM columns are discussed. Under field embankment loading con-ditions, stress concentration ratio, excess pore water pressures generated in the soft clays, total monitored settlement, and lateral soil dis-placement near embankment toes are analyzed and discussed for both treatments. It is concluded that TDM columns have considerableadvantages over conventional DM because they both mitigate settlement and enhance the performance of the embankments while reducingconstruction costs. DOI: 10.1061/(ASCE)GT.1943-5606.0000625. © 2012 American Society of Civil Engineers.

CE Database subject headings: Embankments; Cement; Field tests; Pore pressure; Settlement; Soil mixing; Soft soils.

Author keywords: Embankment; Cement; Field tests; Pore pressure; Settlement; Soil mixing; Soft soils.

Introduction

Deep mixing methodology (DMM) is an innovative in situ soil sta-bilization technique that delivers cement and/or lime additives, ineither slurry or powder form, into the ground to be mixed with thenative soil, using blades that form a hard treated soil column indifferent block, wall, lattice, and column configurations. Deep mix-ing methodology was first used in Japan and the Nordic countries inthe mid-1970s, and then later spread to Thailand, China, the UnitedStates, the United Kingdom, and several other parts of the world[Coastal Development Institute of Technology (CDIT) 2002;Bhadriraju et al. 2008]. The objectives of DMM are to reduce

settlement of soft ground, minimize heaving of expansive soils,seismically retrofit civil infrastructure, enhance the stability ofembankments or slopes, and solidify contaminated soil media(Porbaha 1998; CDIT 2002; Madhyannapu et al. 2010). Deep mix-ing methodology was introduced to China in the late 1970s andspread rapidly throughout the country in the late 1990s (Han et al.2002). Soil cement deep mixed (DM) columns were widely used inChina to strengthen highway and railway embankments built oversoft clayey soils (Lin and Wong 1999; Chai et al. 2002a; Han et al.2002). A detailed discussion of the use of DM columns in groundimprovement projects in China can be found in Han et al. (2002).

Case histories indicate that settlement of surrounding untreatedsoil is always greater than settlement of DM soil under embank-ment loading conditions (Bergado and Lorenzo 2002). This isattributed to the different compressibility behavior of DM and un-treated native soils. The difference in settlement between treatedand untreated soils can be as high as 8 to 20% of the average set-tlement at the ground surface (Bergado et al. 2002). This differen-tial settlement is highly problematic because it can causeembankment instability and also pavement distress in the formof longitudinal cracking. For high-speed rail embankments, wheretolerance is further restricted, differential settlements betweentreated and untreated soil zones needs to be carefully evaluated.

For conventional DM column-supported embankments over softclays, the DM columns need to be closely spaced. In addition, acompacted granular material (e.g., gravel mat) or a geosyntheticreinforced layer is usually placed on top of the DM soil zone, serv-ing as a load transfer platform (LTP). The main function of the LTP

1Professor, Institute of Geotechnical Engineering, Southeast Univ.,Nanjing 210096, China. E-mail: liusyseu.edu.cn

2Professor, Institute of Geotechnical Engineering, Southeast Univ.(SEU), Nanjing 210096, China (corresponding author). E-mail: duyanjun@seu.edu.cn

3Doctoral Student, Institute of Geotechnical Engineering, SoutheastUniv., Nanjing 210096, China. E-mail: yiyaolin@seu.edu.cn

4Professor, Dept. of Civil Engineering, Univ. of Texas at Arlington,Arlington, TX 76019; presently, Visiting Professor, Institute of Geotechni-cal Engineering, Southeast Univ., Nanjing, China. E-mail: anand@uta.edu

Note. This manuscript was submitted on September 3, 2010; approvedon August 26, 2011; published online on August 26, 2011. Discussion per-iod open until November 1, 2012; separate discussions must be submittedfor individual papers. This paper is part of the Journal of Geotechnical andGeoenvironmental Engineering, Vol. 138, No. 6, June 1, 2012. ©ASCE,ISSN 1090-0241/2012/6-718–727/$25.00.

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is to transfer surcharge embankment loads on the DM columns andreduce the vertical loads on the soft clays (Han and Gabr 2002;Porbaha and Dimillio 2004). Though this design reduces differen-tial displacement between treated and untreated soils, it oftencomes with high construction costs because of the larger areareplacement ratios used for the entire soft clay depths and also be-cause of the cost of geosynthetic layers used for LTPs.

In the rigid pile-supported embankment, installation of anenlarged pile cap is an alternative solution to this problem. The in-creased percentage coverage provided by pile caps can consider-ably suppress the differential settlement of ground surface (Hanand Gabr 2002). In Japan, a column–slab system consisting offloating soil cement columns (i.e., columns not fully penetratingthe soft layer) combined with a soil cement slab installed at shallowdepth has been developed to reduce construction costs and enhanceperformance of DM column-supported embankments (Shen et al.2001). The soil cement slab, close to 1 m in thickness, is usuallyinstalled using the shallow mixing method. The column–slabsystem-supported embankment has been reported to perform wellin terms of mitigation of total and differential settlement betweencolumns and surrounding soil (Shen et al. 2001).

Recently, for construction of DM columns for highway engi-neering in China, a new type of soil cement DM column, calledT-shaped deep mixed (TDM) column, was proposed (Liu et al.2007a). The diameter of the TDM column is larger in diameterat shallow depth (enlarged column cap) than at greater depths(deep-depth column), resulting in a column shaped like the letter“T” (see Fig. 1). The area replacement ratio of TDM columns–supported ground at shallow depth is much higher than that of con-ventional DM column–supported ground. Therefore, at greaterdepths, TDM columns can be installed at wider intervals thancan conventional DM columns, reducing the amount of cementused in the construction project. The enlarged column cap in theTDM column–supported embankment is somewhat similar tothe pile cap in the rigid pile-supported embankment except thatthe former is less rigid and greater in length than the latter. It shouldbe noted that the area replacement ratio of the TDM column–supported embankment is generally higher than that of the rigidpile–supported embankment. It is hypothesized that the load trans-fer mechanism of a TDM column–supported embankment is some-what closer to that of a rigid pile–supported embankment becauseof the similarities in their geometrical configurations. Both thehigher column efficacy of TDM columns and the lower additionalstress on surrounding soil under embankment loading are expectedbecause of the higher area replacement ratio used in shallow TDMcolumn–supported ground.

The effect of the enlarged column cap of the TDM column issimilar to the effect of increasing the percentage coverage of pilecaps in rigid pile–supported embankments, which can increasepile efficacy and suppress differential settlement of ground sur-face (Han and Gabr 2002). Thus, both total settlement and

postconstruction settlement of TDM column–supported groundwould be lower than with conventional DM column–supportedground. This behavioral aspect is currently being evaluated withnumerical modeling analyses.

The TDM column was first used in China in 2005, and wasproven to be more effective with respect to both performanceand cost than the conventional DM column (Liu et al. 2007b).Currently, TDM column have been used in more than 20 highwayengineering projects in China, with the total volume of treated softsoil exceeding 220;000 m3. Pinto et al. (2005) reported a solutionfor ground improvement similar to TDM column that uses cementjet grouting columns 1.2 m in diameter for railway construction.The tops of the columns were enlarged to a diameter of 2.5 m.On top of the enlarged caps was placed an LTP consisting of poly-propylene geogrids reinforced with compacted granular fill. Thefull-scale load test showed that Pinto and colleagues' (2005) solu-tion performed well.

In this research study, field tests were carried out on highwayembankments on soft clayey soil supported by both TDM and con-ventional DM columns. Column installation, in situ plate loadingtests, and monitoring results under embankment loading, includingstress concentration ratio, column efficacy, excess pore water pres-sure, ground total settlement, and lateral soil displacement at theembankment toe, are presented and discussed in this paper.

Description of Test Sites

Pilot field tests were conducted along the Husuzhe Highway inJiangsu Province, China. The Husuzhe Highway is located inthe south of the Jiangsu Province, nearing Suzhou City and TaihuLake. Three test sites (A–C) were contiguously planned in themiddle of Husuzhe Highway. Each site was 50 m wide and100 m long. The height of the embankment was approximately 4 m.

Both in situ and laboratory tests were conducted for site char-acterization studies before installation of the TDM/DM columns. Insitu included vane shear and piezocone penetration (CPTu) tests.The piezocone used has a projected area of 1;000 mm2, an apexangle of 60º, and a sleeve surface area of 15;000 mm2. The stan-dard penetration rate during the piezocone test was 20 mm∕s. Porepressure was measured behind the cone (u2), and the cone area ratiowas 0.80. The CPTu testing was conducted as per ASTM (2000).

Undisturbed soil samples for laboratory tests were obtained us-ing stainless thin-wall sampling tubes. Moisture content, plastic

Fig. 1. Illustration of TDM column–supported embankment

Fig. 2. Piezocone penetration test results before TDM/DM column in-stallation: (a) tip resistance, qt ; (b) sleeve friction, f s; (c) pore waterpressure, u2

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limit, and liquid limit of the sampled soils were measured as perASTM (2010). The coefficient of volume compressibility (mv) wasobtained from odometer tests on the soil samples. Soil sampling,CPTu tests, and vane shear tests were conducted along the center-line of each test site at intervals of approximately 10 m.

Results of the in situ CPTu, laboratory, and in situ vane sheartests are illustrated in Figs. 2 and 3. On the basis of the test results,ground soil strata can be divided into four layers: crust, muck, silt,and clay. The CPTu results shown in Fig. 2 indicate that tip resis-tances (qt) in the muck layer and silt layers are lower at Site C thanat Sites A and B, especially at depths from ∼3 to ∼5 m. Overall, onthe basis of the remaining trends noted in Figs. 2 and 3 and theknown geological features of these sites, it can be concluded thatthe soil deposition conditions at all three test sites are similar. Theaverage coefficient of volume compressibility of the muck layer atdepths of 2 to 14 m was about 0:65 MPa�1 [corresponding to anaverage constrained modulus (1∕mv) of 1:5 MPa]. On the basis ofthe value of constrained modulus, the muck layer is classified as ahigh compressible soil as per the method outlined in Lambe andWhitman (1969). As a result, the muck layer represents anundesirable condition that needs to be treated appropriately. Thegroundwater table was recorded at a depth of about 1.0 to1.5 m from the existing ground surface.

Design Considerations

Sites A and B were treated with the soil cement TDM columns,whereas Site C, which was the control section and was treated withconventional soil cement DM columns. The columns were arrangedin a triangular pattern at each site. Column spacing (S) was 2.0, 1.8,and 1.4 m for Sites A, B, and C, respectively Fig. 4(a). The length(L) of all columns was 16.5 m (i.e., penetration into fourth soillayer). The conventional DM column has a uniform diameter(D) of 0.5 m, whereas the TDM column has two diameters: a largediameter for the enlarged column cap (D1 ¼ 1:0m) and a smalldiameter for the deeper part of the column (D2 ¼ 0:5 m). Thelengths of the enlarged column cap (L1) were 4 and 3 m for SitesA and B, respectively. The geometry of the columns at each test siteis illustrated in Fig. 4(b).

The column area replacement ratio (as) is defined as the ratio ofthe column area to the whole area of the influence unit cell,expressed as (Bergado et al. 1996)

as ¼Ac

Ac þ Asð1Þ

where Ac = horizontal area of a column; and As = horizontal area ofthe soil surrounding the column. For the triangular pattern, the areareplacement ratio is calculated as (Bergado et al. 1996)

as ¼π

2ffiffiffi3

p�DS

�2

ð2Þ

where D = column diameter; and S = column spacing (from columncenter to column center).

For TDM column–supported ground, there are two values of as:one at shallow depth and the other at greater depth. Both values

Fig. 3. Soil profiles and soil properties at three testing sites; w = water content; wL = liquid limit; wP = plastic limit; e = void ratio, mv = coefficient ofvolume compressibility; Su = undrained shear strength obtained from vane shear test

Fig. 4. Geometry of columns at test sites: (a) plan view and (b) a–a′cross section; S = column spacing; D = diameter of conventional DMcolumn; D1 = diameter of enlarged cap of TDM column; D2 = diameterof deeper part of TDM column

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were calculated using Eq. (2) by replacing D with D1 and D2, re-spectively. The calculated values of as are listed in Table 1. Becausethere are two different values for TDM columns, it is difficult tocompare the area replacement of TDM column–supported groundwith that of conventional DM column–supported ground. As a re-sult, in this study, an equivalent diameter (De) is proposed for TDMcolumns that assumes that a TDM column and a conventional DMcolumn are equal in volume. The equivalent diameter (De) is thenexpressed as

De ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiD2

1L1 þ D22ðL� L1ÞL

rð3Þ

where De = equivalent column diameter; D1 = diameter of enlargedcap of TDM column;D2 = diameter of deeper part of TDM column;L1 = length of enlarged cap of TDM column; and L = total columnlength.

The equivalent area replacement ratio (aes) is then calculated as

aes ¼π

2ffiffiffi3

p�De

S

�2

ð4Þ

For conventional DM column–supported ground (Site C), aes ¼ as.The values of aes and as calculated at the three test sites are listed inTable 1. The values of as at shallow depth at Sites A and B are 0.227and 0.280, respectively, which are much larger than that at Site C;whereas the as at greater depths at Sites A and B are much lowerthan that at Site C. The aes values at Sites A and B and as value atSite C are 0.098, 0.108, and 0.116 respectively, indicating that theamounts of cement used (i.e., cost of cement used for construction)for Sites A and B are 15 and 7% smaller than that of Site C, re-spectively, resulting in lower construction costs.

In this study, locally produced cement, equialent to Portland ce-ment Type I, was used for the DM and TDM operations. A cementslurry with a water:cement ratio of 0.55 was used for deep mixing.On the basis of laboratory tests on muck soil, a cement content (aw)of 22.5% was recommended and used at the construction site.Cement content (aw) is defined as the percentage weight of cementto weight of dry soil that is to be improved. Field cement content(av) is defined as the percentage weight of cement added to thevolume of soil that needs to be improved, and this term is widelyused (Han et al. 2002). In this study, the av value was 255 kg∕m3

for an average bulk density of 17 kN∕m3 and an average water con-tent of 50% for the treated muck soil during mixing operations. Thesame av value was used in all other soil layers in consideration ofconstruction convenience and quality control assessments in thefield operations.

Column Installation

The double slurry mixing method was followed at all sites. Detailsof the double slurry mixing method can be found in CDIT (2002)and Shen et al. (2003, 2008). During installation of the TDM

columns at Sites A and B, the mixing blades spread outward atdesignated levels where enlarged column caps were constructed,whereas the mixing blades shrunk inward at designated levelswhere the deeper parts of the columns were constructed (Liu et al.2007a). Details of the composition of the construction machine andstructure of the mixing head can be found in Liu et al. (2007a, b). Incolumn installation, the pressure with which the cement slurry wasinjectd was 0.3 MPa, and the rotation speed was maintained at ap-proximately 60 rpm. Installation procedures for both DM and TDMcolumns are given by Liu et al. (2007a, b), and the Jiangsu ProvinceConstruction Department (JPCD) (2007).

The JPCD provides a guideline on how to determine the length,diameter, and spacing of enlarged column caps: the length is lessthan one-third of the whole column; the diameter is 1.8 to 2.4 timesthat of the deeper part of the column (which is greater than500 mm); and the spacing is restricted to the range of 1.8–mto 2.4 m.

Field Testing Program

Quality Control/Quality Assurance Investigations

To evaluate soil cement column quality, at 28 days after installation,six columns at each site randomly selected for coring. One coringposition was set near the center. The other coring position was nearthe edge of the enlarged cap of the TDM column. Core samplescollected were 50 mm in diameter and 100 mm long. The lengthof the constructed column was estimated by measuring the lengthof the sampled cores. The continuity of the cores was also visuallyinspected. Column strength was investigated by performing uncon-fined compressive strength (UCS) tests on the sampled cores.

In Situ Plate Loading Tests

At each test site, three columns were randomly selected for thestatic plate loading test of the single-column composite foundation28 days after installation. A circular steel plate 30 mm thick wasused as the loading plate. Plate area was the same for single-columninfluence areas (or unit cells). Plate diameter was 2.1, 1.9, and1.5 m for Sites A, B, and C, respectively. Before loading, a100-mm-thick sand mat was placed between the composite foun-dation and the loading plate.

The plate was loaded by jacking against the steel beams onwhich the sand bags were placed. A slow maintained-load testmethod was adopted for loading tests. The test load was appliedin increments, and the loading plate was allowed to move undereach maintained-load increment until a certain rate of displacementwas achieved. Load increments in this study ranged from 20 to30 kPa, and each load increment was maintained until two consecu-tive displacement readings in each hour were < 0:1 mm. In China,two failure criteria are commonly used for loading tests (Han andYe 2006): (1) the load was maintained for 24 h but with the rate ofdisplacement still exceeding 0:1 mm∕hr; and (2) the total displace-ment exceeded 10% of the width of the plate. In this study, all fail-ures were determined by the second criterion. Unloading wasstarted immediately after termination of the last load.

Field Instrumentation and Monitoring

After installation of soil–cement columns, but before embankmentfilling, a monitoring section was selected in the middle of each testsite (near the section of site characterization). Monitoring instru-ments include earth pressure cells, piezometers, and inclinometers.Fig. 5 shows the locations of the instruments in the monitoring sec-tion at Site A. The setup of instruments at Sites B and C is the same

Table 1. Area Replacement Ratio at Different Test Sites

Site Column type asa as

b aesc

A TDM 0.227 0.056 0.098

B TDM 0.280 0.070 0.108

C Conventional DM 0.116 0.116 0.116aArea replacement ratio of enlarged column cap.bArea replacement ratio of deeper part of column.cEquivalent area replacement ratio.

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as that at Site A. Earth pressure cells with a diameter of 120 mmwere installed on the tops of columns and on the ground surface. Ateach test site, two earth pressure cells were placed at the point of thesoil surface representing the centroid of three columns, and anothertwo earth pressure cells were placed on the column surface. Twopiezometers were installed 2.0 and 10.0 m below the centroid ineach test, corresponding to the lower levels of the enlarged capand the deeper part of the depth TDM column. Settlement plateswere installed on the surrounding soil surface as shown in Fig. 5.The surface settlement plate was placed in the centerline and at thecentroid of three consecutive column surfaces. For each test site,except for two inclinometers that were installed at the embankmenttoe, the other instruments were installed along the embankmentcenterline. Staged construction and surcharge techniques were usedfor embankment construction. The measured embankment incre-ment is shown in Fig. 6 and is similar for the three test sites.

Results and Discussion

Unconfined Compressive Strength Tests

Fig. 7 shows the variation UCS in core samples with depth at thethree test sites. It can be seen that most of the UCS values variedfrom 1.0 to 1.6 MPa. The mean UCS values are 1.32, 1.24, and1.25 MPa for core samples at Sites A, B, and C, respectively, in-dicating that of the soil cement columns installed at the three sitesare almost the same with respect to quality.

Because of the variability of the native soil, mixing effective-ness, and other factors, the strength of the DM material may behighly variable even at the same construction site (Navin and Filz2005). Burke and Sehn (2005) summarized more than 1,000 UCS

data on DM column core samples from 11 projects, and reportedthat coefficients of variation (COVs) values ranged from 0.22 to0.76, with an average of 0.42. Navin and Filz (2005) analyzed alarge number of UCS data on DM columns from several projectsin the United States, and the COVs ranged from 0.34 to approx-imately 0.74, with an average of apporoximately 0.55. In this study,the COVs values of UCS data were computed using the methodreported by Larsson et al. (2005). The COVs were 0.259, 0.206,and 0.262 for core samples at Sites A, B, and C, respectively, whichare on the lower side of the average values reported by Burke andSehn (2005) and Navin and Filz (2005). The relatively low COVdata for UCS installed soil cement columns in this study is attrib-uted mainly to the use of the double slurry mixing method for col-umn installation, which resulted in enhanced soil cement mixing.The properties of the treated soils were shown to be similar.

The UCS results can be considered representative of qualitycontro/quality assurance (QC/QA) assessments of field treatmentmethods followed for both TDM and DM operations, and are con-sidered to be more than satisfactory for the present treatmentstudies.

In Situ Plate Loading Tests

Fig. 8 illustrates plate loading test results for single-columncomposite foundations at Site A (A-1, A-2, A-3), Site B (B-1,B-2, B-3), and Site C (C-1, C-2, C-3). As the diameters of the loadplates at the different test sites differed, the measured settlement

Fig. 5. Locations of monitoring instruments at Site A

Fig. 6. Embankment height versus time

Fig. 7. Variation unconfined compressive strength of core sampleswith depth

Fig. 8. Load displacement obtained from plate loading test of single-column composite foundation

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was normalized by plate diameter. Under the same surcharge load-ing, the normalized settlement measurements at Sites A and B aremuch lower than that at Site C. The difference increased with in-crease in surcharge loading. There is a slight difference in the loadresponse for triplicates at the same site, which is due mainly to themarginal variability of column quality. From the plate loading test,the ultimate load was determined as the load prior to failure beingreached, as discussed earlier.

The ultimate bearing capacity of the single-column compositefoundation (qult) was then calculated as the average value of theultimate loads determined. The estimated values of qult at SitesA, B, and C are 270, 300, and 126 kPa, respectively. The firsttwo values are nearly 2.1 and 2.4 times the last value, indicatingthat the bearing capacity of the TDM column–supported groundis higher than that of the coventional DM column—supportedground. The surcharge loading influence depth is limited becauseof the limited load plate area; therefore, the bearing capacity ofdeeper composite ground was not well mobilized. However, underthe conditions in this study, it can be concluded that the groundsupported by TDM columns had greater bearing capacity theground supported by conventional DM columns.

Stress Concentration Ratio and Column Efficacy

The average stresses measured on the surface of columns and soilsat each test site were used to calculate the stress concentration ratio(n) and column efficacy (ηc), as illustrated in Fig. 9. The stress con-centration ratio is defined as the ratio of measured stress (kPa) act-ing at the column surface to that acting at the ground surface,expressed as

n ¼ σp

σsð5Þ

where σp = measured average stress on the surface of columns; andσs = measured average stress on the surface of soil. Column effi-cacy, defined as the ratio of the load (kN) acting at the column to theentire load acting at the supported ground, is expressed as

ηc ¼n · as

n · as þ ð1� asÞð6Þ

where as = area replacement ratio.As stress was measured on the ground surface by earth pressure

cells, the area replacement ratio at shallow depth was used to cal-culate column efficacy with Eq. (6). Fig. 9(a) shown that stress con-centration ratio increased with embankment height (loading) at thethree test sites. At the beginning of embankment construction, thatis, during the first 45 days, n was slightly lower at Site C than atSites A and B because of the lower embankment height at Site C.However, n reported rapidly after 45 days.

Shen et al. (2001) and Chai et al. (2002a) reported that n de-pends on several factors, such as modulus ratio of column to soil,area replacement ratio, loading time, and rigidity of the foundation.As the average UCS of the TDM columns and the conventionalcolumns at Site A (1.32 MPa), Site B (1.24 MPa), and Site C(1.25 MPa) are nearly the same, the moduli of the columns at thesetest sites are considered to be nearly the same because that soil ce-ment column modulus is proportional to its UCS (Bergado et al.1996). For a given loading time and loading intensity at the threetest sites (which have nearly same rigidity), the difference in themeasured stress concentration ratio (n) is attributed mainly tothe lower equivalent modulus for TDM columns. The higher areareplacement ratio of the column used at shallow depth contributedto the lower equivalent modulus of the TDM column–supportedground. As the area replacement ratio at Site C (0.116) is muchlower than that of the enlarged cap at Site A (0.227) or Site B(0.280), n increased rapidly and was higher at Site C than at SitesA and B. The final measured n values at Sites A, B, and C are 3.18,3.06, and 4.19, respectively.

Fig. 9(b) illustrates the values of ηc at Sites A and B, which arehigher than that at Site C throughout the monitoring. Again, thisobservation is attributed mainly to the larger area replacement ratioof the shallow enlarged cap at Sites A and B. The higher efficacyvalues at Sites A and B, relative to that at Site C, indicates that alarge portion of the additional stress induced by embankment load-ing was transferred to the column (and, then, to the bearing clay soillayer), and there was less embankment loading acting on the sur-rounding soil.

Excess Pore Water Pressure

Fig. 10 shows the excess pore water pressure measured at depths of2.0 and 10.0 m in the surrounding soil. The excess pore water pres-sure increased with the increase in embankment and decreased withtime owing to the dissipation. Excess pore water pressure at both2.0 and 10.0 m is lower at sites A and B than at Site C because thehigher column efficacy at Sites A and B resulted in lower additionalstress in the surrounding soil at both sites. The excess pore waterpressures measured at a depth of 2.0 m were lower than those mea-sured at 10.0 m for two reasons: (1) The ground surface represents apartial drainage boundary, and the drainage distance is shorter at2.0 m than at 10.0 m. As a result, the excess pore water pressuredissipated more rapidly at 2.0 m than at 10.0 m. (2) At Sites Aand B, where TDM columns were installed, the area replacementratio of the shallow enlarged cap was larger than that of the deeperpart of the column. Therefore, a larger portion of the vertical addi-tional stress in the surrounding soil induced by embankment load-ing was transferred to the enlarged cap at shallow depth, resulting ina lower excess pore water pressure.

Total Settlement Profiles

Fig. 11 shows that settlement increased with an increase in embank-ment height. At Sites A and B, 164 days after completion of

Fig. 9. Change in (a) stress concentration ratio and (b) column efficacytime elapsed

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embankment fill construction, the heights were nearly the same. AtSite C, 193 days after completion of the embankment, the heightwas practically the same as those at Sites A and B. The final readingat 290 days shows that the total settlement of the TDM column–supported ground (Sites A and B) was only about 65% that of con-ventional DM column–supported ground (Site C) (see Fig. 11).

In highway engineering, postconstruction settlement is of majorconcern to engineers. In this study, the hyperbolic method (Lin andWong 1999) was used to predict final and postconstruction settle-ment using the readings taken after embankment construction. Onthe basis of the hyperbolic method, which assumes that the rate ofsettlement decreases hyperbolically with time, a settlement—timecurve can be expressed by

ts¼ αþ βt ð7Þ

where t = time from the start of embankment filling; s = measuredsettlement at the ground surface at any specific time after comple-tion of embankment; and α and β = gradient and intersection of thestraight line between t and t∕s, respectively. As staged constructionwas used in the embankment filling in this study, only the data

collected after completion of embankment were selected for usein Eq. (7).

Final settlement (s∞) is calculated as follows by assumingt ¼ ∞: Postconstruction settlement (SP) is defined as estimatedfinal settlement (S∞) minus settlement measured at the end ofembankment (sc):

s∞ ¼ 1β

ð8Þ

sP ¼ s∞ � sc ð9Þ

Fig. 12 shows the fitting result using the hyperbolical method ex-pressed by Eq. (7). on the basis of Eq. (8), final settlement at thecenterlines of Sites A, B, and C was estimated to be 278, 179, and384 mm, respectively. The values for Sites A and B are 72 and 47%that for Site C. Using Eq. (9), the values of sP are calculated to be123 mm (278–155 mm), 22 mm (179–157 mm), and 177 mm(384–207 mm) for Sites A, B, and C respectively. The first twovalues are 70 and 12% of the third value. The aforementionedanalysis results indicate that the TDM column provides a betterground treatment solution for reducing settlement of soft ground.The final settlement at Site Awas also found to be larger than that atSite B. Because site conditions, column quality, and loading pro-cess at these two sites are similar, the difference in performance isattributed mainly to the relatively higher column efficacy (as shownin Fig. 9) and consequent lower stress acting on the soil at Site B.

Lateral Displacements

Fig. 13 shows the measured profiles of lateral displacement at dif-ferent dates. The southern inclinometer tubes were damaged duringembankment filling as they were quite near the road on which theengineering machine was operated. Lateral displacement measuredfrom southern inclinometers differed slightly from that measuredfrom northern inclinometers, mainly because embankment fillingrate differed between the southern and northern sections. The dataobtained from northern inclinometers reveal that movement wasmaximum at shallow depth (< 5:0 m) and decreased with depth.At a given construction date, lateral displacement measured at SitesA and Site B is lower than that measured at Site C. The maximumlateral displacement values for Site A (20.84 mm) and Site B(12.11 mm) were much less that for Site C (38.72 mm). The resultare consistent with the loading test, total settlement, and excesspore water pressure measurements, mainly because of the higherarea replacement ratio of the shallow enlarged cap as Sites Aand B.

Fig. 10. Dissipation of excess pore water pressure: at depths of (a) 2 mand (b) 10 m

Fig. 11. Measured settlement of ground surface versus time elapsed

Fig. 12. Relationship between time elapsed and time/settlement

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According to Indraratna et al. (1992), Chai et al. (2002b), andLiu et al. (2007c), the ratio of maximum lateral displacement mea-sured at the embankment toe to maximum total settlement (kown asthe lateral displacement ratio) at the embankment centerline is agood indicator of embankment stability. Indraratna et al. (1992)concluded that a small ratio is necessary to maintain embankmentstability during the filling period. Chai et al. (2002b) found thatembankment failure is characterized by a ratio larger than 0.5.Liu et al. (2007c) found that for a geogrid-reinforced and pile-supported embankment, the ratio increases slightly, but less than0.3, during embankment filling. In this study, the lateral displace-ment ratios are 0.07, 0.08, and 0.17 for Sites A, B, and C at com-pletion of half the embankment height. The ratios increased to 0.12,0.09, and 0.22, respectively, at completion of embankment at SitesA, B, and C, respectively. The lower ratios for Sites A and B in-dicate that TDM columns–supported ground enhances the embank-ment stability better than conventional DM column–supportedground.

Advantages of TDM Columns

Performance at all three test sites is summarized in Table 2. Theimproved areas and embankment heights at all three test sites

are the same, as are soil conditions and strength. Table 2 clearlyshows that the total amounts of cement used at Sites A and B(TDM columns) are 85 and 93%, respectively, of the total amountof cement used at Site C (conventional DM columns). Total con-struction time for Sites A and B is 72 and 81%, respectively, of theconstruction time for Site C. In addition, the ultimate bearingcapacity of Sites A and B is 2.1 and 2.4 times that of Site C.

The final settlement values for Sites A and B are 70 and 12%that of Site C; maximum soil lateral displacement values for Sites Aand B are only 44 and 47% that for Site C. The lateral displacementratios for Sites A and B at the completion of embankment filling are55 and 41% that for Site C. These results clearly indicate that per-formance of TDM column–supported ground (Sites A and B) isenhanced compared with that of conventional DM column–supported ground (Site C) under equal embankment loading andrequires less cement and less construction time. These are consid-ered major benefits of TDM columns over conventional DM col-umns where the construction of varying diametral treatments atvarious depths are fully achieved for optimal benefits of larger areatreatments at shallow depths to lower treatment areas at deeperdepths.

Fig. 13. Profiles of lateral soil displacement below embankment toe: (a) Site A; (b) Site B; (c) Site C

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Summary and Conclusions

This paper is a comprehensive summary of the field testing pro-gram conducted to investigate the performance of TDM column–supported soft ground under embankment loads compared withthat of conventional DM column–supported soft ground. On thebasis of results from field tests the field tests on TDM andDM prototype test sections, the following conclusions can bedrawn:1. For treatment of soft soil, installation of TDM columns at Sites

A and B resulted in cement savings of 15 and 7% by weightand construction time savings of 28 and 19% compared withinstallation of conventional DM columns.

2. The UCS test results show that at the three test sites, the ver-tical variability of core samples near the column center is onthe lower side of literature values. The UCS results can be con-sidered representative of QC/QA assessments of TDM and DMcolumn operations.

3. Soil deposition conditions and column strengths at all three testsites are similar, whereas the equivalent area replacement ratiosof TDM column–supported ground (Sites A and B) are lowerthose of conventional DM column–supported ground (Site C).However, ultimate bearing capacities of TDM column–supported ground are more than twice those of conventionalDM column–supported ground. This indicates that TDM col-umns yield a stiffer foundation than conventional DMcolumns, at least in shallow supported ground.

4. The stress concentration ratio of TDM column–supportedground is lower than that of conventional DM column–supported ground, whereas the efficacy of TDM columns isgreater than that of DM columns. Consequently, excess porewater pressures in TDM column–supported ground are lowerthan those in conventional DM column–supported ground.

5. Post construction settlement of TDM column–supportedground is 72% (Site A) and 47% (Site B), respectively, thatof conventional DM column–supported ground (Site C). Also,maximum soil lateral displacement of TDM column–supported ground is 44% (Site A) and 47% (Site B) that ofconventional DM column–supported ground (Site C). This in-dicates that the TDM column–supported embankment per-formed better because it induced lesser settlement in bothvertical and lateral directions under embankment loading.

6. The field study indicates that TDM columns provide a viable,economic, and technologically sound solution for soft groundimprovement under embankment loads when compared withconventional DM columns.

Acknowledgments

This study is partly financially supported by the NationalNatural Science Foundation of China (Grant Nos. 50879011,50878052, and 40972173). The authors are very grateful to

Prof. Z. D. Zhu, Dr. P. S. Xi, Mr. B. F. Zhang, and Mr. Z. H.Zhu for their help during the field testing program.

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Table 2. Comparison of Performance of TDM and DM Columns at Test Sites

Site Column typeTotal amount of cement atconstruction site (×103 kg)

Construction timeof test site (h)

qult(kPa)

s∞(mm)

sP(mm)

δmax(mm)

Lateraldisplacement ratio

A TDM 2057.7 1444 270 278 123 20.84 0.12

B TDM 2272.1 1634 300 179 22 22.56 0.09

C Conventional DM 2430.5 2013 126 384 177 47.69 0.22

Note: qult = ultimate bearing capacity; s∞ = final settlement calculated using hyperbolic method; sP = postconstruction settlement, defined as final settlementminus settlement measured at completion of embankment; δmax = maximum soil lateral displacement below embankment to.

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