strength development in cement admixed bangkok...

239

i) Professor, School of Civil Engineering, Suranaree University of Technology, Thailand (suksun＠g.sut.ac.th).ii) Assistant Professor, Department of Civil Engineering, Mahanakorn University of Technology, Thailand.iii) Post Graduate Researcher, School of Civil Engineering, Suranaree University of Technology, Thailand.iv) Assistant Professor, Construction Technology Research Unit, School of Civil Engineering, Suranaree University of Technology, Thailand.

The manuscript for this paper was received for review on September 29, 2010; approved on December 7, 2010.Written discussions on this paper should be submitted before November 1, 2011 to the Japanese Geotechnical Society, 4-38-2, Sengoku,Bunkyo-ku, Tokyo 112-0011, Japan. Upon request the closing date may be extended one month.

239

SOILS AND FOUNDATIONS Vol. 51, No. 2, 239–251, Apr. 2011Japanese Geotechnical Society

STRENGTH DEVELOPMENT IN CEMENT ADMIXED BANGKOK CLAY:LABORATORY AND FIELD INVESTIGATIONS

SUKSUN HORPIBULSKi), RUNGLAWAN RACHANii), APICHAT SUDDEEPONGiii) and AVIRUT CHINKULKIJNIWATiv)

ABSTRACT

The in-situ deep mixing technique has been established as an eŠective means to eŠect columnar inclusions into softBangkok clay to enhance bearing capacity and reduce settlement. In this paper, an attempt is made to identify the criti-cal factors governing the strength development in cement admixed Bangkok clay in both the laboratory and the êld. Itis found that clay-water/cement ratio, wc/C is the prime parameter controlling the laboratory strength developmentwhen the liquidity index varies between 1 and 2. Based on this parameter and Abrams' law, the strength predictionequation for various curing times and combinations of clay water content and cement content is proposed and veriêd.This will help minimize the number of trials necessary to arrive at the quantity of cement to be admixed. Besides thewc/C, the strength of deep mixing column is controlled by the execution and curing conditions. For low strength im-provement (laboratory 28-day strength less than 1,500 kPa), the êld strength of the deep mixing columns, quf, madeup from both dry and wet mixing methods is higher than 0.6 times the laboratory strength, qul. The quf/qul ratios for thewet mixing columns are generally higher than those for the dry mixing columns. This higher strength ratio is due to thedissipation of the excess water in the column (consolidation) caused by the êld stress. The water to cement ratio,W/C, of 1.0 is recommended for the wet mixing method of the soft Bangkok clay. A fast installation rate was shownto provide high quality for low strength columns. Suggestions are made for improving the deep mixing of soft Ban-gkok clay, which are very useful both from economic and engineering viewpoints.

Key words: Bangkok clay, clay-water/cement ratio, curing stress, deep mixing technique, low-swelling clayey soil, un-conˆned compression test (IGC: D6/H1/K6)

INTRODUCTION

Bangkok clay is well-known as a soft clay with a highwater content close to its liquid limit. It has large poten-tial for settlement with low inherent shear strength. Thisclay is classiêd as non- to low swelling (Horpibulsuk etal., 2007) as per free swelling test (Prakash and Sridha-ran, 2004). Its swelling potential increases with depth. Be-sides Bangkok clay, non- to low swelling soils are general-ly found in many lowlands, such as Ariake bay in Japan(El-Shafei, 2001; Modmoltin, 2002). Data provided byTanaka et al. (2001) indicate that some marine clays(Pusan, Singapore, Drammen, Louiseville clays) are clas-siêd as inactive and normal clays, and are thus non- tolow swelling. Even though the clay mineralogy of manyclayey soils is primarily montmorillonite, the mont-morillonite in those soils might not be the dominantparameter controlling the soil expansivity. Otherparameters, such as other clay minerals (kaolinite, and il-lite, etc), the non-clay fraction (À0.002 mm) and poremedium chemistry, can also play a great role, masking

the role of montmorillonite. As such, it is possible thatsome clayey soils can be classiêd as non-swelling or lowswelling types, even if the primary clay mineral in clayfraction (º2 mm) is montmorillonite (Horpibulsuk et al.,2007).

One of the eŠective ground improvement techniquesfor the soft Bangkok clay is in-situ deep mixing. Cementis commonly used as a cementing agent since it is readilyavailable at a reasonable cost in Thailand. The resistanceto compression and consequent strength development ofthe cement admixed clay increases with curing time. Thecolumnar inclusions in the soft ground transform all suchsoft ground to composite ground.

The fundamental mechanical properties of cement ad-mixed clays have been extensively investigated by Terashiet al. (1979, 1980); Kawasaki et al. (1981), Kamon andBergado, (1992) and Horpibulsuk et al. (2004a, b), etc.Even though many previous investigations (Nagaraj andMiura, 1996; Uddin, 1994; Yin and Lai, 1998; andothers) have been focused on the eŠect of water contentand cement content on the strength development in ce-

240240 HORPIBULSUK ET AL.

ment admixed clays, the combination eŠect of both watercontent and cement content has never been well integrat-ed. Moreover, no dosage methodologies based on ration-al criteria have been suggested like those which are usedin concrete technology, where the water/cement ratioplays a fundamental role in the assessment of targetstrength. In concrete technology, Abrams' law (Abrams,1918) is broadly applied: for a given concrete with certaincontent, age and curing conditions, the strength of thehardened concrete is determined exclusively by the ratioof free water content to the cement content in the mix.Strength is independent of the absolute contents of freewater and cement content in the mix.

As an analogy, Horpibulsuk and Miura (2001), Hor-pibulsuk et al. (2005) and Miura et al. (2001) have identi-êd the clay-water/cement ratio, wc/C as a parameter foranalyzing and assessing laboratory strength developmentin cement admixed clays. It is deˆned as the ratio of claywater content to cement content (both reckoned in per-centage). While the clay water content re‰ects themicrofabric of soft clay, the cement content in‰uencesthe level of bonding of that fabric. Based on thisparameter and Abrams' law (Abrams, 1918), Horpibul-suk et al. (2003) have revealed that for a given set of ce-ment admixed clays, the strength development dependsonly on the clay-water/cement ratio, wc/C. They havealso introduced a generalized equation for predictinglaboratory strength development in cement admixed claysat various water contents, cement contents, and curingtimes.

In addition to laboratory studies, studies on the perfor-mance of composite ground are vital. Experimental in-vestigations have been done by Miyake et al. (1991),Hashizume et al. (1998) and Kitazume et al. (1999). Fieldobservations on the deep mixing column are also necessa-ry to investigate the êld strength development in order toprovide an understanding of the diŠerence between êldand laboratory strengths and consequently, estimates ofthe optimal input of cement to achieve the target êldstrength. Horpibulsuk et al. (2004c) and Nishida et al.(1996) have concluded that the strength diŠerence be-tween êld and laboratory improvement is mainly causedby the non-uniformity in mixing in-situ clay with cement.Horpibulsuk et al. (2004c) have classiêd the deep mixingcolumn into low and high strength (cement content)columns based on the di‹culty in êld mixing. They haveshown that for the low strength column (qul(28days)º1500kPa, where qul(28days) is the laboratory 28-day strength),both fast and slow installation rates yield practically thesame quality. In the case of the high strength column,however, a low installation rate of less than 0.7 m/min isrequired. Its êld to laboratory strength ratio is about0.33 to 0.67, which is much lower than that of the lowstrength column.

In practice, many laboratory trial mixes are needed toarrive at a proper strength before the execution of thedeep mixing column. This laboratory strength must behigh enough to compensate for execution and curing con-ditions. At the service time, the êld strength must meet

the designed strength. To facilitate the determination ofthe proper quantity of cement to be admixed, which com-pensates for strength reduction in the êld, a geotechnicalengineer needs a simple and rotational method to assesslaboratory and êld strengths with time for various com-binations of water content and cement content by mini-mum laboratory trials. In this paper, we analyze thestrength development in the cement admixed Bangkokclay in both laboratory and êld improvements. Theframework of the analysis of the laboratory strength de-velopment is the clay-water/cement ratio hypothesis(Horpibulsuk and Miura, 2001; Miura et al., 2001) andthe Abrams' law (Abrams, 1918). The eŠect of the execu-tion and curing conditions on the êld strength develop-ment of the deep mixing columns is examined. Finally,we make some suggestions to improve deep mixing forsoft Bangkok clay based on the laboratory and êld in-vestigations. This method could also be applied to othernon- to low swelling clay deposits.

EXPERIMENTAL INVESTIGATION

This investigation consists of laboratory and êld stu-dies to formulate a simple and rational method of assess-ing laboratory and êld strength development in the ce-ment admixed Bangkok clay.

Laboratory InvestigationSince Bangkok clay is non- to low swelling, and the

swelling potential increases with depth (Horpibulsuk etal., 2007), kaolin and Bangkok clays, with free swell ra-tios of 0.6 and 1.3, were used to represent non- and lowswelling clays, respectively. The free swell ratio, FSR, isdeˆned as the ratio of equilibrium sediment volume of10-g oven-dried soil passing a 425 mm sieve in distilledwater (Vd) to that in carbon tetra chloride or kerosene(Vk). Soil is classiêd as non-swelling and low swellingwhen the free swell ratio is (less than or equal to) 1.0 andbetween 1.0 and 1.5, respectively (Prakash and Sridha-ran, 2004). Kaolin clay was obtained from a commercialcompany. Its consistency limits were liquid limit, LL＝43z and plastic limit, PL of 34z. Its speciˆc gravity was2.78. Bangkok clay was collected from Ladkrabang dis-trict, Bangkok, Thailand at a depth of 3 to 4 meters. Itsnatural water content was 85z, the liquid and plasticlimits were in the order of 89 and 30z, and the speciˆcgravity was 2.71. The groundwater level was at about 1.0m from the surface. The overconsolidation ratio was 1.2and the eŠective strength parameters in triaxial compres-sion were c?＝0 and q?＝229. The chemical properties ofthe two clays and the Type I Portland cement are shownin Table 1. The grain size distribution curves for the twoclays and the cement are shown in Fig. 1. The speciˆcgravity of the cement was 3.15.

The main aims of the laboratory test are to analyze thestrength development of the cement admixed Bangkokclay using the wc/C as a prime parameter and to developits generalized strength prediction equation. For the ˆrstaim, both Bangkok and kaolin clays were passed through

241

Fig. 1. Grain size distribution of Bangkok and kaolin clays and Port-land cement

Table 1. Chemical composition of the cement, kaolin clay, and Bang-kok clay

Chemicalcompounds

Portland cementType I Kaolin clay Soft Bangkok

clay

zSiO2 20.90 59.79 63.83zAl2O3 4.76 31.84 21.34zFe2O3 3.41 1.59 8.41zMgO 1.25 — 1.54zCaO 65.41 — 0.94zNa2O 0.24 — 0.28zK2O 0.35 3.05 2.45zSO3 2.71 0.05 1.22

Fig. 2. Soil proˆle of the test sites

241DEEP MIXING IN BANGKOK CLAY

a 2-mm sieve to remove any shell pieces and other largerparticles. The water content was adjusted to a range ofliquidity indices (LI), i.e., 1.0, 1.5, and 2.0. The liquidityindex has been used in this investigation as an indicator torefer the initial water content of the clays in relation totheir plasticity characteristics before the cement is admix-ed. This intentional increase in water content is to simu-late water content increase taking place in the wet methodof dispensing cement admixture in deep mixing. Bothclays with their water content corresponding to the abovelevels of LI were thoroughly mixed with the cement atdiŠerent cement contents to attain the target wc/C. Thetarget wc/C values were 3.5 and 6.5 for kaolin clay and 4,7, 12, and 14 for Bangkok clay. The mixing time was ar-bitrarily ˆxed at 10 min as recommended by Miura et al.(2001). This uniform paste was transferred to cylindricalcontainers 50 mm in diameter and 100 mm in height aswell as to oedometer rings of 50 mm in diameter and 20mm in height, taking care to prevent any air entrapment.After 24 hours, the cylindrical samples were dismantled.All the cylindrical samples and the oedometer sampleswith the rings were wrapped in vinyl bags and they werestored in a humidity room of constant temperature (20±29C). After 7 and 28 days of curing, the cement admixedkaolin and Bangkok clays were taken for unconˆnedcompression (UC) test. The rate of vertical displacementin UC tests was 1 mm/min. Consolidation tests were con-ducted after 28 days of curing.

To achieve our second aim, the two clays were used todevelop a simple and rational method of predictinglaboratory strength development. The clays at LI＝1 to 2were admixed with 10 to 30z cement. After 7, 14, 28, 60,90, and 120 days, the UC tests were conducted on the ad-mixed samples. Based on the analysis of the test results,the strength prediction equation for the cement admixedBangkok clay was developed. The trial mix results for theêld investigation were used to verify the developedstrength prediction equation.

Field InvestigationThe êld study investigates the strength reduction due

to several êld factors such as non-uniformity in mixingin-situ clay with cement, and the diŠerence in curing con-ditions between laboratory and êld improvement. Deepmixing was performed in the Sukhaphiban 3, and Lad-krabang districts in Bangkok, and the Bangpee district inSumutphakarn province, Thailand. The soil proˆles forthe three test sites are presented in Fig. 2. It was foundthat the uppermost soil of about 2 meters in thickness,was weathered zone. The soft Bangkok clay was more

242

Fig. 3. mixing machines; a) dry method and b) wet method


than 13 meters thick for the three sites. The maximumthickness was found at the Bangpee district since it isclose to the Gulf of Thailand. Underlying this soft clay,medium to stiŠ clay was found. The medium to stiŠ claywas deposited above the ˆrst sand layer (1st sand) over-lying hard clay (Horpibulsuk et al., 2007). The naturalwater content of the soft Bangkok clay is close to liquidlimit (slightly higher than liquid limit) near the groundsurface, and tends to decrease with depth. There was nosigniˆcant diŠerence in the speciˆc gravity of the softBangkok clay with depth for the three sites: it was about2.67±0.02. The yield stress increased with depth, and theOCR was close to 1.0.

The tested deep mixing columns were made up fromboth the dry and wet methods. The input of cement wasobtained from a trial mix to attain a 28-day strength ofabout 1000 and 900 kPa for Sukhaphiban 3 and Lad-krabang districts, respectively. These tested deep mixingcolumns were thus classiêd as low strength as per Hor-pibulsuk et al. (2004c). Tables 2 to 4 show the laboratorystrength test results from the trial mix at both sites fordiŠerent depths together with the application of theproposed strength prediction equation. The cement con-tents are 150 and 185 kg/m3 for the dry and the wet mix-ing methods, respectively at Sukhaphiban 3 district. It is175 kg/m3 at Ladkrabang district for the wet mixingmethod. In the prediction, wc is the total water content inthe clay while being mixed with cement. For dry mixing,it is equal to the natural water content, wn but for wetmixing, it is the sum of the natural water content and theadditional water content from the cement slurry. The ce-ment content, C, is deˆned as the ratio of weight of ce-ment to dry weight of soil. Figure 3 shows the mixingmachines for both the dry and the wet mixing methods.The dry mixing columns were made up with the penetra-tion rate (PR) of 1.0 m/min, withdrawal rate (WR) of 4.0m/min, and a rate of rotation of 140 rpm. They were ex-ecuted only at the Sukhaphiban 3 district. In the wet mix-ing method, the wing rotation was 40 rpm for all threetest sites. For the Sukhaphiban 3 and Ladkrabang dis-tricts, both the PR and WR were 1.0 m/min and thewater to cement ratio (W/C) was 1.0. The eŠect of W/C,the installation rate, and the cement content on the êldstrength development of the wet mixing columns were in-vestigated at the Bangpee district. The test results werecompared with those previously reported by Horpibulsuket al. (2004c) for Ariake clay in order to provide a betterunderstanding of êld strength development. All the test-ed dry and wet mixing columns of 0.6 m diameter and 12m length were installed with 1.5 m spacing at the threesites. After 3, 14, and 28 days of curing, the cored sam-ples were taken by a coring machine. These samples weretrimmed to a diameter of 50 mm and a height of 100 mm,which are the same as those prepared in the laboratory.Since the samples are hard and carefully cored andtrimmed, the eŠect of sample disturbance on the strengthcan be neglected. The êld curing water content was alsomeasured to compare with the laboratory one.

LABORATORY TEST RESULTS

Figures 4 and 5 show the role of wc/C on the strengthcharacteristics of cement admixed kaolin and Bangkokclays. They show the typical stress¿strain relationshipsin unconˆned compression tests of samples with diŠerentinitial water contents and diŠerent levels of cementingagent but the same wc/C values. It is noted that the lowerthe wc/C, the higher the cementation bond strength,which leads to higher strength. Similar stress¿strain be-havior of all the admixed samples, which had the samewc/C, was found.

Figure 6 presents the compression behavior of the ce-ment admixed Bangkok clay samples, all with a wc/Cvalue of 12 but with diŠerent combinations of water con-tent and cement content after 28 days of curing. The yieldstress was obtained as the intersection point of the twostraight lines which extend from the linear parts on eitherend of the compression curve plotted as log (1＋e) againstlog s?v (Butterêld, 1979; Sridharan et al., 1991). Theclay-cement mixtures were made up from three clay watercontents; namely, 89z, 119z, and 148z. The (ev, logs?v) relationship is plotted so as to take care of the eŠectof the diŠerence in the void ratio for the eŠective verticalstresses less than the yield stress. In this range of the eŠec-tive vertical stress, the cementation component is the

243

Fig. 4. Unconˆned compression test results of kaolin samples, havingthe same wc/C

Fig. 5. Unconˆned compression test results of Bangkok clay samples,having the same wc/C

Fig. 6. Compression behavior of cement admixed Bangkok clay sam-ples for wc/C＝12 after 28 days of curing


dominant factor which resists compression. It is foundthat the yield stress and the deformation behavior at pre-yield stress of all samples with identical wc/C are practi-cally the same. The samples with higher clay water con-tent, however, are stable at higher void ratios and providehigher compression index beyond yield stress. This is dueto the break-up of the cementation bond (Miura et al.,2001, Horpibulsuk et al., 2010), which is similar to thebehavior of naturally cemented clay. This implies that thecement admixed sample with higher clay water contentundergoes higher settlement at the post-yield state. Therole of cement admixture is to increase the yield stress,resulting in an increase in the yield surface and the failureenvelope.

From the strength and the compression test results, itwas found that when the clay water contents are in therange of LI＝1 to 2, the yield stress and the strength arepractically the same as long as the wc/C is identical. Assuch, it is logical to relate the yield stress and the uncon-ˆned compressive strength, as successfully done by Hor-pibulsuk et al. (2004a). The eŠect of the clay water con-tent plays a dominant role on compressibility at the post-yield state in which the cementation bond is broken

down. The lower the wc/C, the greater the strength andthe yield stress. This ˆnding is the same as that reportedby Horpibulsuk et al. (2005) and Miura et al. (2001) forthe cement admixed Ariake clay. As such, it is possible toadopt the wc/C and Abrams' law to analyze the strengthdevelopment in the cement admixed Bangkok clay.

Figures 7 and 8 show an analysis of laboratory strengthdevelopment in the cement admixed kaolin and Bangkokclays for diŠerent curing times. It is found that the wc/Ccan be applied to analyze the strength development of ce-ment admixed clay with diŠerent cement content andwater content at a speciˆc curing time. The test data arerepresented well by a power function in the form:

qu＝A

(wc/C )B (1)

where qu is the unconˆned compressive strength, A and Bare empirical constants. The test data can also berepresented by the exponential function as done by Hor-pibulsuk (2001) and Horpibulsuk et al. (2003) for variousclays. However, it is found from this study that for ce-ment admixed Bangkok clay, the power function is themost appropriate since it has the highest degree of corre-lation. This power function was successfully used to ass-ess the strength development of compacted cement stabi-lized coarse-grained soils by Horpibulsuk et al. (2006). Inall cases, the parameter A varies widely depending uponsoil type and curing time. However, the parameter B only

244

Fig. 7. Analysis of strength development in cement admixed kaolinclay using wc/C

Fig. 8. Analysis of strength development in cement admixed Bangkokclay using wc/C

Fig. 9. Strength development with time for diŠerent low-swellingclayey soils and their generalization


varies in a narrow band between 1.25 and 1.31, irrespec-tive of the swelling potential and curing time considered.The parameter B can thus be taken as a constant for bothcement admixed clays (non- to low-swelling clays) for 7 to120 days of curing. When the parameter B is 1.27, theresult is the following relation:

q(wc/C)1

q(wc/C)2

＝A/(wc/C )B

1

A/(wc/C )B2＝« (wc/C )2

(wc/C )1$1.27

(2)

where q(wc/C)1 is the strength estimated at a clay-water/ce-ment ratio of (wc/C )1, and q(wc/C)2 is the strength value atclay-water/cement ratio of (wc/C )2.

At a particular wc/C, the strength development withtime is controlled only by the value of A since B is re-garded as constant. Even though the parameter A de-pends on clay type, the rate of strength development withtime is identical for various admixed clays since it ispredominantly in‰uenced by the hydration process (Hor-pibulsuk et al., 2003). As such, it is possible to generalizethe strength development using the 28-day strength, q28 ofcement admixed clays as a reference value (vide Fig. 9). Alinear regression analysis gives the following relationship

with a high degree of correlation (0.908):

qD

q28＝0.039＋0.283 ln D (3)

where D is the curing time (days), and qD is the strength atD days of curing. This normalization accounts for theeŠects of diŠerence in clay type, water content, and ce-ment content. It is valid for the range of curing time be-tween 3 and 180 days in which the usual service time ofdeep mixing column is within 30 days. This relationship isclose to that proposed for cement admixed Ariake clay(Horpibulsuk et al., 2003) and for cement stabilized siltyclay (Horpibulsuk et al., 2009).

The generalized interrelationship among strength, cur-ing time, and wc/C for predicting the laboratory strengthdevelopment in the cement admixed Bangkok clay at LI＝1 to 2 for wc/C ranging from 2.5 to 15 is obtained bycombining Eqs. (2) and (3).

{q(wc/C)D

q(wc/C)28}＝« (wc/C )28

(wc/C )D $1.27

(0.039＋0.283 ln D) (4)

where q(wc/C)D is the strength of the cement admixed Bang-kok clay to be estimated at clay-water/cement ratio of (wc

/C ) after D days of curing, and q(wc/C)28 is the strength ofthe cement admixed Bangkok clay at clay-water/cementratio of (wc/C ) after 28 days of curing.

Tables 2 to 4 show that the predicted strengths are in

245

Table 2. Trial mix for determination of input of cement for Sukraphiban 3 district for dry mixing method

Depth(m.)

Input ofcement, C?

(kg/m3)

Curingtime,(days)

Cementcontent,C (z)

Clay watercontent,wc (z)

Clay-water/cement

ratio,wc/C

Laboratorystrength,qul (kPa)

Predictedstrength,qup (kPa)

|qup－qul|

qul×100

z

3 150 7 20.95 106.7 5.09 873 857 1.8314 20.95 106.7 5.09 1167 1142 2.1728 20.95 106.7 5.09 1453 Reference 0

175 7 24.44 106.7 4.37 1008 1042 3.4014 24.44 106.7 4.37 1378 1389 0.7828 24.44 106.7 4.37 1745 1735 0.54

200 7 27.93 106.7 3.82 1262 1235 2.1314 27.93 106.7 3.82 1628 1645 1.1028 27.93 106.7 3.82 1969 2056 4.42

250 7 34.92 106.7 3.06 1596 1639 2.7014 34.92 106.7 3.06 1996 2185 9.4228 34.92 106.7 3.06 2250 2730 21.34

6 150 7 21.45 114.5 5.34 803 807 0.5314 21.45 114.5 5.34 985 1076 9.1728 21.45 114.5 5.34 1246 1344 7.92

175 7 25.03 114.5 4.58 992 982 1.0514 25.03 114.5 4.58 1181 1308 10.8328 25.03 114.5 4.58 1463 1635 11.74

200 7 28.6 114.5 4.00 1224 1163 4.9814 28.6 114.5 4.00 1446 1550 7.1928 28.6 114.5 4.00 1694 1937 14.33

250 7 35.75 114.5 3.20 1676 1544 7.8614 35.75 114.5 3.20 1932 2058 6.5228 35.75 114.5 3.20 2168 2572 18.62

12 150 7 16.12 66.6 4.13 1022 1118 9.3614 16.12 66.6 4.13 1246 1490 19.6028 16.12 66.6 4.13 1549 1862 20.15

175 7 18.81 66.6 3.54 1209 1360 12.5014 18.81 66.6 3.54 1429 1812 26.76

Mean Absolute Percent Error, MAPE (MAPE＝1n

n

Si＝1

|qup－qul|

qul×100) 8.24z

Remark: for dry mixing, the natural water content, wn and the clay water content, wc are the same.


very good agreement with the laboratory ones. In thisprediction, even though index properties of the soft clayvary with depth, it is assumed that this variation insig-niˆcantly aŠects the strength prediction. As such, astrength value for a particular wc/C at a particular depthcan be used as a reference. It is found that the error fromthe prediction is acceptable for engineering practice withthe mean absolute percent error less than 9.6z for bothsites. This reinforces the application of the strengthprediction equation (Eq. (4)) for the cement admixedBangkok clay at various curing times and combinationsof water content and cement content.

FIELD TEST RESULTS

Figure 10 shows the êld 3-day curing water contentfor the dry mixing, compared with the natural water con-tent, wn, and the laboratory 3-day curing water content,w(lab)3days. Figure 11 shows the êld 28-day curing watercontent for the wet mixing, compared with the natural

water content, wn, the clay water content (after mixingclay with water to attain W/C of 1.0), wc, and the labora-tory 28-day curing water content, w(lab)28days. It is foundthat for the whole depth, the w(lab)3days for the dry mixing islower than the wn and the w(lab)28days for the wet mixing islower than the wc. This reduction in water content is dueto cement hydration. It is of interest to mention that theêld curing water contents for the dry mixing are close tothe w(lab)3days while the êld curing water contents of thewet mixing are very much lower than the w(lab)28days. ThediŠerence between the êld and the laboratory curingwater contents for the wet mixing is due to the êld curingstress. This shows that the in‰uence of the êld curingstress is more for the wet mixing. This is a fact that imme-diately after dispensing cement slurry, the natural watercontent suddenly increases. Such excess water must bedissipated to reach the natural equilibrium state under theexisting overburden stress. Due to the dispensing pressureand the wing rotation, cracks on the surrounding clay de-velop. These cracks accelerate the consolidation of the

246

Table 3. Trial mix for determination of input of cement for Sukraphiban 3 district for wet mixing method

Depth(m.)

Input ofcement, C?

(kg/m3)

Curingtime,(days)

Cementcontent,C (z)

Watercontent,wc (z)

Clay-water/cement

ratio,wc/C



|qup－qul|

qul×100

z

3 175 7 27.05 122.62 4.53 631 596 5.5614 27.05 122.62 4.53 758 794 4.7728 27.05 122.62 4.53 1053 992 5.75

185 7 28.78 124.00 4.31 703 635 9.6114 28.78 124.00 4.31 874 847 3.1128 28.78 124.00 4.31 1121 1058 5.60

200 7 31.40 126.07 4.02 793 695 12.3514 31.40 126.07 4.02 920 926 0.6928 31.40 126.07 4.02 1175 1158 1.49

6 175 7 24.63 108.33 4.40 565 619 9.6014 24.63 108.33 4.40 722 825 14.3028 24.63 108.33 4.40 999 1031 3.22

185 7 26.20 109.59 4.18 634 660 4.0814 26.20 109.59 4.18 773 879 13.7628 26.20 109.59 4.18 1119 Reference 0

200 7 28.58 111.48 3.90 668 721 7.9514 28.58 111.48 3.90 918 961 4.6828 28.58 111.48 3.90 1236 1201 2.85

9 175 7 20.87 89.60 4.29 552 638 15.6414 20.87 89.60 4.29 658 851 29.2828 20.87 89.60 4.29 807 1063 31.73

185 7 22.19 90.67 4.09 638 680 6.5214 22.19 90.67 4.09 775 906 16.8628 22.19 90.67 4.09 940 1132 20.40

200 7 24.19 92.28 3.82 705 742 5.1914 24.19 92.28 3.82 900 988 9.8128 24.19 92.28 3.82 1090 1235 13.30


n

Si＝1

|qup－qul|

qul×100) 9.56z

Remark: W/C＝1.0


column before the onset of the hydration (prior to cemen-tation bonds formation) (Miura et al., 1998; Shen andMiura, 1999; Shen et al., 2003; Shen et al., 2008). Thisconsolidation results in a signiˆcant reduction in the êldwater content, as experimentally observed by Consoli etal. (2006) and Rotta et al. (2003).

Figures 12 and 13 show the strength development inthe dry and the wet mixing columns at Sukhaphiban 3district. Figure 14 shows the strength development in thewet mixing columns at Ladkrabang district. For the sameinput of content in kg per cubic meter of clay for thewhole depth, the weight of cement per dry weight of clay,C, decreases with depth since water content decreases(unit weight increases) with depth. The decrease in bothwc and C yield almost the same wc/C value, and thereforepractically the same laboratory strength, qul, for thewhole improved depth. The laboratory strengths shownin Figs. 12 to 14 are thus the average values from diŠerentdepths. It is found that the êld strength is between 0.6and 1.5 times the laboratory strength for the wet mixingand between 0.6 and 1.0 times the laboratory strength forthe dry mixing. It must be kept in mind that the quf/qul

ratios vary wildly, depending on clay types, binder types,executing machines, etc. Consequently, the quf/qul ratiospresented in this paper are valid only for the soft Bang-kok clay and the employed machines. For the wet mixing,the average êld strength is close to the laboratorystrength, while the average êld strength is close to the 0.7times the laboratory strength for the dry mixing. For thedry mixing, most of the êld strengths are lower than thelaboratory strengths for all curing times, possibly due tonon-uniformity of mixing in-situ clay with cement (Hor-pibulsuk et al., 2004c; Nishida et al., 1996). For the wetmixing, even with the non-uniformity of the mixing, mostof the êld strengths are higher than the laboratorystrengths. This high êld strength (qufÀqul) is due to theêld curing stress, resulting in the consolidation beforethe onset of the hydration. This consolidation causes thereduction in the êld void ratio (water content), which in-creases the strength. The eŠect of the reduction in thevoid ratio due to the curing stress (consolidation) on thestrength development is experimentally depicted by Con-soli et al. (2006) and Rotta, et al. (2003).

Generally, for a clay subjected to the same shear stress,

247

Table 4. Trial mix for determination of input of cement for Ladkrabang district for wet mixing method

Depth(m.)

Input ofcement, C?

(kg/m3)

Curingtime,(days)

Cementcontent,C (z)

Watercontent,wc (z)

Clay-water/cement

ratio,wc/C



|qup－qul|

qul×100

z

3 150 7 21.78 110.54 5.08 359 395 10.0114 21.78 110.54 5.08 500 526 5.1928 21.78 110.54 5.08 669 Reference 0

175 7 25.81 113.83 4.41 453 471 4.0314 25.81 113.83 4.41 614 628 2.2628 25.81 113.83 4.41 865 785 9.19

200 7 29.95 117.12 3.91 593 549 7.3114 29.95 117.12 3.91 770 732 4.9628 29.95 117.12 3.91 1010 915 9.47

250 7 38.57 123.70 3.21 762 707 7.2414 38.57 123.70 3.21 940 942 0.1428 38.57 123.70 3.21 1197 1177 1.66

6 150 7 21.16 100.34 4.74 439 430 2.0714 21.16 100.34 4.74 556 573 3.0928 21.16 100.34 4.74 758 716 5.53

175 7 25.08 103.53 4.13 517 513 0.7514 25.08 103.53 4.13 673 684 1.5128 25.08 103.53 4.13 928 854 7.94

200 7 29.12 106.72 3.67 596 596 0.0414 29.12 106.72 3.67 808 795 1.6828 29.12 106.72 3.67 1061 993 6.35

250 7 37.52 113.10 3.01 743 764 2.9214 37.52 113.10 3.01 959 1019 6.2528 37.52 113.10 3.01 1261 1273 0.96

12 150 7 18.69 95.66 5.12 463 390 15.7214 18.69 95.66 5.12 601 520 13.4128 18.69 95.66 5.12 792 650 17.89

175 7 22.13 98.51 4.45 544 466 14.3214 22.13 98.51 4.45 712 621 12.7428 22.13 98.51 4.45 946 776 18.00

200 7 25.65 101.35 3.95 646 542 16.1014 25.65 101.35 3.95 836 722 13.5828 25.65 101.35 3.95 1101 903 18.00

250 7 32.97 107.04 3.25 788 696 11.7314 32.97 107.04 3.25 1051 927 11.7828 32.97 107.04 3.25 1348 1159 14.03


n

Si＝1

|qup－qul|

qul×100) 7.72z

Remark: W/C＝1.0


its excess pore water pressure development decreases withthe increase in eŠective stress (shear strength). For thesoft Bangkok clay deposit where natural water contentdecreases and undrained shear strength increases withdepth, the excess pore water pressure development due towet mixing in the soft Bangkok clay decreases with depth.Hence, the ˆeld strength development due to the consoli-dation (dissipation of excess pore pressure) decreaseswith depth. In other words, the quf/qul value for the wetmixing tends to decrease with depth as shown in Figs. 13and 14.

The eŠect of water to cement ratio (W/C) on the ˆeldstrength development of the wet mixing columns is shown

in Fig. 15. The strength development at three levels ofwater to cement ratio (W/C) is shown. The W/C of 1.0,which is commonly used for soft Bangkok clay, gives thehighest strength. Hence, a W/C of 1.0 is recommendedfor the wet mixing method based on the range of W/Ctested.

The eŠect of the installation rate on the strength de-velopment in the low strength columns is shown in Fig.16. The strength proˆles were almost the same for diŠer-ent installation rates. This result is in agreement with thatreported by Horpibulsuk et al. (2004c), and is typical forlow strength columns. This means that both slow and fastinstallation rates result in practically the same quality

248

Fig. 10. Field curing water content of dry mixing columns at Suk-haphiban 3 district

Fig. 11. Field curing water content of wet mixing columns at Suk-haphiban 3 district

Fig. 12. Strength proˆle of dry mixing columns at Sukhaphiban 3 dis-trict

Fig. 13. Strength proˆle for wet mixing columns at Sukhaphiban 3district

Fig. 14. Strength proˆle of wet mixing columns at Ladkrabang district

Fig. 15. EŠect of water to cement ratio, W/C, on the strength de-velopment of wet mixing columns


columns (quf/qul＝0.6 to 1.5). The same is not for the highstrength columns where the reduction in water contentdue to hydration has a remarkable eŠect on the eŠective-ness of the mixing: a very low installation rate is required

in this case.Figure 17 shows the strength development of the high

strength columns (CÀ220 kg/m3) compared with that ofthe low strength column (C＝220 kg/m3) for the sameW/C and installation rate (PR＝WR＝1.0 m/min). It isclearly noted that the strength of all the wet mixing

249

Fig. 16. EŠect of installation rate on the strength development of wetmixing columns

Fig. 17. EŠect of cement content on the strength development of wetmixing columns

Fig. 18. Suggested procedure of wet mixing method for soft Bangkokclay


columns made up from diŠerent cement contents is prac-tically the same. In other words, the strength is irrespec-tive of the cement content for such a high installationrate. This result is the same as that reported by Horpibul-suk et al. (2004c) for improvement with high cement con-tents with installation rates close to and higher than 1.0m/min. As such, it is not advantageous or economical toopt for the high cement content columns particularly athigh installation rate. Horpibulsuk et al. (2004c) conclud-ed that an enhancement of strength does occur when theinstallation rate is less than 0.7 m/min. The higher the ce-ment content, the greater the reduction in water due tolarger hydration, making mixing di‹cult.

From this study and the work of Horpibulsuk et al.(2004c), wet mixing is suitable for executing low strengthcolumns since the fast installation rate can be adoptedand the quf/qul is high. For high strength columns, highinput of cement is required with the very low installationrate. The quf/qul value is about 0.33–0.67, which is muchlower than that of low strength columns (Horpibulsuk etal., 2004c). As such, for a particular design (dead andlive) load on the soft Bangkok clay, it might be more eco-nomical to improve soft ground with many low strength

columns than a few high strength columns.

SUGGESTED METHOD FOR DEEP MIXINGIMPROVEMENT

Based on the laboratory and êld study, our sugges-tions for improving deep mixing for the soft Bangkokclay are summarized and presented (vide Fig. 18). Themethod below is for low strength columns (qul(28days)

º1500 kPa) by the wet mixing method.Determination of input of cement compensating for êldfactors

1. From the soil proˆle and the design load on the softground, determine the diameter, length, spacingand êld strength at the service time of the deepmixing column.

2. From the target êld strength at service time (opensquare symbol), estimate the target êld strength at7 days of curing (Target quf(7days)) (cross symbol),which can be approximated using Eq. (3).

3. Determine the laboratory strength at 7 days of cur-ing (qul(7days)), using the êld strength reduction of1.7 (quf/qul＝0.6) (black circle symbol).

4. Determine the cement content to attain the labora-tory strength at 7 days of curing (qul(7days)) and serv-ice time. This task can simply be done using Eq. (4).

Field execution and examination of êld strength5. Execute the deep mixing column with a W/C of 1.0

and an installation rate (PR and WR) of about 1.0m/min.

6. Core the soil-cement samples from a selectedcolumn at diŠerent depths to determine the êldstrength at 7 days of curing for every section (every3000 column).

7. If quf(7days)À0.6 qul(7days), then the tested section meetsthe requirement.

8. In the case that quf(7days)º0.6 qul(7days),8.1 If the actual quf(7days) is slightly lower than the

target quf(7days) (black square), the service time ofthe section should be postponed to increase cur-ing time.

8.2 If the actual quf(7days) is much lower than target


quf(7days), conduct a pile load test.8.2.1 If the measured load capacity is twice

higher than the design load capacity (fac-tor of safety, FSÀ2.0), the requirementis met.

8.2.2 If FSº2.0, the section must beredesigned and reimproved to compen-sate for the strength loss.

CONCLUSIONS

This paper deals with the laboratory and êld strengthdevelopment in cement admixed Bangkok clay. An analy-sis of the laboratory strength development based on theclay-water/cement ratio hypothesis and the Abrams' lawis carried out. The êld strength development in the deepmixing column is studied. The following conclusions canbe drawn.1. For the cement stabilization of soft Bangkok clay, in

which its water content varies in the range of liquidityindex between 1 and 2, the wc/C is the primeparameter governing the strength and the compres-sibility at the pre-yield state. The cementation bondstrength increases as the clay-water/cement ratio,wc/C, decreases.

2. Based on the clay-water/cement ratio and theAbrams' law, we proposed a relationship between theclay-water/cement ratio, the curing time and thestrength for the cement admixed Bangkok clay. Thisproposed relationship is useful in estimating thelaboratory strength of the cement admixed Bangkokclay wherein the water content and cement contentvary over a wide range by using the test results of a sin-gle trial. The formulation of the proposed relationshipis on sound principles. It is possibly applicable toother non- to low-swelling clays, in which a constant Bof 1.27 is used. For other medium to high swellingclays, this constant can be further reˆned with theanalysis of more data generated for this speciˆc pur-pose.

3. The êld to laboratory strength ratio (quf/qul) is higherthan 0.6 for both the dry and wet mixing methods.The average strength ratio is about 0.7 and 1.0 for thedry and the wet mixing methods, respectively. Theconsolidation of the wet mixing column before the on-set of the hydration leads to an increase in the êldstrength.

4. From the êld test results, a water to cement ratio,W/C, of 1.0 is recommended for the wet mixing im-provement of soft Bangkok clay. For low strengthcolumns (qulº1500 kPa), a high installation rate ofabout 1.0 m/min should be adopted to execute thequality column.

5. The suggested procedure for the wet mixing improve-ment for soft Bangkok clay is useful from both an en-gineering and economical viewpoint. The procedurecan save on sampling and laboratory testing and there-fore cost. It can be applied to the other non- to lowswelling soft clay deposits.

ACKNOWLEDGEMENT

The authors would like to acknowledge the ˆnancialsupport provided by the Suranaree University of Tech-nology. The êld test results of the soil-cement columnsobtained from the Bureau of Materials Analysis and In-spection, Department of Highways, Thailand are ap-preciated.

LIST OF NOTATIONS

A＝empirical constantB＝empirical constantC＝cement content in terms of percentage

C?＝input of cement in terms of kg/m3

qu＝unconˆned compressive strengthquf＝êld strength of the column

quf(7days)＝êld strength of the column at 7 days of cur-ing

qul＝laboratory strengthqul(28days)＝laboratory 28-day strength

q(wc/C)1＝strength to be estimated at clay-water/cementratio of (wc/C )1

q(wc/C)2＝strength value at clay-water/cement ratio of(wc/C )2

q(wc/C)D＝strength of the cement admixed Bangkok clayto be estimated at clay-water/cement ratio af-ter D days of curing

q(wc/C)28＝strength of the cement admixed Bangkok clayclay-water/cement ratio after 28 days of cur-ing

wn＝natural water contentw(lab)3days＝laboratory 3-day curing water contentw(lab)28days＝laboratory 28-day curing water content

wc/C＝clay-water/cement ratioW/C＝water to cement ratio

FS＝factor of safety

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