Triaxial shear behavior of a cement-treated sand-gravel mixture

Younes Amini,Amir Hamidi

School of Engineering,Kharazmi University,Tehran,15614,Iran

1.Introduction

Cementation occurs due to the various geological processes that create bonds between soil particles like aging,chemical reactions,carbonates,silicates,iron oxides,and natural cementing agents.Due to the difficulties of in situ sampling,the mechanical characteristics of cemented soils are usually studied using artificial samples prepared in laboratory and cured by different cementing agents.

The mechanical behavior of cemented soils is influenced by a number of parameters including cement content,cement type,density,con fining stress,grain size,and stress-strain history(Saxena and Lastrico,1978;Clough et al.,1979,1981,1989;Acar and El-Tahir,1986;Leroueil and Vaughan,1990;Airey,1993;Coop and Atkinson,1993;Das et al.,1995;Malandraki and Toll,1996;Cuccovillo and Coop,1997,1999;Huang and Airey,1998;Consoli et al.,2000,2007,2009,2010,2011;Schnaid et al.,2001;Ismail et al.,2002;Rotta et al.,2003;Lee et al.,2010;Park,2010;Baxter et al.,2011;Hamidi and Hooresfand,2013;Shahnazari and Rezvani,2013).According to the previous studies,the cementation can increase brittleness,shear strength,and dilative behavior of sands.However,it should be noted that most of the previous studies have focused on the mechanical behavior of cemented fine sands,rather than the mechanical behavior of coarse grained gravels or gravely sands.

In the last decade,a number of studies have been performed to investigate the mechanical behavior of cemented gravely sands or sandy gravels(Haeri et al.,2005a,b,2006),in which a series of triaxial compression tests were performed on a representative gradation of the Tehran coarse grained alluvium.Cemented samples were prepared with different cementing agents including lime,Portland cement,and gypsum.They concluded that the strain associated with the peak deviatoric stress decreases as the cementation increases.Also it was indicated that the maximum rate of dilation and negative pore water pressure occur after the maximum shear strength is obtained.

Review of the literature shows that there is rarely particular study on investigation of the mechanical behavior of cemented poorly graded sand-gravel mixtures.Indeed,all previous studies on the behavior of cemented gravely sands concern fine sands or well graded gravely sands as the base soil.Therefore,the objective of present research is to investigate the mechanical behavior of a cement-treated poorly graded sand-gravel mixture.In this regard,a number of new features of the mechanical behavior of cemented soils are reported.

2.Testing program

Thirty groups of conventional triaxial compression tests were performed,among which 24 groups are considered in this study.Six groups of additional tests(25%of the total)were performed to check repeatability of the experiments and results.Also,12 groups of unconfined compression tests were conducted and reported.Cement content and confining pressure were considered as the variables of testing program,and the triaxial compression tests were performed in the consolidated drained and undrained conditions.

2.1.Soil and cementing agent

Clean and uniform quartz beach sand with sub-round to subangular particles from the shores of the Caspian sea(specifically Babolsar,Iran)was first sieved using a#30 sieve and then was mixed with 30%uni-sized(9.5-12.5 mm)gravel grains.The mixed soil can be named as SP in unified soil classification system and was used as the base material.Gradation curves and physical properties of the base material are shown in Fig.1 and Table 1,respectively.In Table 1,Gsis the specific gravity;D10is the effective diameter;CUand CCare the uniformity and curvature coefficients,respectively;and γd,minand γd,maxare the minimum and maximum unit weights,respectively.All physical characteristics were determined according to the ASTM(1998)standard methods.

Portland cement(Type II)with a setting time lasting for about 4 h was used as the cementing agent.It was first sieved using a#100 sieve and then added to the base soil.

Fig.1.Gradation curve of tested sand-gravel mixture.

2.2.Sample preparation and test procedure

The under compaction method was used for sample preparation proposed by Ladd(1978).Required weight of the soil was mixed with desired cement content and about 7%distilled water.Samples were prepared using a split mold,100 mm in diameter and 200 mm in height,and were compacted in eight layers.Each layer was poured into the mold and compacted using metal hammer until the desired height was reached.Cemented samples were stored at a(25±3)°C humid roomwith＞90%relative humidity for 24 h.After that,samples were extracted from the mold and were kept for6 d at humid room.On the 7th day,the diameter,height and weight of the samples were measured.For unconfined compression tests,samples were prepared with curing times of 7 d and 28 d.The variables considered in sampling process are listed in Table 2.

A computer-controlled triaxial cell was used to test the samples at the confining pressures(CPs)of 50 kPa,100 kPa,and 150 kPa.The outer surface of samples was soft enough to minimize the effect of membrane penetration.As a result, flexible membranes do not affect pore pressure generation in saturated condition.Membranes with average thickness of 1 mm were used and corrections such as membrane thickness and cross-sectional area were considered according to Bishop and Henkel(1969).

All samples were fully saturated in two stages prior to shearing.At the first stage,de-aired water was flushed from the bottom of sample under a very low pressure difference of 10 kPa for 24 h.After that both cell and back pressure were ramped simultaneously to 310 kPa and 300 kPa for complete saturation at the second stage.Saturation procedure was considered to be completed until Skempton’s B value of 0.9 was reached.

The samples were consolidated up to the desired confining pressures.Shear loading was applied at an axial strain rate of 0.1 mm/min for drained tests and 0.3 mm/min for undrained ones.Cell pressure,volume change,pore pressure,load and displacements were measured during triaxial compression tests by electronic transducers and a calibrated data acquisition system.All the variables considered in testing program are listed in Table 3.

Table 1Physical properties of the base soil.

3.Unconfined compression tests

Some specifications of the unconfined compression tests are described in Table 3.Fig.2 indicates the variation of unconfined compressive strength(UCS)with cement content at different curing times.Peak strength occurred at small strains between 0.2%and 0.7%.It can be observed from Fig.2 that the UCS increases with increasing cement content and elapsed curing time.The lines intersect horizontal axis at cement content about 0.5%,which is the minimum cement content to mobilize the shear strength of cemented soil and formation of cemented bonds.

Table 2Summary of samples and test conditions.

4.Analysis of the results

A summary of triaxial test results at failure and residual state is shown in Table 4.Deviatoric stress(q),mean effective stress(p′)and specific volume(ν)are defined using the following equations:

Table 3Variables in testing program.

Fig.2.Variation of the unconfined compressive strength with cement content at different curing times.

whereσ′1is the major effective principal stress,σ′3is the minor

effective principal stress,and e is the void ratio.

Table 4Summary of triaxial test results at failure and residual state.

4.1.Mode of failure

Fig.3 shows the typical failure modes of the uncemented and cemented samples.Although dilation occurred at different confining pressures,all uncemented samples in the drained and undrained tests showed barreling mode without shear plane formation.In lightly cemented samples(CC=1%),failure mode was a combination of barreling shape and shear plane,although shear band was not obvious and barreling was the predominant mode.Increase in cement content increased the thickness of the shear band.Cemented samples with more cement content(CC＞1%)experienced a mode of brittle failure and underwent significant dilation with apparent peak point in stress-strain curve.In the drained and undrained conditions,cemented soils showed a shear band with a thickness of 3-6 cm.Inclination of the shear band with horizontal axis decreased from 70°to 55°with increase in confining pressure from 50 kPa to 150 kPa.

4.2.Stress-strain behavior

Fig.3.Failure modes of tested samples.(a)Barreling mode in the uncemented samples;(b)Shear zone failure in the cemented samples.

Fig.4.Triaxial test results of uncemented samples.

Deviatoric stress-axial strain curves are depicted in Figs.4a,5a,6a and 7a for the consolidated drained and undrained conditions.The drained and undrained tests are indicated by the symbols“D”and “U”,respectively,and are followed by a number that represents the value of confining pressure in kPa.Six tests were repeated twice to check the repeatability of experiments,and the average stress strain diagram is plotted in these figures.All cemented samples in drained and undrained conditions showed an apparent peak point associated with the failure.After that,slope of the stress-strain curve decreased to its residual value in an axial strain of about 20%.For the uncemented samples under undrained condition,the peak stress was not obvious and the softening behavior was not as clear as the drained tests.Increase in cement content and reduction in confining pressure caused more softening in stress-strain curve.Comparison of the results in the drained and undrained conditions showed that the strain associated with the peak strength was larger in undrained tests than that in drained ones.It can be concluded that the cemented soil behavior is more brittle in the drained condition and bond degradation occurs easier when volumetric strains can freely occur in the soil(Malandraki and Toll,2001).

Fig.5.Triaxial test results of cemented samples with 1%Portland cement content.

Fig.6.Triaxial test results of cemented samples with 2%Portland cement content.

In order to understand the effect of confining pressure on the shear strength of cemented soil,the maximum deviatoric stress(qmax)is normalized to the uncemented one in drained tests in

Fig.8.It can be seen from Fig.8 that,for lightly cemented sample(CC=1%),normalized shear strength was approximately constant under different confining pressures.However,the effect of confining pressure on the shear strength ratio increased for other cement contents.

Fig.7.Triaxial test results of cemented samples with 3%Portland cement content.

Fig.8.Variation of the normalized shear strength with confining pressure.

Fig.9.Comparison of test results for cemented poorly graded and well graded mixtures.(a)Drained condition;(b)Undrained condition.

4.3.Volume change and pore pressure

Figs.4b,5b,6b and 7b indicate the variation of the specific volume with mean effective stress for the uncemented and cemented samples in consolidated drained condition.We can know that contractive behavior at the start of shearing is followed by large dilation in all samples.Increase in cement content attributed to the dilative behavior at the final state.In addition,increase incon fining pressure increased contractive behavior at the start of the test.

Figs.4c,5c,6c and 7c indicate the variation of the excess pore pressure with the mean effective stress in the consolidated undrained tests.Positive pore pressure occurred at the beginning of loading,followed by significant negative pore pressure at the final state.Same as the volume change,increase in cement content and decrease in con fining pressure increased negative suction at the end of loading process.

Haeri et al.(2005b)reported the results of consolidated drained triaxial tests on a well graded gravely sand(containing 45%gravel)cemented with Portland cement.The gradation curve is shown in Fig.1 and the maximum gravel size is 12.5 mm.Fig.9 shows the comparison between the dilation and excess pore pressure values for the present study and well graded gravely sand tested by Haeri et al.(2005b).According to the previous studies,increase in gravel content increases dilation in sand-gravel mixtures(Evans and Zhou,1995;Simoni and Houlsby,2006;Hamidi et al.,2009).Although gravel content is larger inwell graded samples,dilation in drained state or negative pore pressure in undrained condition is lower compared with the poorly graded mixture tested.For the same gravel content,it can be concluded that dilation in drained condition or negative pore pressure in undrained state is larger in poorly graded sand-gravel mixtures compared with the cemented well graded gravely sands.

Fig.10.Strain contours for the cemented samples containing 2%cement.(a)Drained condition;(b)Undrained condition.

Fig.12.Failure envelopes for the drained and undrained conditions based on different criteria.(a)Drained failure envelope using qmax=0 criterion;(b)Undrained failure envelope using qmax=0 and A=0 criteria.

Table 5Shear strength parameters in drained and undrained conditions.

4.4.Stress paths

Stress paths for different cement contents are shown in Figs.4d,5d,6d and 7d.The stress path moved linearly with a slope of 3:1 inq-p′space in the consolidated drained tests.It reached a peak point which has been marked on the figures.After that,softening caused reversal of the stress path with the same slope until residual stress state was reached.In undrained condition,the peak point of the stress path was higher than the drained one due to generation of significant negative pore pressure.However,the difference between peak points of stress paths for the drained and undrained tests decreases by increase in confining pressure.The same trend has also been reported for a gypsum cemented well graded gravely sand(Haeri et al.,2005a);however,peak points of the stress path in undrained condition were lower than the drained ones at high confining pressures(over 300 kPa).

Fig.10 shows the axial strain contours of cemented samples containing 2%cement in the drained and undrained conditions.Peak shear stress has been observed in an axial strain level about 1%in drained condition and 3%in undrained condition.Results of triaxial tests on the cemented well graded gravely sand indicate that peak shear stress is associated with axial strains of 2.5%and 4.5%in drained and undrained states,respectively(Haeri et al.,2005b).It confirms more brittle behavior of cemented poorly graded sand-gravel mixture compared with the cemented well graded gravely sand in the same conditions.

Fig.13.Failure envelopes and residual state lines.(a)CC=0%;(b)CC=1%;(c)CC=2%;(d)CC=3%.

4.5.Failure envelopes and shear strength parameters

Fig.11 plots failure envelopes for all the tests.Numbers after the symbols“D”and “U”show the value of cement content.Previous studies have suggested a curved failure envelope for cemented materials(Malandraki and Toll,2001;Asghari et al.,2003;Baker,2004;Sharma et al.,2011).However,the failure envelopes were not curved for the studied soil.Moreover,the failure envelopes of drained tests were lower than those of undrained ones which can be related to the large negative pore pressures induced during undrained shearing.

Fig.14.Variation of effective principal stress ratio at failure with confining pressure.(a)Drained condition;(b)Undrained condition.

Different criteria can be used to determine the shear strength parameters for the soil,including peak deviatoric stress at failure,maximum principal stress ratio,maximum excess pore pressure,and zero Skempton’s pore pressure parameter （A=0）.The peak deviatoric stress is a commonly used method for determination of the failure envelope in drained condition.In undrained state,Baxter et al.(2011)recommended using A=0 as the failure criterion for cemented soils,because it can eliminate the effects of large pore pressure gradients on the shear strength.

Fig.12a plots failure envelopes for drained condition based on the peak deviatoric stress and Fig.12b shows undrained failure envelopes based on both peak deviatoric stress and A=0 criteria.Table 5 also lists the shear strength parameters,c′and φ′.According to Table 5,using A=0 criterion in undrained condition yields consistent friction angle and cohesion intercept values with drained state.Baxter et al.(2011)mentioned that increase in cement content increases the differences between cohesion intercept values calculated in the drained and undrained conditions using A=0 criterion.However,results of the present study show that increase in cement content does not particularly influence the relation between shear strength parameters in the drained and undrained conditions.It can be related to the difference in gradation of the cemented sandy gravel used in the present study and the fine cemented sand tested by Baxter et al.(2011).

Fig.15.Variation of the stiffness with cement content and confining pressures.(a)Drained condition;(b)Undrained condition.

Fig.16.Variation of normalized absorbed energy with cement contents at different confining pressures.(a)CD(CP=50 kPa);(b)CD(CP=100 kPa);(c)CD(CP=150 kPa);(d)CU(CP=50 kPa);(e)CU(CP=100 kPa);(f)CU(CP=150 kPa).

Fig.13 depicts the failure envelopes and residual state lines for different cement contents in drained condition.Residual state line shows the relation between deviatoric stress and mean effective stress in the final stage of loading.It can be seen from Fig.13 that the difference between the two lines increases with increasing cement content,which confirms more brittle behavior as cement content increases.

Fig.14 shows variation of the effective principal stress ratio at failure （σ′1/σ′3）ffor drained and undrained conditions with confining pressure. The effective principal stress ratio at failure decreases with increasing confining pressure and increases with increasing cement content. Also, it is much higher in undrained state than drained state, especially at lower confining pressures

5.Stiffness and energy absorption

Stiffness of cemented soil was determined by calculating the secant modulus for half shear strength in different confining pressures as shown in Fig.15.We can note that the stiffness increases with increase in cement content and confining pressure.The drained stiffness is larger than the undrained one;however,the difference is less than 10%.

Required energy to induce deformation in cemented soil can be calculated by the area under stress-strain curve.In the present study,absorbed energy for different axial strains is normalized to the absorbed energy at 10%axial strain.Fig.16 shows the variation of the normalized absorbed energy for different cement contents and confining pressures in the drained and undrained conditions.A major change in the slope of the curve can be observed at 2%axial strain in drained state.As the cement content decreases,slope of the curve became flattened and reached to the curve of the uncemented samples.For the undrained condition,the slope of the curve was lower than that of the drained one.Also,the change in the slope of energy absorption curve can be observed at axial strains more than 2%.

6.Conclusions

The present study deals with the engineering characteristics of cemented poorly graded sand-gravel mixtures.The following conclusions can be drawn based on the test results:

(1)Under the similar conditions(the same cement type,cement content,gravel content,and maximum gravel size),dilation and negative pore pressure induced in cemented poorly graded sandgravel mixture were larger than those of well graded gravely sand.

(2)Brittle behavior of cemented soils was more obvious in drained condition than in undrained state.The axial strain at the peak shear strength was smaller in drained condition than in undrained state.Failure envelopes were lower in the drained condition than in the undrained state.

(3)Shear strength parameters calculated under the drained and undrained conditions were consistent when undrained shear parameters were calculated based on zero Skempton’s coefficient（A=0）criterion.Consistency between the results can be observed for all cement contents.

(4)A major change in the slope of the absorbed energy curve was observed at axial strain of about 2%in the drained condition.As cement content decreases,the slope of the curve became flattened and reached to the curve of the uncemented soil.The change in the slope of energy absorption curve was minor in undrained condition at axial strains more than 2%.

Conflict of interest

The authors wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.

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