Collapse Behavior of Unsaturated Lateritic Soils in Taiwan

B. L. Chu Department of Civil Engineering, National Chung-Hsing University, Taichung, Taiwan blchu@ mail.ce.nchu.edu.tw

L. M. Huang Department of CADD, Chung-Lu Construction Co., Taichung, Taiwan danny@cctjv250.com.tw

Y. W. Jou China Engineering Consultants Inc., Taipei, Taiwan cww29160@ceci.org.tw

Abstract: In an effort to obtain more understandings on engineering behavior of the problematic soil, the authors have investigated col-lapse behavior of laterites in Taiwan with emphasis on the influence of initial water content, compaction effort, degree of saturation, and soil plasticity. Both laboratory tests and analytical works were performed in this study. Results obtained show that: (1) Saturation process of lateritic soil can be modeled by using a three-stage curve, namely stable suction segment, near saturation collapse segment, and full saturation collapse segment. (2) The collapse behavior of lateritic soil is found to be a three dimensional behavior. However, relation between quantities of collapsed settlement at different degrees of saturation and controlled time factor cannot be established. (3) The collapse problem can be effectively reduced under the compaction effort equivalent to energy level of modified Proctor com-paction. (4) Plastic lateritic soil possesses higher collapse potential than that of non-plastic one.

1 INTRODUCTION

The lateritic deposit is widely distributed throughout the western terrace and foothills of Taiwan, which covers almost 65% of the usable sloping lands. Due to its flat slope and integrity, the ex-ploitation of lateritic soil is becoming more important for urban developments. In Taiwan, lateritic soil is usually used as a back-fill material especially for freeway, airport, high-speed railway, and dam constructions. Furthermore, many projects in Taiwan is planned to build on the laterite terrace. Therefore, to evaluate the characteristics of lateritic soil is an important task for civil engi-neering constructions.

One feature of this lateritic soil is that most of the materials are used in an unsaturated condition and it possesses apparent stability due to negative pore water pressure and matric suction. However, the unsaturated condition would also lead to wetting induced collapse problems, which is also called hydro-collapse problems in this paper, caused by rapidly water content increase as a result of heavy rainfall or ground water intrusion. Engineer-ing behaviors of such borrowed material is therefore found to be of great influence by its water content, plasticity and degree of compaction. Needs of understanding unsaturated engineering properties of such lateritic material has increased.

For unfavorable or unexpected working conditions relating to type of materials, weather and topography, that make the quality of backfilling difficult to control. There have been cases when continuous rainfall in north or central Taiwan that led to col-lapsed roads, uneven ground settlement, surface crack, and even tilting structures. The worst case is hydro-collapse induced ground settlement.

Study of lateritic soil as compacted backfill material has mostly been concentrated more on its change of strength than on its collapse behavior. Research carried out by Rao & Revansi-

dappa (2002) shows that lateritic soil can bring about collapse behavior and in turn will weaken the function of earthwork struc-ture and may even cause disasters. In order to ensure a safe con-struction of backfilling, more understanding of collapse behavior on lateritic soil is necessary.

The engineering properties of compacted soils will depend greatly on soil type, principal stress ratio, initial water content, the method or type of compaction and the compaction effort ap-plied. Of which, water content and initial dry density are the ma-jor factors. Usually the water content of compacted soils is refer-enced to the optimum water content (OMC) for a given type of compaction. Depending on their position, soils are called dry of optimum, near or at optimum, or wet of optimum. In geotechni-cal engineering practice, if water content of compacted soil in-creases dramatically, this will cause detrimental effect on stabil-ity of earthwork structure. On the other hand, in the course of backfilling, insufficient compaction will also cause the initial dry density of lateritic soil unable to meet the specification and cause collapse. Until now, studies associated with hydro-collapse be-havior for lateritic soil carried out in Taiwan is still limited.

In this study, two types of lateritic soils with different plastic-ity (PI=0 and 22) were collected from the sites in central Taiwan. First we examined the physical properties of the collected sam-ples and carried out compaction tests and then, remolded speci-men with 8 types of different initial water content and density are compacted under different level of energies. Conventional triax-ial cell for testing saturated soils was modified to simulate the rise of groundwater table in the field, which can change the water content and reduce the suction of soil to its full saturation condi-tion. Herein, a new developed triaxial hydro-collapse test appara-tus, which can control soil suction through changing water con-tent were used to study the collapse behavior of lateritic soils. Finally, results of oedometer and triaxial collapse tests of these

lateritic soils were compared. Result of this study is hope to be helpful of obtaining more understanding on engineering behavior as well as improving construction control of lateritic soil in tropi-cal area.

2 LITERATURE REVIEW

Houston (1988) found that oedometer collapse test provides a fast and accurate prediction for wetting induced collapse of the soils at site. The shortcoming of oedometer test is that only the effect of vertical strain/stress on collapse potential is taken into account. Hence, the oedometer test may underestimate the col-lapse potential of the soil.

Rao & Revansidappa (2002) used red soils in India as testing material. The bonded and un-bonded soil specimens were con-ducted with load wetting oedometer test, and the vertical con-solidation stress applied were range from 6.25 to 800 kPa, re-spectively. The result shows that collapse induced volume changes of bonded and un-bonded soil is increasing as consolida-tion stress increases. The maximum amount of collapse occurred while consolidation pressure was applied at 200 kPa.

Kezdi, A. (1979) concluded that the stress condition of a soil at site is not only one-dimensional, but also three-dimensional. Therefore, the actual collapse behavior can only be simulated by triaxial test to obtain complete information for both vertical and radial strain.

Yen (2003) conducted three types of laterite in Taiwan with oedometer collapse test to evaluate the effect of soil plasticity on wetting induced settlement. He found that laterite with higher plasticity has higher collapse settlement. The collapse strain ver-sus vertical stress curve is concave in nature. The collapse strain is decreasing as the initial water content increasing.

3 TEST MATERIALS

Lateritic samples taken from two different sites with plastic index PI= 0 and 22 were tested in this study. Field density tests for these two soils were conducted with Sand Cone Method. The procedure for triaxial hydro-collapse test is similar to that of tra-ditional triaxial test. A series of remolded specimens with differ-ent initial water content were compacted under 50% or 100% of

the Standard Proctor Compaction effort, to control their relative compaction. Table 1 shows the physical properties of the test ma-terials. Specimen LAT-0 and LAT-20 represents lateritic soil with different plasticity, respectively.

4 TEST PROGRAM

With reference to Chu & Huangs research in 1993, the modified triaxial test apparatus was utilized in this study. A steel cover is put on the high air entry ceramic disc, which was installed into the pedestal of the triaxial cell, to avoid possible breakage of the discs caused by uneven contacts. Fig. 3 shows a photo of such device. After many tests, the modified test apparatus has been proved that it could be used in unsaturated triaxial test success-fully.

In this paper, the pressure regulator system developed by Fredlund (1978, 1988) was applied to conduct the triaxial hydro-collapse test. The triaxial apparatus may control pore air pres-sure and pore water pressure separately as shown in Fig. 2. The pressure control system may be applied both for unsaturated tri-axial and three-dimensional hydro-collapse tests.

Fig. 1 Schematic drawing for the base of modified triaxial cell.

Table 1 Physical property of test materials.

Accumulated percent passing sieves Soil Classification Sample Group

4# 200# 0. 002mm

Liquid Limit LL

Plasticity Index

PI

Activity Ac AASHTO USCS

LAT -0 100 51. 1 24. 2 - NP 0. 0 A-4 ML

LAT-20 100 83. 6 47. 6 37. 2 22 0. 46 A-6 CL

Fig. 2 Schematic set up for triaxial hydro-collapse test.

Fig. 3 Specimen loaded vertically through a rigid frame.

Before the soil is soaked, the specimen was applied with an isotropic confining pressure of 100, 200, 300 and 400 kPa respec-tively, to simulate the stress condition on site. Furthermore, a rigid loading frame was put on the top of a ram of triaxial cell, and then the specimen was applied with static axial load as shown in Fig. 3. Additional axial stress of 100, 200, 300 and 400 kPa was applied on the specimens mentioned above, respec-tively. The collapse tests for unsaturated lateritic soils were con-ducted with a constant air pressure applied on top of specimen, and with stepped reduced back water pressure to decrease the suction in specimen. The applied suctions were 200, 100, 50, 33,

20 and 10 kPa, respectively. While the suction is too low to apply the suction into specimen, it still allows water entering and satu-rating the specimen with a minor amount of back water pressure larger than pore air pressure (the typical pressure difference be-tween back water pressure and pore air pressure is 10 kPa). The degrees of saturation of the specimens were measured after col-lapse tests. Fig. 4 shows the stress path diagram used in this study.

Fig. 4 Stress path diagram in triaxial hydro-collapse test

5 TEST RESULTS

5.1 Hydro-collapse Behavior of Unsaturated Laterites

Fig. 5 shows laboratory results of soil specimens saturated gradu-ally from the dewatered condition. The collapse curve can be di-vided into three obvious stages: (1) stable suction segment: while the applied suction is ranged from 200 to 10 kPa, there is almost no volumetric strain take place. The soil structure is in a stable condition; (2) near saturation hydro-collapse segment: while the suction is lower than 10 kPa, it is difficult to apply a minor suc-tion by existing pressure regulator system. Therefore, the degree of saturation is considered as a variable in this stage. Fig. 5 also shows as the degree of saturation increase, the collapse strain in-crease. However, there is no any collapse failure was observed in this stage; (3) full saturation collapse segment: while the speci-men is fully saturated, some of specimens were observed that the collapse strain is continuously increasing until collapse failure occurs.

During the experimental process, the specimen will reach a transient collapse strain as degree of saturation varied. Although the specimen was kept in a constant degree of saturation, no fur-ther strain occurred. From the laboratory result, it is evidenced that collapse strain is independent to time variable. As the ratio of principal stresses of specimen is kept at 2, the maximum col-lapse strain occurs while deviatoric stress (1 3) is 400 kPa.

5.2 Anisotropic Behavior of Lateritic Soils

This paper define anisotropic index as the ratio of radial collapse strain Ich and vertical collapse strain Icv (i. e., Ich / Icv). Fig. 6 shows the anisotropic index varies as the degree of saturation change. While the degree of saturation or vertical load is lower, the anisotropic index varies violently. On the other hand, if the degree of saturation or the vertical load is higher, the anisotropic index varies in a gentle manner and reaches a constant value, which is approaching a critical value of 0.4.

Fig. 5 Collapse strains of unsaturated laterite at different depths.

Fig. 6 An-isotropic collapse behavior of unsaturated laterites at different depth and degree of saturation

5.3 Comparison of Oedometer and Triaxial Collapse Tests

Fig. 7 shows the vertical collapse strain of triaxial collapse test is always higher than that of oedometer tests conducted by Yen (2003). When the initial water content of the specimen is lower, the difference of volumetric strain becomes higher. The collapse strain in triaxial collapse test is even 2.3 times higher than that of oedometer test. The result shows oedometer test may underesti-mate the collapse strain.

Fig. 7 Vertical collapse strains of one-dimensional oedometer test and three-dimensional triaxial collapse test

5.4 Effect of Initial Water Content

In this paper, the initial water content of the specimen was con-trolled at OMC-6%, OMC-4%, OMC-2% and OMC, respec-tively. These two types of laterite show the collapse strain de-creasing as initial water content increasing as shown in Fig. 7. While the initial water content is near OMC, the collapse strain will be the minimum value; this phenomenon can be explained as cementation between fine and coarse particles of laterite caused by effective stress and capillary.

5.5 Effect of Principal Stress Ratio K

Fig. 8 shows the effect of principal stress ratios of K=1 and K=2 on collapse strain. The result indicates collapse strain Icv at K=2 is larger than that at K=1. While the laterite with PI =22, initial water content Wi = OMC-6%, and specimen prepared by Stan-dard Proctor Compaction effort, the axial collapse strain Icv at K=2 is approximate 1.2 time of that at K=1.

5.6 Effect of Plastic Index

Table 2 & 3 show the collapse strains of laterite soils with plastic index of PI=0 and PI=22 under 50% and 100% of Stan-dard Proctor Compaction effort. The result indicates collapse strain Icv at PI=22 is always larger than that at PI=0. Therefore, plastic lateritic soil possesses higher hydro-collapse poten-tial than that of non-plastic one.

Table 2 Collapse strain Icv (%) under Standard Proctor Compac-tion effort

OMC OMC-2% OMC-4% OMC-6%LAT-0 0.69 5.79 7.55 9.99

LAT-20 1.53 6.05 7.81 10.91 Table 3 Collapse strain Icv (%) under 50% of Standard Proctor Compaction effort

OMC-2% OMC-4% OMC-6% LAT-0 4.15 7.10 9.82

LAT-20 8.61 9.50 13.34

Fig. 8 Collapse strains at principal stresses ratio of K=2 and K=1.

6 CONCLUSIONS

Preliminary conclusions drawn from the laboratory results are:

1) Saturation process of lateritic soils can be modeled using a three-stage curve, namely stable suction segment, near satu-ration collapse segment, and full saturation collapse segment. The collapse failure may or may not occur, and can only ap-pear on full saturation collapse segment.

2) Despite of the soil plasticity, no collapse strain is observed, as suction applied on lateritic soil decrease from 200 kPa to 10 kPa.

3) Wetting induced collapse of the lateritic soils is first increas-ing with vertical load then decreasing. The critical overbur-den pressure is found to be around 400 kPa.

4) Collapse of lateritic soil is found to having an anisotropic be-havior through triaxial hydro-collapse test. The anisotropic index varies as overburden stress and degree of saturation changes. Low degree of saturation and shallow soil deposit exhibits big variation of anisotropic index. While degree of saturation is over 66% or soil deposit is located deeply, the anisotropic index is approaching a critical value of 0.4.

5) The collapse strain of lateritic soil is decreasing as initial wa-ter content increasing. While initial water content is near op-timal moisture content, the collapse strain tends to become smaller. The main reason for this phenomenon is while ini-tial water content lies on the dry side of OMC, the laterite

exhibit more potential to absorb water. Consequently, the water intruding into the laterite may cause more collapse strain.

6) Comparing the laboratory results between oedometer and tri-axial collapse tests, the conclusions can be drawn below.

(i) The vertical collapse strain Icv is decreasing as initial wa-ter content increasing in triaxial collapse test, which is the same as the oedometer collapse test results conducted by Yen (2003).

(ii) The collapse strain of oedometer test is always lower than that of the triaxial collapse test. Therefore, oedometer col-lapse test may underestimate the collapse potential of soil.

7) Higher principal stress ratio K may cause higher collapse strain. The collapse strain of soil at K=2 is 1. 2 times higher than that at K=1.

REFERENCES

Rao, S. M. & Revansidappa, K. 2002. Collapse Behaviour of a Residual Soil, Geotechnique, Vol. 52, No. 4: 259- 268.

Houston, S. L., Houston, W. N. & Spadola, D. J. 1988. Predic-tion of Field Collapse of Soil Due to Wetting. Journal of Geotechnical Engineering, ASCE, Vol. 114, No. 1: 40-58.

Kezdi, A. 1979. Stabilized Earth Roads. Developments in Geotechnical Engineering, New York.

Yen, C. N. 2003. Effects of Soil Plasticity on One Dimensional Hydro-collapse Behavior of Lateritic Soils in Taiwan. Mas-ter Thesis, National Chung-Hsing University, Taichung, Taiwan.

Chu, B. L., Huang, C. Y., Lee, T.M., & Huang, L. M. 1993. Shear Strength Behavior of Unsaturated Compacted Gravel Soil. The 5th Conference on Current Researches in Geotech-nical Engineering in Taiwan: 165-172. Fulong, Taiwan.

Fredlund, D.G. & Morgenstern, N.R. 1978. The Shear Strength of Unsaturated Soils. Canadian Geotechnical Journal, Vol. 15, No. 3: 313-321.

Gan, K. J. & Fredlund, D. G. 1988. Multistage Direct Shear Test-ing of Unsaturated Soils. Canadian Geotechnical Journal, Vol.11, No.2: 132-138.

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