Effect of Compaction Degree on Soil-water Characteristic Curve of

Chongming Clay

Jingsong Qiana, Hang Lub

(Key Laboratory of Road and Traffic Engineering of The Ministry of Education, Tongji University,

Shanghai,201804)

aqjs1001@126.com, bluhang1989@yahoo.com.cn

Keywords: Compaction Degree, SWCC, Filter Paper Method.

Abstract.The soil-water characteristic defines the relationship between the soil suction and

gravimetric water content, w, or the volumetric water content, θ, or the degree of saturation, S. It is

a convenient method to predict water content in the subgrade using the curve. But in the field tests

of subgrades, the compaction degree of soil became lower with time than initially designed. With

the purpose of finding out effect of compaction degree on soil-water characteristic curve, a study to

the SWCC (soil-water characteristic curve) of Chongming low liquid limit clay using filter paper

method was carried out and is presented in this paper. Specimens of different water contents were

prepared by absorbing different amount of water, in order to better simulate the process of wetting

of subgrade soil. After the filter paper test, the soil-water characteristic curve was fitted with two

models, and then the effect of compaction degree on the curve was analyzed. The figures show that

the compaction degree of the specimen will decrease with higher water content, and from the

gravimetric water content-matric suction curve, it is found that compaction degree has an effect on

air-entry value and water storage capacity.

Introduction

The constitutive equations for volume change, shear strength, and flow through unsaturated soil

have received general acceptance in geotechnical engineering applications [1]. The distinguishing

features of the soil-water characteristic depend on several factors such as soil structure (and

aggregation), initial moulding water content, void ratio, type of soil, texture, mineralogy, stress

history, and method of compaction. Kawai et al. (2000) [2] studied the effect of initial void ratio on

the SWCC. The effect of initial water content, soil structure and stress history has been presented

[3]. In the field, due to its depositional history, soil normally experiences a certain stress, which is

recognized to have some influence on SWCC [1]. Vanapalli et al. [3,4,5] studied the influence of

total stress state on the SWCC of a compacted fine-grained soil indirectly. However, experimental

evidence of effect of compaction degree on soil-water characteristic of unsaturated soil is very

limited.

The filter paper method is a soil suction measurement technique. It is a laboratory test method

that covers the full range of suction, and is inexpensive and relatively simple.

The aim of this study was to evaluate the influence of compaction degree on SWCC using filter

paper method.

Applied Mechanics and Materials Vols. 90-93 (2011) pp 701-706Online available since 2011/Sep/08 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMM.90-93.701

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Material and Preparation of Specimens

The sample was sandy clay obtained from Chongming, Shanghai. This soil is classified as clay with

low liquid limit. The optimal water content, maximum dry density, liquid limit, plastic limit and

grading properties are given in Table 1.

Table 1 Summary of the soil properties

Wopt

(%)

ρdmax

(g/cm3)

WL

(%)

Wp

(%)

15.4 1.822 34.3 18.9

Specimens for the test were first compacted in a compaction mould with inside diameter of

100mm and height of 100mm at optimum water content and compaction degree of 87%, 90%, 94%,

and 96%.

Immediately after extruded, specimens were put on the permeable stone in the water tank. Water

level in the tank was about 1mm below the top surface of the permeable stone (Fig.1). Specimens

absorbed water from below. The preparing method can better simulate the process of wetting in the

real subgrade. The specimens were weighed to calculate water content, based on the following

relationship:

(1)

where w1 is water content, m1 is the weight of the specimen, V is inner volume of the mould, and D

is compaction degree.

The increasing rate of water content was high at first and then slowed down to 0. Fig.2 shows

that the water content changed with time and tended to be stable in 10 hours. When the specimens

were taken out of the tank, diameter and height were larger than 10 hours before. Fig.3 shows the

change of volume of a specimen. Therefore, compaction degree was measured using cutting ring

method, and the result is shown in Fig. 4. It can be seen that compaction degree decreased, which

means that when it came to soil-water characteristic curve, the water content could only be

represented by gravimetric water content, w.

Fig.1 Specimens absorbed water from below Fig.2 Water content was changing with time in

10 hours

702 Advances in Civil Engineering, ICCET 2011

Fig.3 The specimen’s volume increased Fig.4 Compaction degree decreased

Measurement of Matric Suction

Measurement of the matric suction of the soil using the filter paper method is based on the

procedure of ASTM 5298-94. The diameter of filter (whatman No.42) is 11cm. Containers with a

volume of 1900ml were used to contain the soil specimen and filter papers. The contact method

proposed in the ASTM procedure was used to estimate the matric suction of the soil. The calibration

formula recommended by the procedure of ASTM 5298-94 were used to calculate the matric

suction of the soil and given as

(2)

(3)

where (kPa) is the matric suction of the soil; is the moisture content of the filter paper.

Test Results and Analysis

Fig.5 shows SWCCs based on semilogarithmic model, and the model is expressed as:

(4)

where (kPa) is the matric suction of the soil; is the water content of the specimen; a, b are

fitting parameters. The values of the parameters after fitted are given in Table 2.

Table 2 Value of the parameters in Eq.4

Compaction degree a (%) b R2

87% 30.836 -2.682 0.9333

90% 28.325 -2.568 0.9819

94% 27.640 -2.323 0.9650

96% 24.889 -1.751 0.9287

Applied Mechanics and Materials Vols. 90-93 703

Table 2 shows that R2 of the curves are greater than 0.9, prooving the fitting effective. From all

the curves it can be seen that water content increases with the decrease of matric suction. When the

specimen is compacted at a higher compaction degree, the slope of the SWCC is smaller, and so is

the parameter ‘a’. It can also be seen that when the matric suction is relatively low (lower than

80kPa) and stays at a certain value, water content is larger when the compaction degree is lower.

Then the curves were fitted based on Fredlund & Xing（1994）equation with three parameters [6],

which is expressed as:

(5)

where (kPa) is the matric suction of the soil; is the water content of the specimen; a, b, c are

fitting parameters; ws is saturated water content.

Since the compaction degree of the specimen decreased, ws could not be calculated based on the

original compaction degree. But the parameter a in Eq.4 is related to saturated water content, so set

ws equals to a in Eq.4. Fig.6 shows SWCCs based on Fredlund & Xing（1994）equation. The values

of the parameters are given in Table 3.

Table 3 Value of the parameters in Eq.5

Compaction

degree a b c R

2

87% 9.2728 0.8268 0.6583 0.9684

90% 7.6698 0.7948 0.6789 0.9844

94% 7.7027 0.9981 0.5316 0.9716

96% 5.1531 1.8599 0.2531 0.9532

Fig.5 Fitted SWCC based on Eq.4 (compaction degree is represented by ‘D’)

704 Advances in Civil Engineering, ICCET 2011

Fig.6 Fitted SWCC based on Eq. 5 (compaction degree is represented by ‘D’)

The fitting effect based on Fredlund & Xing（1994）model is better than that based on

semilogarithmic model. Fig.6 shows that the air entry value increases if the compaction degree

increases. This phenomenon can be explained by the change of the shape of the hole in soil. When

the compaction degree is high, holes in the soil is smaller and have worse connectivity, so a larger

suction is needed to absorb water in. At high water content (>20%) zone, high compaction degree

leads to lower water content when the matric suction are the same. So soil with higher compaction

degree has larger water storage capacity. Therefore compaction degree should be strictly ensured in

the process of construction, or the deficiency will lead to large increase of water content, which will

largely influence the performance of subgrade.

Conclusions

This paper presents a study to the SWCC of Chongming low liquid limit clay using filter paper

method. Before measure the suction, a method better simulating the process of wetting is used in the

preparing of specimen. The SWCCs shows the compaction degree’s influence. The air entry value

increases if the compaction degree increases, and high compaction degree leads to lower water

content at high water content(>20%) zone. Therefore the compaction degree should be strictly

ensured in the process of construction.

Acknowledgements

The financial support of NSFC(50908176) and Shanghai Educational Development

Foundation(10CG22) are gratefully acknowledged.

Applied Mechanics and Materials Vols. 90-93 705

References

[1] Fredlund, D. G.&Rahardjo, H. (1993). Soil mechanics for unsaturated soils. New York: Wiley.

[2] Kawai, K，Karube, D.，Kato, S.，2000. In: Rahardjo, H.，Toll, D.G.，Leong, E.C.(Eds)，

Unsaturated Soils for Asia. Balkema, Rotterdam, p.329-334.

[3] Vanapalli, S.K，Pufahl, D.E.,Fredlund, D.G.,1999. Geotechnique,49(2):143-159.

[4] Vanapalli, S.K，Fredlund, D.G，Pufahl, D.E，Clifton, A.W., 1996. Can. Geotech. J., Ottawa,

33:379-392.

[5] Vanapalli, S.K，Pufahl, D.E.，Fredlund, D.G.，1998. The Meaning and Relevance of Residual

Water Content to Unsaturated Soils. Proceedings of 51st Canadian Geo- technical Conference,

Edmonton, AB, p.101-108.

[6] Fredlund, D. G., and Xing, A.(1994). Can. Geotech. J., 31, 533-546.

706 Advances in Civil Engineering, ICCET 2011

Advances in Civil Engineering, ICCET 2011 10.4028/www.scientific.net/AMM.90-93 Effect of Compaction Degree on Soil-Water Characteristic Curve of Chongming Clay 10.4028/www.scientific.net/AMM.90-93.701

DOI References

[6] Fredlund, D. G., and Xing, A. (1994). Can. Geotech. J., 31, 533-546.

http://dx.doi.org/10.1139/t94-062