Abstract—Construction of structures always involves

excavation, and recompaction following foundation installation. In some cases foundations are placed on compacted sandy soils. The influence of relative compaction on the strength of sandy soils in Kuwait is examined herein by a comprehensive laboratory testing program on surface samples. The program includes basic properties, compaction and, direct shear tests. Various soil parameters were determined including the compaction characteristics, the cohesion c, angle of friction φ. The results indicate that as the relative compaction increases towards 100%, the strength increases and the soil compressibility decreases. A comparison was made between the soil parameters for the samples compacted on the dry side, and on the wet side of optimum moisture content at several relative compaction ratios. The difference in the values obtained for the two cases are explained. The effect of using 1% by weight cement additive is also examined.

Index Terms—Relative compaction, shear strength, cohesion, angle of friction, silty sands.

I. INTRODUCTION The soil profile in Kuwait consists of a surface layer of

windblown dune sand underlain by cemented sands that are locally known as gatch. The properties and behavior of the surface layer has been examined in detail [1]-[3]. It varies in thickness from site to site from 0 to 7m and consists predominantly of fine sand or silty sand. Cemented sands are formed due to the excess of evaporation over rainfall and the precipitation of cementing agents such as carbonates and sulphates in the soil matrix. This deposit is usually dense to very dense and the degree of cementation varies with depth from weak or uncemented to moderately or strongly cemented. The characteristics of cemented sands in Kuwait were determined at several sites [4], [5].These natural soil deposits are encountered during excavation for foundation construction.

With the recent population growth, and economic development in Kuwait, several cases were encountered where construction on compacted fills was necessary. The housing authority had a large area for construction of two storey villas where loose sand extends from ground surface to a depth of 5m to 6m. The question posed was if this layer is removed and compacted to 95% relative compaction or as specified in the contract, and the foundations are placed on it, what is the allowable soil pressure for foundation design?, and what is the effect of the relative compaction on the shear

Manuscript received July 3, 2013; revised September 1, 2013. Nabil F. Ismael is with the Civil Engineering Department, Kuwait

University, Kuwait (email: nabilismael@hotmail.com). Mariam Behbehani is with Ministry of Public Works, Kuwait.

strength characteristics of the compacted fill. This paper aims to answer these questions and to learn

more about the properties of compacted sand fills by means of a laboratory-testing program. Testing included basic characteristics, compaction tests, and direct shear tests. In view of recent work indicating significant soil improvement with artificial cementation [6], the effect of using 1 % by weight ordinary Portland cement additive (Type I) on the shear strength parameters of compacted surface sands is also examined.

II. TESTING PROGRAM The testing program consisted of:

1) Collecting large bulk sample of surface sand from one site in Kuwait.

2) Determine the soil composition and grading characteristics and classification by mechanical analysis and Atterberg limit tests.

3) Carry out the Modified Proctor Compaction test. 4) Carry out 5 sets of direct shear tests on samples

compacted at 100% relative compaction, 95% relative compaction, and 90% relative compaction. Two sets of samples were tested at 95% relative compaction and two sets at 90% relative compaction. One set were samples compacted at moisture content on the dry side of optimum, and the other set were samples compacted at moisture content on the wet side of optimum. Relative compaction is defined as the ratio of the compacted dry density to the maximum dry density.

5) Carry out 3 sets of direct shear tests on samples compacted to 100% and 95% relative compaction as described above but with 1% by weight Portland cement additive after curing for a period of one week prior to testing.

A. Site Investigation and Soil Classification Bulk surface soil samples from a site in Al Rai area were

collected from excavated test pits. The soil came from excavation during the making of a roadway project. The samples were taken directly to the laboratory at Kuwait University for examination, analysis and testing. A sieve analysis test was first done to classify the sample and show its particle size distribution. Fig. 1 shows the particle size distribution curve which indicates that there was no gravel and that the sample is predominantly fine sand with about 10% fines.

After the sieve analysis test was done, the sample was tested for its Atterberg limits (liquid and plastic limits). The sample was classified as well graded sand with clay (SW-SC)

Influence of Relative Compaction on the Shear Strength of Compacted Surface Sands

Nabil F. Ismael and Mariam Behbehani

International Journal of Environmental Science and Development, Vol. 5, No. 1, February 2014

8DOI: 10.7763/IJESD.2014.V5.441

according to the Unified Soil Classification System (ASTM D-2487).

Fig. 1. Particle size distribution curve

B. Test Procedure Modified Proctor Compaction tests were carried out on the

samples to determine the compaction curve. Different dry unit weights were calculated at different water contents. Fig. 2 shows the plot of dry unit weight versus water content indicating the maximum dry unit weight and optimum water content and the dry unit weights and water contents at 90%, 95% and 100% relative compactions.

Fig. 2. Modified Proctor Compaction curve for the test sand

Later, laboratory direct shear tests were carried out on the

compacted samples at 90%, 95% and 100% to find the shear strength parameters. The compacted samples were prepared by static compaction using hammering and were divided into layers depending on their density; the denser the sample the more the layers. First, tests on the 90% and 95% relative compaction samples were done for both the dry and wet side of the optimum. Then tests were done on samples compacted to 100% relative compaction and optimum moisture content. Tests were then repeated on samples prepared with 1% Portland cement additive at 95% and 100% relative compaction. These tests were carried out to show the effect of relative compaction on the shear strength parameters (cohesion and angle of friction) of the compacted specimens

prepared with and without the cement additive.

C. Test Results and Discussion Table I shows a summary of the results of the direct shear

tests. Fig. 3 shows the variation of the angle of frictionφ, and cohesion c with the relative compaction.

TABLE I. SUMMARY OF THE DIRECT SHEAR TEST RESULTS

Relative compaction Cohesion (kPa) Angle of friction (degrees)

90 % dry 43.7 35

95 % dry 37 37

100 % 41 41

90 % wet 43 32

95 % wet 72 34

95 % dry with 1% cement 47.3 42.2

95 % wet with 1% cement 83.6 46

100 % with 1% cement 41 51.4

From Table I and Fig. 3 it can be seen that as the relative

compaction increased the angle of friction also increased for both cases with and without cement additive. Without cement the angle of friction on the wet side is lower than the dry side whereas with the 1% Portland cement the angle of friction in the wet side is higher. On the other hand, the cohesion component of strength c has not been greatly affected by the degree of relative compaction or the addition of cement. All values of c were about 45 kPa expect at 95% relative compaction on the wet side of the optimum where a larger value of c was measured.

Fig. 3. Variation of the angle of friction and cohesion with the relative

compaction.

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Fig. 4 shows larger stiffness for samples compacted on the dry side of optimum as compared to samples compacted on the wet side of optimum. As shown, the horizontal shear displacement is smaller at different shear stress levels for the samples compacted on the dry side of the optimum indicating a stiffer and less compressible sample as compared to samples compacted on the wet side of optimum.

Fig. 4. Shear stress versus horizontal displacement from direct shear tests on samples compacted at 90% relative compaction at different normal pressures.

Fig. 5 shows the shear stress versus displacement curves

for samples compacted at 100% relative compaction. As shown, larger stiffness and strength were measured with the addition of 1% cement additive.

Fig. 6 shows the shear stress vs displacement curves from direct shear tests on samples compacted at different relative compactions and at different normal pressures. Shown compaction on the dry side of the optimum shows that as relative compaction and normal stress increased, the strength and stiffness increased. Similar findings were obtained from the tests carried out on the wet side of optimum.

Fig. 5. Shear stress versus horizontal displacement from direct shear tests on

samples compacted at 100% relative compaction at different normal pressures.

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Fig. 6. Shear stress versus horizontal displacement from direct shear tests on samples compacted at different relative compaction and at different normal

pressures on the dry side of optimum.

III. CONCLUSION A laboratory testing program was carried out to determine

the influence of relative compaction on the shear strength of surface sands. Direct shear tests were carried out on compacted sand at different relative compaction. Based on test results the following conclusions were reached: 1) As the relative compaction increased the angle of friction

φ increased from 35º at 90% relative compaction on the dry side of optimum to 41º at 100% relative compaction. The angle of friction decreased on the wet side of the optimum reaching 32º at 90% relative compaction on the wet side of the optimum. For relative compactions 90%, 95% and 100%, the angle φ was lower on the wet side of the optimum.

2) The cohesion c was fairly constant expect for one point at 95% relative compaction on the wet side which showed a large value of c.

3) The addition of 1% by weight cement additive substantially increased the angle of friction φ by 5º to 12º whereas it had no effect on the cohesion.

4) The stiffness, as determined from the slope of the shear stress versus shear displacement curves, generally increased with the degree of relative compaction, and with the addition of cement additives.

5) The shear stress versus displacement curves indicate that with the addition of 1% by weight cement additive, samples on the wet side of the optimum show larger

strength and stiffness compared to samples on the dry side of the optimum.

ACKNOWLEDGMENT The authors acknowledge the College of Graduate Studies

for supporting this research project. Thanks to Mr. Sulaiman Al-Haji of the civil engineering department, Kuwait University for his help in performing the laboratory tests.

REFERENCES [1] N. F. Ismael. “Allowable pressure from loading tests on Kuwaiti soils,”

Canadian Geotechnical Journal, vol. 2, no. 22, pp. 151-157, 1985. [2] N. F. Ismael, A. M. Jeragh, M. A. Mollah, and O. Al-Khalidi. “A study

of the properties of surface soils in Kuwait,” Journal of the Southeast Asian Geotechnical Society, Bangkok, Thailand, vol. 1, no. 17, pp. 67-87, 1986.

[3] N. F. Ismael, O. Al-Khalidi, and M. A. Mollah. “Saturation effects on calcareous desert sands,” Transportation Research Record 1089, 1987.

[4] N. F. Ismael, A. M. Jeragh, M. A. Mollah, and O. Al-Khaldi. “Expansive behavior of subgrade soils in arid areas,” Transportation Research Record 1288, 1991.

[5] N. F. Ismael, “Properties and behavior of cemented sand deposits in Kuwait,” Soils and Foundations, Japanese Geotechnical Society, Japan, vol. 4, no. 39, pp. 47-59, 1999.

[6] N. F. Ismael, “Influence of artificial cementation on the properties of Kuwaiti sand,” Kuwait Journal of Science and Engineering, Kuwait University, Kuwait, vol. 27, pp. 59-76, 2000.

Nabil F. Ismael was born in Egypt, 1946. He obtained his B.Sc.in civil engineering from Cairo University, Egypt 1966, M.Sc., 1970 and Ph.D, 1974 from Duke University, N.C.,U.S.A. His major field of study is geotechnical engineering. His is presently a Professor and Chairman of civil engineering at Kuwait University, Kuwait. He has previous experience as a senior engineer, Ontario Hydro, Toronto, Canada, and as a civil

engineer at the Port Authority of N.Y & N.J, USA. He has about 100 publications in International Journals and Conference Proceedings. Professor Ismael is a member of many professional societies including the Egyptian society of engineers, the deep foundations institute and the Canadian geotechnical society Mariam Behbehani has obtained her B.Sc.and M.Sc. degrees from Kuwait University, Kuwait. She is a civil engineer with the Ministry of Public Works, Kuwait

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