Effect of fine content on drained shear strength of mix materials.
The drained shear strength of normally consolidated cohesive soils is defined by friction angle [phi]'. The friction angle is mainly a function of the clay mineral content and clay mineralogy of the composition. Kenney  and more recently in the book by Terzaghi, Peck and Mesri  showed that the friction angle [phi]' for undisturbed and disturbed normally consolidated cohesive soils reduces with plasticity index. Furthermore, according to torsional ring shear tests by Stark and Eid  the value of [phi]' may decrease by 4' as the clay size fraction increases from less than 20% to more than 50% or as effective normal stress increases from less than 50 kPa to more than 400 kPa. The drained shear strength of an over consolidated clay should ordinarily be greater than the drained strength of the same constituents in a normally consolidated state, mainly because the overconsolidated clay has a smaller preshear void ratio. According to Mesri and Abdel-Ghaffar  the drained strength of an over consolidated clay is better expressed in terms of the drained strength of the same constituents in the normally consolidated state as S = [sigma]'.tan([phi]'). [OCR.sup.1-m] In which [sigma]' the effective normal stress on the plane of shear, OCR is over consolidation ratio and m is an empirical factor depending on the structure of soil.
Casagrande and Hirschfield  for the first time showed that for compacted clay as initial degree of saturation increases the changes of shear strength with total vertical stress become smaller. In unsaturated soil suction is imposed on the soil. In unsaturated soils with air voids connected to the atmosphere, the total normal stress acts as an effective confining pressure and push the soil particles together. According to data shown by Escario et al  on a series of suction controlled direct shear tests on an unsaturated clay soil, the increase in shear strength with increasing total normal stress is determined by the friction angle [phi]'. For their data at zero suction for saturated undisturbed clay it displays a cohesion intercept C'. The data shown by Escario et al (1989) indicated a linear relationship between shear strength and vertical total stress.
Data regarding shear strength characteristics of mix materials are lacking in literature . In this regard a research program is being conducted at the Civil Engineering department of Shahid Chamran University. Part of the results of testing that has been performed on the compacted samples at optimum water content is presented in the following sections.
The clay that was used in this research was commercial type commonly applied in oil wells exploration in Ahwaz. The sand used is local sand from shooshtar region in northeast of Ahwaz that is usually used in concrete. The sand used in this research passed the No. 10 sieve was well graded.
2.1 Sample Preparation:
The samples were prepared by mixing clay with 10, 30, 50 and 70 percent by weight of sand (Table 1). Then in order to obtain the optimum water content of each sample the standard proctor compaction test (ASTM) were performed. The result of compaction tests are shown in Table 2. The results of index tests on mixtures are shown in Table 3. As expected, liquid limit and plastic limit of mixtures decreased with increase in percentage of sand.
2.2 Consolidation tests:
In order to determine consolidation characteristics of samples such as the time required for the end of primary consolidation and the over consolidation pressure, each sample were prepared according to following procedure before performing the consolidation test. Before compacting each sample at the optimum water content they were kept in plastic bags for 24 hours. After compaction, samples were removed from compaction mold. Then the consolidation ring (ID = 56 mm, HT. = 20mm) was placed on top of the sample and with the help of a surgical blade the sample was pushed and cut into the ring. After weighing, the sample is placed into consolidation cell. The samples were loaded in increment from 12 kPa to 1540 kPa in the presence of water until the end of primary which was determined according to the Taylor's method. In Fig. 1 the end of primary e-log[sigma]'v' relationships for mixtures are shown. For samples with 0, 10 and 30 percentage of sand after the pressure of about 380 kPa a distinct change in the slope of the curves is observed. This observation for samples with 50 and 70 percentage of sand is not clearly seen.
2.3 Direct shear tests:
The samples for direct shear tests like the samples for consolidation tests were prepared from the compacted samples and with the help of a cubic sampler 50x50mm in size and 20mm in height. After the sample was placed into the cubic sampler, it was carefully transferred into the square shear box. Samples for each mixture were sheared under normal stresses of 38, 76, 152 and 304 kPa after primary consolidation was reached under each normal stress. The time of the end of primary for the samples ranged between 1 to 15 minutes depending on the percentage of sand and the normal stress. According to Bishop and Henlkle , the rate of shear for a drained condition must be such that a time of failure [t.sub.f] = 50.[t.sub.p] is achieved. This means that the time to failure for the samples tested should be between 50 to 750 minutes. If it is assumed that the failure occurs at 10% horizontal strain then the rate of shearing for a complete drained condition becomes 0.06 to 1 mm/min. However the lowest rate which could be used in the machine was 0.5 mm/min. This rate of shearing was used for all samples.
In Figure 2 typical shear stress versus shear displacement relationship for all samples are shown. As it can be seen, the behaviors of all samples look like a ductile material. This is due to lack of any cementation or digenetic bonding that is characteristics of many over consolidated natural clays and shales. Visual examination of some of the samples after the test indicated that these samples showed no distinct shear plane unlike brittle materials.
In Figure 3 typical. Horizontal versus vertical displacement for all samples are shown. In all samples the dilatant behavior during shearing process is observed, indicating the dense and compacted nature of the samples. This behavior for all the mixtures was the most under normal stress of 76 and 152 kPa and was the least under normal stress of 38 and 304 kPa. The reason for a lower tendency for dilation for samples under lower normal stress of 38 kPa may be due to a lower local density for these samples as compared with others. The samples in this group of tests were taken from the upper part of the compacted sample. The dilatancy of samples under normal stress of 304 kPa were stopped, increased with a constant rate or decreased with further horizontal displacement.
Under normal stress of 38 kPa and at the percent sand of 0 and 10 the dilatancy behavior of samples changed to a contractive behavior with further horizontal displacement. At the percent sand of 30 and 50 and under normal stress of 38 kPa, the dilatancy behavior of sample were stopped or decreased with further horizontal displacement. At the percent sand of 70 and under normal stress of 38 kPa, the dilatancy behavior of sample continued with further horizontal displacement.
The dilatancy behavior of all samples under normal stress of 76 and 152 kPa continued at a constant rate with further horizontal displacement and then at the end the rate decreased. If it is assumed that a linear relationship between shear stress and normal stress exists, the effect of the percentage of sand on the cohesion intercept c' and on the angle of internal friction [phi]' is shown in Figs 4 and 5 respectively. The data trend in Fig.4 shows a decrease in c' with increase in the percentage of sand in consistent with the conclusion made with respect to natural materials . On the other hand, the results shown in Fig. 5 indicate the increase in [phi]' with the percentage of sand. Although the amount of [phi]' obtained in this manner is not an indication of the mineralogy of material , however, when the data are plotted against the liquid limit, [W.sub.L] as shown in Fig.6, a general decrease in [phi]' with [W.sub.L] is observed.
Based on the results of this study following conclusions is reached:
1. stress--strain relationship of compacted samples of clay--sand mixtures at optimum water content looks like a ductile material
2. All samples showed dilatancy behavior during shear displacement.
3. The cohesion intercept c' decreases with the increase in the percentage of sand.
4. The angle of internal friction
5. [phi]' Increases with the increase in the percentage of sand.
Received 22 October 2013
Received in revised form 14
Accepted 20 January 2014
Available online 1 March 2014
Financial support for the research described in this communication has been provided by the Islamic Azad University Ahvaz Branch. This support is greatly appreciated.
 Bishop, W. and D.J. Henkle, 1962. The Measurement of soil properties in the triaxial tests, 2nded., London, Edward Arnold, 228 pp.
 Casagrande, A. and R.C. Hirschfield, 1960. Stress Deformation and Strength characteristics of a clay compacted to a constant dry unit weight. Proc. Research conf. On shear strength of cohesive soils, ASCE, pp: 359-417.
 Escario, V., J.F.T. Juca and M.S. Coppe, 1989. Strength and deformation of partly saturated soils. Proc. 12th int. conf. On soil Mech. And Found. Eng., Rio de Janeiro, 1: 43-46.
 Kenney, T.C., 1959. Discussion, proc., ASCE, vol.85, No. SM3, pp: 67-79.
 Khayat, N., 2003. Evaluation of shear strength properties of bentonite--sand mixtures, M.S. thesis, Shahid Chamran University, Ahwaz, Iran.
 Mesri, G. and M.E.M. Abdel-Ghaffar, 1993. Cohesion intercept in effective stress stability analysis, J. Geotech. Eng. ASCE, 119(8): 1229-1249.
 Stark, T.D. and H.T. Eid, 1994. Drained residual strength of cohesive soils, J. Geotech. Eng., ASCE, 120(5): 856-871.
 Terzaghi, K., R.B. Peck and G. Mesri, 1996. Soil Mechanics in Engineering practice 3rd Ed., John Wiley and sons, 549.
Navid Khayat and Seeyed Majid Alboshoke
Department of Civil Engineering, Islamic Azad University Ahvaz Branch, Ahvaz, Iran
Corresponding Author: Navid Khayat, Department of Civil Engineering, Islamic Azad University Ahvaz Branch, Ahvaz, Iran E-mail: KHAYAT@IAUAHVAZ.AC.IR
Table 1: Proportions of sand and clay used in samples tested. Mixture name Percentage Percentage of sand of clay C 0 100 C90 S10 10 90 C70 S30 30 70 C50 S50 50 50 C30 S70 70 30 Table 2: Standard compaction test data for sand-clay mixtures. Mixture [omega] [[gamma].sub.d] [[gamma].sub.wet] name (%) (kN/m^3) (kN/m^3) C 24.3 16.21 13.05 C90 S10 23.48 18.22 14.75 C70 S30 20.73 19.47 16.13 C50 S50 15.43 20.7 17.93 C30 S70 14.33 19.6 17.14 Table 3: Index properties of sand-clay mixtures tested. Index properties\ LL (%) PL (%) PI (%) CF (%) [A.sub.C] Mixture name C 107 47.7 60.1 92.7 0.65 C90 S10 85.5 45.7 39.8 80.2 0.5 C70 S30 69.1 40 29.1 60.9 0.48 C50 S50 63.8 38.8 23 44.3 0.5 C30 S70 39.8 26.3 13.5 31.7 0.43
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|Author:||Khayat, Navid; Alboshoke, Seeyed Majid|
|Publication:||Advances in Environmental Biology|
|Date:||Jan 1, 2014|
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