The Effect of Scale Direct Shear Test on the Strength Parameters of Clayey Sand in Isfahan City, Iran
The direct shear test measures shear strength parameters of soils and other particulate materials. In this study the effects of box size on the strength parameters of soil is studied in Isfahan city area. The soil samples are classified as SC according to the unified soil classification system. The samples were well graded with dry density ranges in the between 1.67 and 1.82 g cm-3. The tests on the shear strength properties of SC soils were carried out by using large, medium and small scale direct shear equipment with shear box dimensions of 60, 100 and 300 mm. The undistributed samples, having almost the same properties, were tested in three square shear boxes of varying sizes. Forty five sets (each set with three samples) of direct shear tests at a constant rate of 1 mm min-1 were performed to study the influence size of the shear boxes and soil density on the strength parameters. The results show the effect of scale on the test. The large and medium scale direct shear produces a higher cohesion and lower friction angle compared with the results of the small-scale direct shear test. The tests indicate that the friction angle and cohesion increase when soil density in each of the three boxes increases. Therefore, these observations suggest that strength parameters are controlled by the scaling effect and physical properties of the soil. The present study also shows the relationship between the shear strength parameters of small and large scale direct shear tests.
Received: April 19, 2010;
Accepted: June 10, 2010;
Published: July 05, 2010
The testing of soils by applying a shear load has resulted in a worldwide revival
of interest over the last few decades. In geotechnical engineering, the engineering
properties of soil layers must be known to the required depths. Engineering
properties can be determined by means of tests carried out in the field and
laboratory (Bowles, 1997). Different shear tests are used
in soil mechanics to measure the mechanical properties of soils. Direct shear
tests are favored by many geotechnical engineers because of their simplicity
and repeatability (Zhou et al., 2009). This test
is one of the experiments that are used to determine the shear strength of soil
for more than 50 years (Cerato and Lutenegger, 2006).
First time, Coulomb (1976) used the direct shear test
with different materials including wood blocks to determine the equation used
to express shear stress as a function of normal stress and friction angle. The
direct shear test was not used for testing the shear strength of soils until
the early 1800's when Alexandre Collin studied slop stability (Skempton
and Collin, 1949). A historical account of the engineering application of
direct shear testing is provided by Matthews (1988).
Today this test has been widely used to measure the shear strength of soils
(Maaitah and Allah-Mahadin, 2004; Liu
et al., 2005; Monkul and Ozden, 2007; Pakbaz
et al., 2008; Wu et al., 2008; Shafiee
et al., 2008; Härtl and Ooi, 2008; Yan,
2009). The specimen size or scale effects on direct shear test were, first
time, discussed by Parsons (1936). He presented test
results for crushed quartz and clean uniform sand. It was showed that a larger
shear box produced lower values of friction angle. Few researchers investigated
the influence of specimen size and scale effects shear box testing on the friction
angle and cohesion. It was seen that size and scale shear box (length, width
and height) affected both the resulting strength parameters (Cerato
and Lutenegger, 2006; Simoni and Houlsby, 2006;
Wang et al., 2008; Bareither
et al., 2008; Liu et al., 2009;
Zhou et al., 2009). Previous studies have shown that different shear
strength parameters can be obtained in direct shear test, using different size
shear boxes (Parsons, 1936; Cerato
and Lutenegger, 2006; Bareither et al., 2008;
Wu et al., 2008).
Isfahan is one of the most important historical cities of Iran; therefore,
the construction in this city is importance. The soil samples used in this study
was taken from downtown of the Isfahan city area central Iran (Fig.
||Map of Iran and location of study area
These samples are classified as SC according to the unified soil classification
system. In this study, the experiments on the clayey sands have been studied
in detail to explore the influence of the shear box on the friction angle and
cohesion. In this study, the tests on the shear strength properties of SC soils
were carried out by using large, medium and small scale direct shear equipment
with shear box dimensions of 60, 100 and 300 mm. Earlier studies have also performed
similar direct shear test with different size shear boxes in different countries
(Parsons, 1936; Cerato and Lutenegger,
2006; Bareither et al., 2008;
Wu et al., 2008).
MATERIALS AND METHODS
Clayey sand: Tests were conducted on 45 naturally occurring soils having
almost the same geologic origins. The soils were obtained from borrow pits in
natural ground. The soils used in this investigation were characterized using
grain-size, density, moisture content, liquid limit, plastic limit and plasticity
index according to ASTM Standard test methods (ASTM, 2009).
The soil samples classified according to Unified Soil Classification System (USCS). Grain-size analyses were performed in general accordance to ASTM D 422-63 Standard test method for particle-size analysis of soils. The particle size distribution curves for the sample soils are shown in Fig. 2. These soils could be classified as SC according to the USCS (ASTM D2487-06E01). Soils used in the tests were well graded and the maximum and minimum dry density of the soils was 1.67 and 1.82 g cm-3. Table 1 shows that the other physical properties also exhibit considerable variation:
Moisture content (W%) = 5.6 to 8.3
Liquid Limit (LL) = 22 to 33
Plastic Limit (PL) = 17 to 23
Plasticity Index (PI) = 5 to 14
Direct shear test: Soil strength is the resistance to mass deformation
developed from a combination of particle rolling, sliding and crushing. It is
reduced by any pore pressure that exists or develops during particle movement.
This resistance to deformation is the shear strength of the soil as opposed
to the compressive or tensile strength of other engineering materials (Bowles,
1997). The oldest method for investigating the shearing resistance (e.g.,
cohesion and internal friction angle) of soils is the direct-shear test (Terzaghi
et al., 1996). The direct shear test is used to estimate the shear
strength of a laterally confined sample when breaking along a prefixed horizontal
plane (Oyanguren et al., 2008). According to
ASTM D 3080-04, the direct shear box test has several particle size to box-size
requirements when preparing specimens for testing. It is recommended that the
minimum specimen width should not be less than ten times the maximum particle-size
diameter and the minimum initial specimen thickness should not be less than
six times the maximum particle diameter.
|| Grain-size distribution for the soils used in this study
|| Physical properties of tested samples
|USCS: Unified soil classification system, SC: Clayey sand
It should be mentioned that, the ASTM D 3080-04 standard test method for direct
shear tests of soils stipulates the apparatus size to be at least ten times
the size of the largest particle size and the horizontal dimension of the apparatus
to be at least twice the vertical dimension. Research on the particle-size to
box-size requirement is becoming quite popular. For example, Taylor
and Leps (1938) performed comparative large shear box (305x305 mm) versus
Shear box (76x76 mm) on the oven-dried Ottawa sand. They observed that friction
angle measured in the small shear box was 0.5° larger, on average, than
friction angle measured in the large box. Ingold (1982)
concludes that the friction angle obtained from a 60x60 mm shear area was 2-3°
higher than that obtained from a 300x300 mm shear area. Jewell
and Wroth (1987) suggested a ratio of shear box length to average particle
size in the range of 50 to 300. It is very important to prevent the soil sample
moisture content losing during the shear test. Therefore, the direct shear box
was wrapped with nylon stretch film and covered with moisturized cloth after
placing the sample in the direct shear machine. Three different sizes of shear
boxes were used. The movement of the lower shear box in the horizontal direction
is controlled by a set of gears which are mobilized by an electric motor. The
size of the shear box can influence the test results. The test results from
the Cerato and Lutenegger (2006) studies showed that
there is a scale effect present in direct shear tests. They observed that small
particle-size to box-size ratios provided a smaller friction angle. They also
showed that not only the relative size of the particle to the box but also the
relative density affected the friction angle. The friction angles difference
corresponding to peak strength measured in small-scale and large-scale direct
shear box were reported by Bareither et al. (2008)
ranges between 4 to 2° for sand backfill. Zhou et
al. (2009) presented the ASTM recommended box size in fact may result
in shear band reaching the box boundaries and smaller particle/box ratio may
either decrease or increase the bulk friction. The test results of Toyoura sand
presented by Wu et al. (2008), showed the lower
friction angle value for large scale shear box.
The friction angle and cohesion play a major role in design of many engineering structures in soil, such as foundations, slopes stability and other structures. The direct shear test one of tests is used to determination these parameters. This research project lasted more than six months, from October 2009 to March 2010. The undistributed samples, having almost the same properties, were obtained from borrow pits in center of Isfahan city.
In this study, three different size shear boxes were used. The first square shear box (small box) had a width of 60 mm and a depth of 20 mm (aspect ratio, H/L = 0.34). The samples were sheared at a constant rate of 1 mm min-1. In this test program 15 sets of direct shear tests with three increasing normal stresses (1, 2 and 3 kg cm-2) were performed using a dead-weight system. The second square shear box (medium box) had a width of 100 mm and a depth of 30 mm (aspect ratio, H/L = 0.3). The samples were sheared at a constant rate of 1 mm min-1. In this test program 15 sets of direct shear tests were performed. The third square shear box (large box) had a width of 300 mm and a depth of 100 mm (H/L = 0.34). Also in this box, fifteen tests were performed with the different normal stresses using dead-weight.
The size of the shearing device can influence the direct shear test results.
Generally, the boundary effect and device friction are more significant for
a smaller shear box. A total of forty five tests were performed on the soil
samples. The results are summarized and discussed in this section. It is well
known that the failure of a granular mass can usually be adequately described
by the Mohr-Coulomb failure criterion (Lambe and Whitman 1979).
The friction angle and cohesion were determined either as a regression best
fit to each test series over the normal stress range and by calculating these
parameters for each test.
In this study, direct shear tests were performed in three different size shear
boxes. The friction angle and cohesion for each sample soil as a function of
box size is shown in Fig. 3 and 4. As these
Fig. 3 and 4 show by increasing the box
size, the friction angle decreases and cohesion also increases in most cases.
|| Influence of box size on friction angle
|| Influence of box size on Cohesion
Also Fig. 3 and 4 show the range in friction
angle for small, medium and large box is from 27.6 to 33 and 24.6 to 30.7 and
23.1 to 29.4, respectively. Also the range in cohesion for small, medium and
large box is from 0.04 to 0.19 and 0.14 to 0.26 and 0.17 to 0.28, respectively.
|| Influence of density on friction angle
It can be argued that the friction angle obtained from the smallest shear
box (60 mm), show higher friction angles and lower cohesion in soils tested
and also the results of the largest shear box (300 mm) show higher cohesion
and lower friction angles in soils tested. The friction angles obtained from
the two larger boxes (100 and 300 mm boxes) are almost similar.
Density factor affecting the strength parameter: This study evaluated
the effect one of physical characteristics (density) on the strength parameters
of clayey sand from Isfahan. Density, friction angle and cohesion were determined
for 15 samples. The tests results also show that the friction angle and cohesion
increase with increasing density in each of the three boxes (Fig.
5, 6). The test results in this study show that there
is a scale effect present in direct shear tests. These results help to explain
the previous finding that investigated scale effects in direct shear boxes.
Comparison of large, medium and small scale direct shear test: Large direct shear test (300x300 mm) on the undisturbed soil is costly, difficultly and time consuming.
|| Influence of density on cohesion
Therefore, these tests on the small projects are not feasible usually. A comparison
of friction angles and cohesions of large and small direct shear boxes are presented
in Fig. 7 and 8. The test data of small,
medium and large scale shear boxes were investigated. To distinguish relationship
between scale size direct shear and shear strength parameters of clayey sand
regulation analysis was conducted. There are a highly significant relationship
between friction angle of small and large shear box (R2 = 0.9218),
as well as cohesion obtained from small and large scale direct shear test results
(R2 = 0.7968). So, we can use the results of small scale direct shear
box equations to find shear strength parameters for large direct shear box.
Comparison of results: The results from this study show that there is
a scale effect present in direct shear tests and that the strength parameters
depend on the density. The friction angles and cohesion obtained from the small
scale tests were also compared to those from the large and medium scale tests
to determine if statistically significant differences existed between the friction
angles and cohesion measured in small scale and another scale tests. The medium
and large scale direct shear box test show a higher cohesion and lower friction
angle compared with the results of the small-scale direct shear test results.
||Relationship between friction angles of large and small direct
||Relationship between cohesion of large and small direct shear
Parsons (1936) showed that a larger shear box produced
lower values of friction angle. Cerato and Lutenegger (2006)
performed direct shear tests on five sands at varying densities in 60, 102 and
305 mm square shear boxes. They proposed the friction angle was seen to increase
with increasing relative density in each of the three boxes and that the constant
volume (residual) friction angle decreased or remained constant, with increasing
box size depending on the type of sand and the relative density. Friction angles
measured in this study differed by no more than 4.5° and in most cases differed
by less than 3.5°. Previous studies conducted by Cerato
and Lutenegger (2006), Bareither et al. (2008)
and Zhou et al. (2009) and also the results on
clayey sand reported in this study have shown that different shear strength
parameters can be obtained in direct shear test, using different size shear
Forty five samples of clayey sand, having almost the same properties, were tested in three square shear boxes of varying sizes. The results show that the friction angles and cohesion were most affected by shear box size. The direct shear tests show that the friction angle and cohesion can be dependent on shear box size. The medium and large scale direct shear box test, having almost the same results, show a higher cohesion and lower friction angle compared with the results of the small-scale direct shear test results. A good relation exists between large scale and small scale direct shear test. The following equations for large and small scale direct shear are obtained.
The friction angle and cohesion show an increase with increasing density in each of the three boxes.
The authors would like to express our sincere thanks the support of Zamin Afraze Naghshe Jahan CO. and Ferdowsi University of Mashhad in the carrying out of this study.
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