International Research Journal of Applied and Basic Sciences © 2012 Available online at www.irjabs.com ISSN 2251-838X / Vol, 3 (S): 2815-2821 Science Explorer Publications

The shear strength analyses of soil with various compactions under vertical load in direct shear test Amir Kalhor Department of civil Engineering,Shal Islamic Azad University,Tehran,Iran Corresponding author email: [email protected] ABSTRACT: One of the oldest tests to determine the shear strength of soil is direct shear test. With regard to this test, the stresses on a surface which failure occurred are controlled during the test directly. The purpose of compaction is soil density increment because of more compact of soil particles and reduction of empty space of inter granular. There are many different methods for compaction. Various compactions are applied in many researches which is more close to workshop conditions. In this paper, we tried to obtain the shear strength of soil under vertical load due to drained direct shear test by various compactions and also to evaluate compaction's effect on final shear strength. The unit failure line obtained for the samples of different compaction at the end of shear tests. Keywords: various compaction, direct shear test, shear strength. INTRODUCTION Direct shear test is one of the oldest tests for determination of shear strength of cohesive consolidated soil. With regard to this test, the stresses on a surface which failure occurred are controlled during the test directly. The direct shear machine is essentially included a box divided to two sections; low section with no move is connected to machine basic constantly and upper section is free and is able to move into both horizontal and vertical directions which the momentum measured by gage. Shear strength of soil mass is internal strength of that soil's surface unit which can be displayed to deal with failure or slip at the length of each internal page. It is necessary to identify shear strength well for analyzing some cases such as compressive capacity, slopes stability and lateral pressure on soil retaining structures. Direct shear test is an examination to determine shear strength of soil. It is tested on a consolidated-drained sample to determine C and T in soils. The diagram increased and reached to a pic in compact sandy soil, and then the soil will be soften and reduced. More compact sandy caused more diagram slop, so compact sandy soil tends to dilation with regard to its hardness increment. Dilation means increasing the compaction of shear stress. With process of soil shear, it first be compacted slightly, then the particles pushed each other (consider a person is going to pass a crowd, so he should pressure around) so it tended to increase volume and declining friction and faced to low shear strength and softening effect appeared more. Therefore, compact sandy soil tended to dilation. The compaction occurs in slack

Intl. Res. J. Appl. Basic. Sci. Vol., 3 (S), 2815-2821, 2012 sandy soils when influenced by shear process and more compact. The compaction of soils is an important parameter for building roads, foundations and all ground structures. The stability of this structure depended on favor soil density so much. The amount of water will not be changed through soil compaction process and inter granular's empty space reduced through removing the air, large pores will be changed to small one and water moved into soil. The purpose of compaction is to improve shear strength and bearing capacity of soil. Also, compaction caused for reduction of shrinkage potential, subsidence and permeance. So, shearing various parameters of compressed soil with Geotechnical issues are so important such as foundation bearing capacity, soil lateral pressure and slopes stability. Moisture and compaction rates of shear strength parameters (cohesion and angle of internal friction) are obtained through some tests on soil specimens which are made as same as workshop conditions. With regard to soils compaction, they are divided into two methods; cohesion and cohesionless. Cohesionless soils usually are included sand and silt particles and their behaviors are depended on the particles (contact and arrangement). Cohesion soils -which created by clay minerals- are distributed and integrated by their construction. The construction of compacted cohesive soils is depended on inter granular force, external applied force for compaction and also the history of applied stress on soil (Marchi,1972). The effect of soil construction is important as the history of applied stress and primary porosity of soil. The special feature of cohesive soils is due to compaction which led to shear strength increment (Marsal,2000). This feature is based on Proctor statements (Marsal,1973) and the result of the theory led to standard Proctor test which maximum specific weight of soil is estimated. Today, there are various methods for cohesive soils compaction in laboratory and also in workshop's acts. The influence of moisture rate in specific dry weight is equal for all these methods. An optimal moisture rate obtained through each methods of density which led to maximum specific dry weight. According to in-situ properties, compaction is done by means of SMOOTH STEEL DRUM, TAMPING FOOT ROLLERS, PNEUMATIC ROLLERS, VIBRATORY MESH ROLLERS. Usually, VIBRATORY MESH ROLLERS are used for granular soils with more thickness. As a common method, the compaction method of standard Proctor in laboratory is applied to obtain maximum specific dry weight and optimal moisture rate of soil. Also, the method of various compactions under applied vertical load is used to provide different tests include direct shear, triaxial and consolidation. It is appropriate to use of various compactions by roller in workshop activities and need for mentioned tests in relevance to these activities. The errors are occurred by following: •

Because of adsorption properties between water and soil particles, so there is occurred some false cohesion in

cohesionless soils. If the value of cohesion was between 1 and 2, so it ignored, but if it was more and ignored, so the error occurred. •

If the gages for energy measurement and deformation are not regulated exactly.

•

If the drainage is not performed correctly.

•

If tester made mistake during gages reading and/or had computational error.

Disturbed (saturated) samples are led to low angle of obtained friction. But undistributed (unsaturated) samples and its fast shear are led to obtain high results. ASTMD3080-98(2000) standard about the limitation of direct shear test indicated that: there is main stresses rotation during direct shear test which may be existed in- situ status or not. Also, the failure may be occurred and it is possible never happened on lowest surface when the forces gathered in centre on vertical page and/or near it. It is an advantage for constant position of shear page during shear test to determine shear strength in pre-specified weak page and/or to evaluate the contact level of two different materials. 2816

Intl. Res. J. Appl. Basic. Sci. Vol., 3 (S), 2815-2821, 2012 Shear stresses and displacements are distributed non-uniform into sample and there is no appropriate height for shear strain determination. The low speed of displacements led to loss of pore water pressure, but it also caused to plastic flow of soft cohesive soils. It should be considered that the test's conditions are representative of studied status. If sand have large particles of gravel, so the gravel is able to increase shear strength of sand artificially, because of low height of sample. Therefore, the size of sample particles is another limitation of direct shear test. For example, there are two criteria mentioned by ASTM D3080-98 (2000) . First one is, the sample's diagonal should be at least 10 times larger than largest soil particle. Second criteria are, the sample's height should be at least 6 times larger than largest soil particle. It means that the soil particles for a sample with diagonal 2.63 mm and height 4.25mm must not equal to gravel size. Based on these limitations, the direct shear machine usually led to the values of effective friction with appropriate accuracy. This test is almost fast and easy and is the most important reason for generalization this test. MATERIALS AND METHODS In this section, our purpose is to study about the shear strength of soil analysis under vertical load with different compactions during direct shear test. According to that, differentiate moisture samples of soil are compacted to achieve specific weight. Direct shear test is done on provided samples with different specific weight. First, the samples are distributed (saturated) and then vertical and shear preloading are done. The tests are performed very slowly to allow for drainage. In this paper, the applied soil is located in digging location from 15 meter depth above ground surface. The most of soil is made by Silty clay which located in CL category through unified classification system. However, usually the value of Silt in these soils is more than clay, but it located in clay category through the plastic properties. The soil put out to dry. After crushing lumps, the soil passed by sieve number 10(2mm). According to ASTM D 854-02 , density of solid soil particles analysis(Gs) done on 20g of soil by sieve number 10 and dried in stove which obtained 2.70 density for the soil (Hirshfield). Also, the particle size analysis through sieve and hydrometer were done based on ASTM D 422-63 standard (Baladi,2006). The results of particle size analysis are shown in figure 1: Table 1. Density of solid soil particles analysis Percent Finer

Size

0

0

20

0.001

55

0.01

70

0.05

90

0.1

95

0.7

96

1.8

97

3.7

98

5.6

99

9.6

100

10

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Intl. Res. J. Appl. Basic. Sci. Vol., 3 (S), 2815-2821, 2012 With regard to Proctor method, compaction test done on soil based on ASTM D 698-00a (Jager,2006) through sieve number 10. The results indicated that optimal moisture and maximum specific dry weight respectively obtained about 18% and 1.82 Mg/m3 (Jager,2006). Various compactions test is done by next stage. For this purpose, there is used a system includes the machine with adjustable speed, force transductor for measurement of compaction force and data recorder system to transfer the information about force to computer. The compaction tests done by two different energy levels include 45 and 200kPa. In order to obtain these results, the samples with different moisture rates include 16% and 26% are compacted through related compaction energies with 1mm/min rate. The conclusions of these tests are used for making decision about preparation conditions of direct shear samples. This test done by ASTM D 3080 standard which presented a method to determine shear strength of soil in consolidated-drained conditions(Jewel,2007). It done under controlled strain on provided page by changing table. Under different vertical preloading, generally, there are tested three or more samples to determine the influence of this factor on shear strength and displacement and finally, the failure envelop strength parameters of Mohr Coulomb will be provided. In this test, non-uniform shear strains and displacement are distributed in sample and shearing process must be so slow to be ensured of the sample drainage. The failure is not related to the weakest page because the machine will be created a failure for horizontal page in middle of sample. In despite of that, due to provided failure page, it is possible to use for shear strength determination of provided weak pages or shear strength at the contact surface of different materials. Non-uniform shear stresses and displacements are distributed into samples and corresponding failure with shear stress is considered in 20% of lateral relative displacement. In order to preparation of samples, the soil should be provided three similar samples. There are needed at least three necessary tests to determine the angle of internal friction and cohesion of soil. The samples are prepared through primary moisture about 21% and under compaction energies are applied in compaction tests (45 & 200 kPa). For this purpose, there are located two half of shear box which are fixed on each other and moisture porous rock in bottom of shear box. The amount of soil is determined with regard to specific dry weight of samples and based on compaction results. The samples are compacted by means of different compaction with 1mm/min speed. The sample box and shear box are installed into shear machine after sample structure. Around of sample should be filled with distilled water during test. Each distributed (saturated) shear test included three stages. The first stage is to distribute (saturate) the sample which allowed the sample with no load to distribute (saturate) completely during 24 hours and inflated. The second stage is consolidation. In this stage we have three distribute (saturate) tests and vertical stresses about 50,100 and 150kPa are selected for sample consolidation. Preloading is occurred when this stage started and the sample consolidation is done during 24 hours. Third stage is shearing the sample. After consolidation, with no excess pore's water pressure in failure, the sample with appropriate rate of shear will be cut by minimum speed (about 0.133mm/min). With regard to the consolidation results, the speed of shear is selected. The following approximate equation is suggested by ASTM D3080 in order to determine minimum time for testing since start to failure occurred (Maslov,1987): Tf= 50 * T50 •

Tf is approximate time of test to failure based on minute(min)

•

T is time required to achieve 50% of consolidation under considered vertical stress based on minute(min).

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Intl. Res. J. Appl. Basic. Sci. Vol., 3 (S), 2815-2821, 2012 RESULTS The results of compaction are obtained based on using various dynamic and static compaction tests. The results are shown in figure 2. It is obvious that if different compaction energy is changed, so the results of density will be changed. WC(water content)rate is declined and maximum specific dry weight is increased based on compaction energy increment. Table2. The results of different dynamic and static tests WC

%

yd (mg/m3)

yd (mg/m3)

yd (mg/m3)

yd (mg/m3)

10

2.20

1.60

1.28

1.07

12.5

2.10

1.62

1.29

1.09

15

2.00

1.70

1.3

1.10

17.5

1.92

1.73

1.4

1.15

20

1.88

1.80

1.6

1.20

22.5

1.76

1.90

1.7

1.40

25

1.62

1.80

1.6

1.50

27.5

1.61

1.76

1.5

1.49

30

1.60

1.60

1.48

1.40

Line

Proctor

Static 45

Static 200

As it noted, the samples are made with moisture about 21% and by two level of energy include 45 and 200 kPa through direct shear test. Therefore, specific dry weight of samples is respectively 1.33Mg/m3 and 1.61Mg/m3. According to the compaction results in both shear tests, the primary samples compacted in dry section of results. The diagram of shear strength is obtained with different compactions for both samples during shear strength test (table 3). Table 3. Normalized Horizontal Displacement (NHD) versus shear stress(SH) by two densities of 1.61Mg/m3 and 1.33Mg/m3 NHD

50kpa

100kpa

150kpa

50kpa

100kpa

150kpa

0

0

0

0

0

0

0

0.5

23

45

80

25

43

80

1

36

65

95

39

60

94

1.5

38

68

99

40

66

101

2

38

65

99

40

68

102

1.33 Mg/m3

1.33 Mg/m3

1.33 Mg/m3

1.61 Mg/m3

1.61 Mg/m3

1.61 Mg/m3

With regard to these results, the final obtained shear strength from these two tests are so close to each other and primary slope of results and elasticity modulus and it is more for the sample by more compaction. With any shear test, if normalized horizontal displacement (NHD) increased, so the shear stress (SH) will be increased. According to obtained results of shear stress, it is obvious that the similar final shear stress (strength) is provided through two samples with different compactions when failure occurred. For each tests, it is necessary to pass void ratio(VR) in order to study about the subject. 2819

Intl. Res. J. Appl. Basic. Sci. Vol., 3 (S), 2815-2821, 2012 According to vertical forces during the shear (50,100 and 150 kPa), and compaction force which obtained for the samples with specific germ about 1.61Mg/m3 (200kPa), the slopes of unloading results in order to study about the stress way, should be obtained by consolidation results. The results of consolidation test are shown in table 4. It has been done on a sample by primary compaction about 1.61Mg/m3 and unloading is done after preloading. To ensure of obtained compaction force through compaction test, it is possible to provide pre-consolidation stress by diagram through Casagrand method (Mosaid,2009). Here, the value of pre-consolidation stress is 200kPa (table 4). Table 4. The diagram of sample consolidation with primary specific weight about 1.16Mg/m3 Qv

VR

VR

Log(10)

0.66

0.43

Log(100)

0.65

0.4

Log(1000)

0.35

0.35

Loading

Unloading

Using the slope of unloading results, it is possible to obtain the stress of shear tests. Also, the critical failure line comparative is drawn in page of e-Log (Qv)net (table 5-A). There are obtained two stress path with regard to compaction and vertical forces influenced on samples during the shearing. The results slope is provided through table 5 and preloading and unloading results. The consolidated normal lines and unloading are drawn by using lines slope and primary conditions of samples. The points of two shear tests which made by primary conditions of different compaction (density) are shown in table 5-A. The diagram of shear strength versus vertical stress in failure status are shown in table 5-B for two series of shear tests. A unit failure line obtained with regard to the value of final shear strength (table 3) which is equivalent to schematic (comparative) line in relevance to crisis status in table 5-A. The diagram 5-A is drawn based on logarithm because the value of vertical stresses be comparable in both tables 5-A and B.

Table 5-A. The results of direct shear test on distributed (saturated) soil- changes in porosity ratio, y = 0.655x + 2.528 , R² = 0.990 Qv

VR

VR

VR

80

1

0.75

1

100

0.8

0.74

0.8

200

0.7

0.73

0.7

NCL

URL

IS

Table 5-B. The results of direct shear test on distributed (saturated) soil- final shear strength changes, y = 0.655x + 2.528 , R² = 0.990 Qv

SHS

SHS

80

40

40

100

65

65

200

100

100

yd=1.33

yd=1.61

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Intl. Res. J. Appl. Basic. Sci. Vol., 3 (S), 2815-2821, 2012

Similar final shear strength obtained at the end of shear tests on the samples with different primary compaction. The only difference of samples to achieve failure line occurred during the stress path. The samples which have more compaction rate (yd= 1/61 Mg/m3) have pre-consolidation behavior with regard to their compaction and applied vertical force on them during shear. The soil pre-consolidated behavior is low based on condition of shear strength diagrams (table 34) and no softening. The samples with less compaction rate (yd= 1/33 Mg/m3) have normal consolidated behavior and they under constant vertical stress reached from normal consolidated line to failure line. CONCLUSION There are different methods of soil compaction between laboratory and workshop acts. However, the compaction usually is used by roller and with different method, but it is also used by many researches. The purpose of compaction is to increase the specific weight, to enhance the shear strength features and bearing capacity of soil. The direct shear tests in this paper done on samples of soil through two different compactions. According to the results of shear tests, primary difficulty of the samples with higher compaction is more. Also, there was different behavior during failure based on various compaction energies and vertical force entered to samples during shear test. The sample with more compaction has pre-consolidation about 200kPa. When it lying under vertical load (about 50,100 and 150 kPa), so failure occurred from unloading line under constant stress. But, the failure occurred in the samples with low compaction from normal consolidated conditions under constant stress. Therefore, both samples with different compactions have unit failure line during direst shear test. The only difference is different stress path during failure. Also, the results indicated that during the crisis status (cohesion and angle of internal friction), the parameters of shear strength will not change under compaction effect. But, volumetric strain and different in porosities have much more dependent on the history of stress under compaction effect. REFERENCES Al-Hussaini M. 2009. “Evaluation of Rockfill Material”, Jounal of Soil mechanic and foundation division Dec, 2009, SM12. Baladi F, Gilbert T.Wu. 2006. “Interpretatoin of traxical test result of Cohessionless soils” ;Advanced trixical testing of soil and rock. Hirshfield RC, Polous SJ.(eds.), John Wily & Sons Inc., New york ,pp.109-200. Jager J. 1994.“Influence Of Grain Size and Shape On dry Sand Shear Behaviour”, XIII ICSMFG ,New Dehli, India, PP.13-16. Jewel RA. 2007. “Direct Shear test on Sand”, Geotechnique, Vol. 39,No.2, pp. 309-322. Marchi ND, Chan CK, Bolton Hb.1972. “Evaluation of properties of rockfill material.” J. of the Soil mechanics and foundations Division, vol.98, no. SM1. Marsal RJ. 1973. “Mechanical Properties of rockfill” Embankment –Dam Engeering , Casagrando Volume. Marsal RJ. 2000. “Large scale testing of rockfill material.” J. of the Soil mechanics and foundations Division, vol.93, no. SM2. Maslov NN. 1987. “Basic Engineering Geology and soil mechanic”, Translated from the russian by V.V. Kuznetsov first published 1987 revised from the 1982 russian edition. Nich Barton and Bjorn Kjaernski . 1981. “Shear Strenght of Rockfill.”Journal of geotechnical division vol, 107, num. G.17 , July 1981.

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