INVESTIGATION OF SAMPLING ERROR ON SOIL TESTING RESULTS

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International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 13, December 2018, pp. 579-589, Article ID: IJCIET_09_13_057 Available online at http://www.iaeme.com/ijciet/issues.

1 International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 13, December 2018, pp , Article ID: IJCIET_09_13_057 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed INVESTIGATION OF SAMPLING ERROR ON SOIL TESTING RESULTS Mohammed Abdullateef Al-Neami Civil Engineering Department, University of Technology, Iraq ABSTRACT Laboratory errors may occur in different ways, errors in the volumetric subsampling of soils in the laboratory, rounding off readings to the nearest whole number and human error. A special manufactured device was made to prepare and collect the samples at different angles (0, 5, 10, 20, 30 and 45 o ). The direct shear test was carried out on the poorly graded sand (SP) with three different relative densities, 40% for loose sand, 60% medium sand, and 80% dense sand. The results showed that the unit weight of sand varies according to the direction of the collected samples. The error in calculating friction angle due to sampling is about 7.2% in medium dense and 9.7% in loose sand comparing with friction angle in ordinary direction. The percentage error in shear strength is about (-2 to %) for loose sand and (-1.8 to +11.3%) for medium sand while in dense sand the error in obtained the shear strength about (0.7 to 3.4%) and may consider a marginal. Keywords: Sampling error, orientation angle, Shear strength, Orientation angle. Cite this Article: Mohammed Abdullateef Al-Neami, Investigation of Sampling Error on Soil Testing Results, International Journal of Civil Engineering and Technology, 9(13), 2018, pp INTRODUCTION Geotechnical investigation is performed to collect the information on the soil properties used in engineering works as well as in earthworks or to support the foundations of the proposed structures and to concern the damages or distress underneath the foundation which caused by the subsurface conditions. So to obtain this goal, a subsurface investigation consists the soil sampling and laboratory tests of soil samples retrieved. A wide variety of laboratory tests can be conducted on soil samples to obtain various soil properties. Some properties of soil are intrinsic to the soil matrix composition and are not affected by disturbance of samples, while in the other hand some of them depend on the soil structure as well as its composition, and can only be effectively tested on relatively undisturbed samples. Under some conditions of soil (soft soil or loose sand, etc..), undistributed samples cannot be obtained therefore remolded samples are used instead of undisturbed samples. Remolded soil (a soil that has had its natural editor@iaeme.com

2 Mohammed Abdullateef Al-Neami structure modified by manipulation) is usually substituted for undisturbed soil and characterized that is convenient to use, simple in structure, low in cost remolded samples with uniform water content and accurate and reliable data results. The remolded soil sample can be used for preparing samples according to preset water content and density. In 2012, Sadrekarimi and Olson [17] stated that the shearing sand behavior shear testing is not only affected by the method of specimen preparation but also by compressibility and particle damage. Zhao and Tong (2018) [20] stated that the conventional process of the unsaturated soil remolding may create to the wide difference in the soil strength and therefore fail to represent the properties of natural soil strength. The importance of a particular type of disturbance of soil modeling depends not only upon the processes of sampling that used but also on the soil type need to be sampled and quality of testing. However, the influence of modeling error will be investigated in this paper from studying its effect on the behavior of shear strength of sandy soil. 2. SOIL DISTURBANCE Soil disturbance may have occurred in all stages of soil exploration, (the drilling process, soil sampling, transportation, and method of storage, or during testing preparation. Any soil sample collected from the site and taken to the laboratory, and prepared for testing will be subjected to the disturbance. The mechanisms accompaniment with the soil disturbance may be classified as: Clayton et. al., (1997) [6] 1. Changes in stress conditions: reduction in values of the vertical stresses and total horizontal (equal to zero, in the laboratory). 2. Mechanical deformation: shear distortions applied to the soil sample. 3. Changes in voids ratio and water content: as an overall consolidation or swelling of or moisture redistribution in the soil sample. 4. Chemical changes may occur in the soil or the pore water. Some of the disturbance mechanisms occur very quickly while others take a considerable time. Some types of disturbance cannot avoidable, while many of them can minimize or even eliminate. Rocchi et. al., (2013) [16]. 3. ERROR INFLUENCE OF SAMPLING ON SOIL BEHAVIOR Laboratory errors may be created in various ways, errors in the volumetric sub- soils sampling in the laboratory and tester error. (Mountier, 1966) [12]. Errors may occur at the several stages therefore, the errors in the soil sampling for use in the laboratory exploration cannot be avoided entirely. (Pleysier, 1995) [15]. Most soil sampling techniques are calibrated with standards and against each other, but the error introduced by the individuals performing each technique (sampler error) is often not addressed. Kulmatiski and Beard (2004) [11] studied reducing sampler error in soil research. They showed that the error in sampler explains a minimum of 5.5% variation in bulk density of soil when a composite coring technique is used. The non-uniformity of stress and strain across the samples may be masked the true element behavior, possibly causing an error in the soil properties evaluation such as the strength (Nishimura, 2005) [13] editor@iaeme.com

3 Investigation of Sampling Error on Soil Testing Results Sadrekarimi and Olson, (2012) [17], studied the effect of sampling preparation on the behavior of the critical state of sand. They concluded that the reconstituted sand sample does not represent that of in-situ deposit. 4. METHODOLOGY AND EXPERIMENTAL 4.1. Materials Used Air-dried poorly graded sand (SP) was used in this study with three different relative densities 40% for loose sand, 60% medium sand, and 80% dense sand. Various experimental tests were performed on both soils to clarify their properties according to the standard specifications as depicted in Figure 1 and Table 1. Figure 1 Particles distribution of soil used Modeling Preparation To simulate the soil exploration and modeling process as close as possible to the field, a manufactured apparatus by Al-Neami et. al., (2018) [1] was used for this purpose. It consists of a hammer device, steel model, steel plate with dimensions ( ) cm as shown in Figure (2). Table 1 Properties of sand used. Index Property Value Specification Effective size, D10, D30, D , 0.46, 0.87 ASTM D Coefficient of uniformity, Cu 2.9 Coefficient of curvature, Cc 0.81 Classification (USCS) SP ASTM D Specific gravity, Gs 2.66 ASTM D Relative density, % Loose Medium Dense 40% 60% 80% Dry unit weight (γ d) kn/m Angle of internal friction (φ), deg ASTM D editor@iaeme.com

Mohammed Abdullateef Al-Neami Organic Matter, O.M% 0.25 ASTM D 2974-00 SO3%

4 Mohammed Abdullateef Al-Neami Organic Matter, O.M% 0.25 ASTM D SO3% 1.75 BS 1377 test No. 9 TSS % 1.15 BS 1377 test No. 10 ph, % 8.7 BS 1377 test No. 11 Figure 2 Test apparatus Preparation of Bed Deposit and Testing setup The bed deposit of sand was compacted using a modified compactor manufactured in a steel tank with dimensions ( m) (length width depth). Three relative densities of sand selected to investigate, so the weights required for achieving the required relative density is predetermined. The soil weight was divided into equal six weights, (each height layer is 10 cm). To simulate the sampling error courses through collected samples from the boreholes, the samples obtained from the soil model with five angles (0, 10, 15, 20 and 30 degree) using a circular pipe with dimensions ( ) cm. So to achieve these angles, the hammer device is set to the desired position and an inclination angle. A Thin-walled sampling pipe was put in guideline pieces (Figure 3A) then pushed down slowly. The empty length of pipe is measured (Le), then after, the soil length inside the tube is calculated (Ls = L Le). If (Ls) is less than embedded tube length, the test is repeated with less penetration (Figure 3B). A movable gate plate was fixed at the end of the pipe to close the pipe end and prevent the sampling slipping through drawl up, (Figure 3C). Finally, the sample inside the tube is putting in the direct shear box with dimensions of (6 6mm) to determine the shear strength parameters and compressibility editor@iaeme.com

5 Investigation of Sampling Error on Soil Testing Results Figure 3 Testing procedure. 5. VERIFICATION OF SAMPLING METHODS USED A core cutter and sand cone tests are common methods used field density test, the procedures for both were explained in ASTM D1556 and IS 2720-Part-29, respectively. To verify which method is the best to use in the laboratory, verification is made for three methods (core device, core cutter, and sand cone) for vertical unit weight and the result indicated that sand cone was the best accuracy then core device and core cutter as shown in Figure (4). The percentage of error in density calculated using the following equation: Where: γ field γ I Error% = 100 γ field γfield = field unit weight (17.3, 18.0, and 18.8 kn/m 3 ) for (loose, medium and dense sand) respectively. γi = unit weight determined by sampler used. (1) editor@iaeme.com

6 Mohammed Abdullateef Al-Neami Error % Losse Medium Dense Core Device Core Cutter Sand Cone Field Method 6. RESULTS AND DISCUSSIONS Figure 4 Field methods verification Effect of Sampling Angle on soil density To clarify the variation of density with directions, inclined density test is carried out on soil used through taken the samples in different angles. It was noted form the inclined density test that the density increased as the angle increased up to 20 then decreases as the angle increases as shown in Figure (5). After the analyzing results, the error in soil density equals (4.1, 3.5 and 1.7%) for (loose, medium and dense sand) respectively. This variation in the soil density related to the void ratio in soil samples which vary according to the sampling angle as shown in Figure (6). It is worth noting that the void ratio calculated using the following equation: Where: e = void ratio Gs =specific gravity of sand (2.66) γw = unit weight of water (kn/m 3 ) G. e = s γ γ sand 6.2. Stress-Strain behavior The direct shear is not suitable to determine stress-strain properties of soils (Olson, 1989) [14]. Also, Shear stress and displacement are non-uniformly distributed within the soil specimen. An appropriate height cannot be defined for calculation of shear strain. Therefore, a stressstrain relationship cannot be determined from this test. So, this test was employed to determine the failure envelopes for soils. w 1 (2) editor@iaeme.com

7 Investigation of Sampling Error on Soil Testing Results Figure 5 Variation of unit weight with sampling angle. Figure 6 Void ratio and sampling angle. Direct shear tests were performed in accordance with ASTM D 2166 with a constant rate of 0.5 mm/min. Generally, in this test the samples were sheared with a horizontal bedding plane. The sand sample is extruded from the sampling tube then directly trimmed in the ring of the shear box to ensure that the sampling angle is unchanged. The results as depicted in Figures (7) and (8) reveal that in the friction angle of loose and medium sand is increased with the increasing of bedding angle then decreased and this behavior may be attributed to the effect of anisotropy that causes an error in calculating friction angle about 7.2% in medium dense and 9.7% in loose sand. Also within the range of angles studied, it can be concluded that the dense sand is nearly insensitive to the orientation of sampling angle. This behavior may occur because the interparticle bonding and their arrangement were unchanged or little changed due to the small editor@iaeme.com

8 Mohammed Abdullateef Al-Neami of a representative structure of the mass of soil has been included in the size of the samples tested. These results agreed with Farhadi and Lashkari (2017). Therefore, it can be concluded that the (within the range of angles studied) dense sand is nearly insensitive to the orientation of sampling angle. Figure 7 Influence of sampling angle on friction angle of sand samples Figure 8 Reduction of friction angle with the sampling angle. Figures (9) to (11) define the effect of sampling angle changing on the values of shear strength of sand samples examined under different normal stresses (27.77, and kpa) respectively [7]. τ = c + σ n tanφi (3) editor@iaeme.com

9 Investigation of Sampling Error on Soil Testing Results Where : τ = shear strength, kpa c = cohesion (equals 0 ) σn= normal stress, kpa φi = angle of friction of samples. Figure 9 Effect sampling angle shear strength for loose sand. Figure 10 Effect sampling angle shear strength for medium sand editor@iaeme.com

10 Mohammed Abdullateef Al-Neami Figure 11 Effect sampling angle shear strength for dense sand. It can be marked that the changing in direction of the collected sample has an influence on the magnitude of shear strength of loose and medium sand while this influence is considered a marginal on the dense sand. The percentage error in calculated shear strength fluctuated in between about (-2 to %) for loose sand and about (-1.8 to +11.3%) for medium sand while in dense sand the error in obtained the shear strength about (0.7 to 3.4%). Therefore in dense sand, the changing in sampling angle has a negligible impact on the shear strength. This behavior may be returned to the change the principle stresses applied within the soil type. This result agreed with Ting and Meachum (1995) [18] who stated that the soil samples exhibit the highest shear resistance when the bedding orientation is normal to the direction of principal stress. 7. CONCLUSIONS Based on the experimental results, the following conclusions can be drawn: 1. The unit weight of the sand increases corresponding to the increase of sampling angle then decreases after the angle of anisotropy equals 20 o. The sampling error increased according to relative soil density and equals (4.1, 3.5 and 1.7%) for (loose, medium and dense sand) respectively. 2. The void ratio changes corresponding to the change of sampling direction which related to dry unit weight. 3. The friction angle of loose and medium sand is increased with the increasing of sampling angle then decreased. The error in calculating friction angle about 7.2% in medium dense and 9.7% in loose sand while in the dense sand, the changing in sampling angle has a negligible impact on the shear strength; therefore, the dense sand is nearly insensitive to the anisotropy angle. 4. Shear strength of loose and medium sand is decreased or increased according to the direction of soil sampling while the dense sand the effect soil sampling is marginal. It was found that the percentage error in shear strength is about (-2 to %) for loose sand and (-1.8 to +11.3%) for medium sand while in dense sand the error in obtained the shear strength about (0.7 to 3.4%) editor@iaeme.com

11 Investigation of Sampling Error on Soil Testing Results REFERENCES [1] Al-Neami, M.A., Rahil, F.H. and Al-Bayati, K.S., (2018) A Laboratory Driving System to Simulate the Insertion of Piles Models with Various Angles, Patent No.5334, the IPC E02D7/08. [2] ASTM D1556, A. Standard Test Method for Density and Unit Weight of Soil in Place by Sand-Cone Method. West Conshohocken, PA, ASTM International. [3] ASTM D (2007), American Society for Testing and Materials Standard test method for particle size analysis of soils. West Conshohocken, PA. [4] Bowels, J.E (1996), Foundation Analysis and Design, 5th edition. Mc Graw-Hill Book Company, New York. [5] British Standard B.S: C.P (1986), Code of Particle for Foundation, British Standard Institution, London. [6] Clayton C. R. I., Matthews M. C. and Simons N. E., (1997), Site Investigation, Second Edition. [7] Das, B.M (2004), Principles of Foundation Engineering, 5th Edition Thomson learning Academic Resource, United State of American. [8] Davis, T., (2008), Geotechnical Testing, Observation, and Documentation, Second Edition, ASCE Press. [9] Farhadi B. and Lashkari A. (2017), Influence of Soil Inherent Anisotropy on behavior of Crushed Sand-Steel Interfaces, Science Direct, Soils and Foundations 57, pp [10] IS 2720 Part 29, (1975), Determination of Dry Density of Soil in Place by the Core-Cutter Method. [11] Kulmatiski A. and Beard K. H. (2004), Reducing Sampler Error in Soil Research, Soil Biology & Biochemistry 36 pp [12] Mountier, N. S., Griggs J. L., and Oomen. G. A. C. (1966), Sources of Error in Advisory Soil Tests: I. Laboratory Sources, New Zealand Journal of Agricultural Research. [13] Nishimura S. (2005), Laboratory Study on Anisotropy of Natural London Clay, Ph.D. thesis, Department of Civil and Environmental Engineering, Imperial College London. [14] Olson R. E., (1989), Direct Shear Testing Advanced Geotechnical Laboratory Department of Construction Engineering. Chaoyang University of Technology. [15] Pleysier J. L., (1995), Soil Sampling and Sample Preparation International Institute of Tropical Agriculture (IITA). [16] Rocchi G., Vaciago G., Fontana M., and Daprat M., (2013), Understanding Sampling Disturbance and behavior of Structured Clays Through Constitutive Modeling, Soils and Foundations, Vol. 53, Issue 2, pp [17] Sadrekarimi A. and Olson S.M., (2012), Effect of Sample-Preparation Method on Critical- State Behavior of Sands Geotechnical Testing Journal, Vol. 35, No. 4. [18] Ting J.M. and Mwachum L.R. (1995), Effect of Bedding Plane Orientation on The behavior of Granular Systems. Conference on Mechanics of Materials with Discontinuities and Heterogeneities, Los Angeles, California. [19] Terzaghi, K. and Peck R.B (1976): Soil Mechanics in Engineering 2nd Particle edition, John Wiley and Sons, New York. [20] Zhao Y. and Tong F., (2018), A Sample Preparation Method Enhancing Shear Strength for Remolded Unsaturated Soil, Proceedings of GeoShanghai International Conference: Multiphysics Processes in Soil Mechanics and Advances in Geotechnical Testing. GSIC. Springer, Singapore editor@iaeme.com

Sci.Int.(Lahore),28(2), ,2016 ISSN ; CODEN: SINTE

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