THE ULTIMATE SKIN RESISTANCE OF CONCRETE PILE IN PARTIALLY SATURATED COHESIVE SOIL BY MODIFIED Β METHOD

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 10, October 2018, pp , Article ID: IJCIET_09_10_187 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed THE ULTIMATE SKIN RESISTANCE OF CONCRETE PILE IN PARTIALLY SATURATED COHESIVE SOIL BY MODIFIED Β METHOD Hadi Mohammed, Mohammed F. Abbas and Ameera Mohamad Civil engineering department, Engineering College, Muthanna Unversity, Iraq ABSTRACT The ultimate skin resistance of concrete pile in partially saturated in cohesive soils is affected by many factors such as the degree of saturation of the surrounding soil through the contribution of the matric suction. In addition, the surface ' s roughness of concrete pile (expressed as British Pendulum Number BPN) also have their contribution to the interface shear strength of the concrete pile hence its bearing capacity. Modified β methods are used to estimate the total shaft resistance of piles in partially saturated soils by including the influence of matric suction. The modified β method can also be used for estimating the variation of shaft capacity of single piles with respect to matric suction using the Soil-Water Characteristic Curve (SWCC) and the saturated soil shear strength parameters. In this paper, the results show that the shaft capacity of concrete piles is significantly influenced by the adhesive factor, roughness of concrete surface and the contribution of matric suction depended on degree of saturation. depended on the laboratories data and modified β equation, the carrying capacity of a concrete pile resulting from estimated value by modified β equation and measured value by modified direct shear box, the comparison between these values were an acceptable very small percentage of error, and can be developed to other types of soils. Relationships among degree of saturation, matric suction, shear strength and surface's roughness, were very important for design a pile embedded in partially saturated cohesive soil. The value of drained interface shear strength parameters of the partially saturated cohesive soil depended on many factors such as adhesive factor, and pile surface roughness and other aforementioned factors. Relationship between skin resistance and the concrete surface roughness showed that the skin resistance increases with the increase of surface roughness. Keywords: partially saturated soils, pile design, modified β method, SWCC, matric suction editor@iaeme.com

2 Hadi Mohammed, Mohammed F. Abbas and Ameera Mohamad Cite this Article: Hadi Mohammed, Mohammed F. Abbas and Ameera Mohamad, The Ultimate Skin Resistance of Concrete Pile in Partially Saturated Cohesive Soil By Modified Β Method, International Journal of Civil Engineering and Technology, 9(10), 2018, pp INTRODUCTION The partially saturated soil which is the part of soil lies above ground water table.the soil that is covered by the water table is saturated and pour water pressure is always positive. In the partially saturated soil the pore water pressure is generally negative with respect to atmospheric pressure.the engineering properties of soils such as shear strength and volume change are affected by the variation of water table. The variation of water table is mainly due to the climatic factors such as precipitation, evaporation and transpiration. In the arid or semi arid regions which usually have a deep groundwater table, the ground water table decreases slowly with the time because of high upward flux (evaporation and transpiration) during dry season. The pore water pressure distribution with depth can take on a wide variety of shapes as a result of environmental changes. The relationship of pore water pressure with depth gives reason for understanding how unsaturated soils affect pile foundations. In this paper, the β method by Chandler and Burland [1],[2], is modified such that it can be used for estimating the ultimate shaft resistance of piles in partially saturated soils. The modified β method is similar to the classical method used in the design of piles in geotechnical engineering practice. This method is presented in a functional form such that it can be used for estimating the variation of the shaft capacity of the single piles with respect to matric suction using the saturated soil properties and the Soil-Water Characteristic Curve (SWCC). The proposed modified equations take the classical form of the β method used for saturated soils when the matric suction value is set to zero. The objective of this paper is to study the variation of the carrying capacity of frictional piles in fine grained soils through studying the effect of the variation of the degree of saturation on the adhesive forces in the soil concrete pile interface; the research covers the following topics: a. To study the variation of degree of saturation on the cohesive shear strength of soil. b. To study the variation of degree of saturation on the soil concrete pile interface shear strength. 2. BACKGROUND OF THE BEARING CAPACITY OF PILE IN PARTIALLY SATURATED SOIL A pile foundation is an integral part of many civil engineering structures such as highway bridges and high rise buildings. The soil pile is a complex system and the complexity is further increased when the soil is in an unsaturated state. Various studies have been performed on the influence of unsaturated soils on pile behavior. The axial capacity of a pile was studied by [3], in which they determined that the ultimate pile load increases as the degree of saturation decreases. Vanapalli et al. [4] investigated the influence of matric suction on the shaft capacity of jacked piles in coarse-grained soils. Results obtained from the study shows the contribution of matric suction towards the shaft capacity was significant (35-40% of the total shaft capacity of silty sand). Vanapalli and Taylan [5] their study shows that the shaft capacity of single piles is significantly influenced by the contribution of matric suction. Based on the experimental results, the conventional (λ, α, and β) methods were modified to estimate the total shaft resistance of piles in unsaturated soils by including the influence of editor@iaeme.com

3 The Ultimate Skin Resistance of Concrete Pile in Partially Saturated Cohesive Soil By Modified Β Method matric suction. Uchaipichat [6] where simulations were performed results on a pile with diameter of 0.40 m. installed in unsaturated clay layer. The length of pile ranges from 5 to 20 m. The matric suction ranges from 10 to 10,000 kpa. The simulation results show a decrease in pile capacity and factor of safety with decreasing matric suction. 3. EXPERIMENTAL PROGRAM Soil samples were founded from one site within Samawah city region at depth (4 m 4.5 m). The physical properties of soil samples were studied by conducting series of tests in the laboratory, these tests included (Specific gravity, Grain size distribution by sieve analysis and hydrometer, Atterberg's limits, Direct shear test, and Interface between clayey soil and concrete pile surface by modified direct shear box test).for each sample, the matric suction were measured by the filter paper method (Whatman No.42) at different degrees of saturation. The soil surrounded the Concrete surfaces has the properties shown in Table (1), which were calculated from laboratory tests carried out on undisturbed samples. The soil contains silt and clay consequently the soil is classified as CL according to (USCS). Table 1 Mechanical and physical properties of used soil property value Type of test standard L.L % 39 P.L% 91 Atterberge limits ASTM 4318 P.I 02 G.S 07.9 Specific gravity of solids ASTM D854 Sand content % 0.7. ASTM D422- ASTM grain size distribution test Clay and silt %.077 D421 Total uint weight (kn/m3) 20.1 Natural moisture content Field Unit weight (ASTM-D % Cohesion (kpa) 9.71 Parameters of shear Angle of internal friction 9177 strength(direct shear box),ø (degree) ASTM-D3080 Classification according to (USCS) CL 3.1. Concrete Interface Two concrete samples were prepared by using a pre-mixed cement-fine sand grout with a 1:3 ratio of sand to cement, and 40% water cement ratio by weight. The sample was cast in the lower portion of a shear box device, and allowed to cure for 14 days prior to initial testing. The British Pendulum Number (BPN) values and skid number are shown in Tables (2) and (3). Table 2 The British Pendulum Number (BPN) values and Skid Number Interface Surface BPN Name Used in This Research Concrete (1) 49 C49 Concrete (2) 52 C editor@iaeme.com

4 Hadi Mohammed, Mohammed F. Abbas and Ameera Mohamad Interface Surface Table 7 Concrete types Information. BPN (SN) Skid Number Ra(µm) Rt( µm) Concrete (1) Concrete (2) Soil-Interface Specimen Preparation Soil-interface materials were tested: C49 and C52. The C49 interface was made by filling the lower half of the shear box with a concrete mix, the internal dimensions of shear box (6*6)cm. The box was reassembled and a clay specimen was fitted, ensuring complete interface contact. Interface testing was carried out in accordance with [7]. The modified shear box device was placed within a metal container which was laid upon a set of linear ball bearings allowing unrestricted horizontal displacements. The normal loading was applied through a steel bearing arm connected to the top section of the shear box. Three different normal pressures of (50, 66, and 75) kpa were applied to simulate typical lateral earth pressures along the pile shaft at a moderate driving depth. The shearing rates applied were achieved through the use of a precise screw type actuators calibrated to 0.5 mm/min in order to simulate drained condition Matric Suction of Soil Measurement by Filter Paper Method The filter paper method has long been used in soil science and engineering practice and it has recently been accepted as an adaptable test method for soil suction measurements because of its advantages over other suction measurement devices. Basically, the filter paper comes to equilibrium with the soil either through vapor (total suction measurement) or liquid (matric suction measurement) flow. At equilibrium, the suction value of the filter paper and the soil will be equal. After equilibrium is established between the filter paper and the soil, the water content of the filter paper disc is measured. Then, by using a filter paper water content versus suction calibration curve, the corresponding suction value is found from the curve by [8]. 1: Non-contact filter paper 3- soil specimen 2: PVC O-ring 4- Contact filter paper Figure 1 Arrangement of filter papers and samples for the filter Paper method editor@iaeme.com

5 The Ultimate Skin Resistance of Concrete Pile in Partially Saturated Cohesive Soil By Modified Β Method 4. RESULTS AND DISCUSSION To determine the effect of the interface roughness on the shear resistance properties of different piles surfaces, interface concrete tested in this study include C49 and C52, surfaces which have the properties defined in Table (2), the factor of BPN which is related to the total and average roughness of the concrete pile surface Table (3) will be considered as the most representative factor though this study. The British Pendulum Number (the higher the number the smoother the surface) will be considered as a factor that has its contribution on the shear strength of the soil-concrete interface hence the bearing capacity of the pile. This work focuses on using a direct shear box tools to obtain interface shear strength parameters expressed as the apparent interface adhesion c a by testing each interface specimen under drained conditions. The results will help in calculating the suitability of concrete materials in piling and other applications where soil-pile frictional interface is of importance. The shear resistance between soils and an interface surface is of significant interest for the design and performance of many geotechnical systems such as friction piles and retaining walls. These interface shear resistance depends on the soil type, grain size distribution, interface material, surface roughness, normal stresses at the interface and rate of shear displacement [9] Results of the Experimental Test The drained shear strength (c, ) of soil was measured by carrying out direct shear test through remolding the samples at different degree of saturation (100%, 90%, 80%, 70%, and 60%). The results demonstrate that the drained shear strength (c, ) and the angle of internal friction (Ø) increases with the decrease of the degree of saturation (S), this was also noticed by [10]. The results of direct shear test (drained and consolidated) are shown in Table (4) and fig. (2). Table 4 The results of direct shear test (drained and consolidated) conditions. S (%) 100% 90% 80% 70% 60% c, (kpa) Ø Figure 2 The relationship between degree of saturation and cohesion of soil editor@iaeme.com

6 Hadi Mohammed, Mohammed F. Abbas and Ameera Mohamad 4.2. Direct Shear test for (clay- concrete) samples Direct shear tests were conducted on two types of concrete materials that represent two different degrees of roughness according to Tables( 2 )and (3). Through these tests, it is required to determine the shear strength of the clay concrete interface and to determine the location of failure from direct observations (i.e. either soil failure or interface adhesion bonds failure). Split samples are prepared having one half filled with concrete and the other half is filled with soil. The direct shear test results (drained condition) are shown in Tables (5 and 6) which are represented on Fig. (3 and 4). Table 5 The shear strength of the clay concrete interface for C52 surface. S (%) 100% 90% 80% 70% 60% C a (kpa) , δ Figure 7.The relationship between degree of saturation and adhesion of clay concrete interface for C52 surface. Table 6 The shear strength of the clay concrete interface for C49 surface. S (%) 100% 90% 80% 70% 60% C, a (kpa) δ editor@iaeme.com

7 The Ultimate Skin Resistance of Concrete Pile in Partially Saturated Cohesive Soil By Modified Β Method Figure. The relationship between degree of saturation and adhesion of clay concrete interface for C49 surface Calculations of Some Essential Parameters There are some essential parameters and relationships that are required to be used in this study dealing with partially saturated soil characteristics. Some of these parameters are general like the SWCC, and the others are required for the pile bearing capacity calculations, like β and parameters values SWCC Suction Versus Degree of Saturation Matric suction of soil sample was measured by remolding the samples at different degrees of saturation (100%, 90%, 80%, 70% and 60%) using the filter paper method (Whatman No.42). Fig. (5) Shows the relationship between the matric suction and degree of saturation. Figure 5 Relationship between the matric suction and the volumetric water contents using Fredlund and Xing fit to original measured data editor@iaeme.com

8 Hadi Mohammed, Mohammed F. Abbas and Ameera Mohamad Determination of β and values The conventional β method combines the total (i.e., undrained) and effective (i.e., drained) stress approaches for calculating the shaft capacity of piles driven into fine-grained soils. This technique is useful in reducing the sensitivity of the shear strength parameters measured using the total stress approach and effective stress approach. The total shaft capacity is calculated using the relationship shown in equation (1) c a v k u u s tan a w As (1) where β = Burland-Bjerrum coefficient is a coefficient which is equal to K o tan δ, δ = effective angle of friction along the soil/pile interface, As = surface area of the pile, σ v = vertical effective stress at the mid of the pile shaft. The coefficient, β values typically vary from 0.30 to 0.60 for fine and coarse-grained soils [1], [2]. = fitting parameter used for shear strength. The relationship between the fitting parameter, and plasticity index, PI for predicting the shear strength of unsaturated soils [28] can be used for estimating the ultimate shaft capacity of the pile. The fitting parameter, κ = 2 is used for the soil tested in the present study. More details of this method are available in [11] Estimation of the ultimate skin resistance of concrete pile in partially saturated fine grained soil by β method Using the modified β method for estimating the ultimate shaft capacity of single pile in unsaturated drained condition,implementing modified direct shear test for two different concrete pile's surfaces, and different degree of saturation First Concrete type C52 The modified β is proposed in a functional form such that it can be used to predict the variation of shaft capacity of the pile with respect to matric suction under drained loading conditions. Similar to the approach used for saturated soils, the ultimate shaft capacity of a pile is related to the drained shear strength, c, by introducing a dimensionless parameter which is β In other words, the method is based on the effective stress approach. Based on Equation (1) for estimate shaft capacity of concrete pile Calculations for C52 The direct shear test uses C52 concrete material; the results are shown in Table 6 Table 6 Estimated and measured ultimate shaft capacity for C52 surface using the modified β method S u a -u w C a, (sat) / unsat. β, τ f(us) τ f(us) Meas. Est. % kpa kpa ---- kpa kpa editor@iaeme.com

9 The Ultimate Skin Resistance of Concrete Pile in Partially Saturated Cohesive Soil By Modified Β Method Second Concrete type C Calculations for C 49 Results are show in Table 7 Table 7 Comparison between the measured and estimated ultimate shaft capacities for C 49 surface using the modified β method.,, S u a -u w C a (sat) / unsat. β τ f(us) Est. τ f(us) Meas. % kpa kpa ---- kpa kpa DISCUSSION According to the experimental data results obtained from the tests conducted on the samples of soil and a soil concrete combination that describes the interaction between the soil and the pile surface material it can be noted that: 5.1. Variation of the drained shear strength and degree of saturation From Table (4) and fig. (2), it can be noted that there is a variation in the drained shear strength of the soil and the degree of saturation. This relationship may describe the variation of the drained shear strength of the soil and degree of saturation. The increase of the drained shear strength with decreasing degree of saturation was noticed by many investigators [5], and is explained due to the contribution of the matric suction to the shear strength. The increase of drained shear strength with decreasing degree of saturation may be explained as the contribution of matric suction that may increase the effect of adhesion factor due to increasing the confining pressure with depth, hence affecting the angle of friction θ and increasing the factor tan θ. However, since the soil used is a cohesive fine grained soil, the contribution of (tan θ ) is observed to be very small compared to the contribution of the degree of saturation S Modified β method for C49 and C52 The modified β method is used to estimate the shear strength of pile shaft in partially saturated soil, according to equation (1). The comparison between the measured values and the estimated values showed in the Tables (6 and 7). It can be observed generally that the value of C ' is lower at higher degree of saturations and higher at low degree of saturation, in opposite to the value of δ where it could be observed to be low at higher degree of saturation and high at lower degrees of saturation. A transient range can be observed between about S=60% to 70% that the lower values between τ f measured and τ f estimated values. The failure usually occurs when the weaker value of the shear resistance is reached, and since the weaker value changes with the degree of saturation then it is worth to note that the correct value of the shear resistance should be used during design depending on the degree of saturation. Varying degree of saturation during seasons makes it unavoidable for the designer to editor@iaeme.com

10 Hadi Mohammed, Mohammed F. Abbas and Ameera Mohamad establish a relationship between the weaker shear resistance and the degree of saturation for the soil type, and pile surface roughness used for the pile foundation design. In this paper, this method is modified such that it can be used to estimate the variation of ultimate shaft capacity of single piles with respect to matric suction using the SWCC and the classical shear parameters. in this paper show significant increase in shaft capacity due to the contribution of matric suction. REFERENCES [1] Chandler RJ, The shaft friction of piles in cohesive soils in terms of effective stress, Civil Engineering and Public Works Review, vol. 63, 1968, pp [2] Burland JB, Shaft friction of piles in clay-a simple fundamental approach, Ground Engineering, vol. 6-3, 1973, pp [3] Georgiadis, K.; Potts, D. M. & Zdravkovic, L., (2003), ''The influence of partial soil saturation on pile behaviour'' Geotechnique 53, No.1, pp [4] Vanapalli, S.K., Eigenbrod K.D., Taylan Z.N., Catana C., Oh W.T. and Garven, E.,(2010), ''A technique for estimating the shaft resistance of test piles in unsaturated soils'', Fifth International Conference Unsaturated Soils, Barcelona, Spain, Vol. 2,PP [5] Vanapalli S.K. and Taylan Z.N.,(2012),'' Design of Single Piles Using the Mechanics of Unsaturated Soils'', Int. J. of GEOMATE, Vol. 2, No. 1 (Sl. No. 3), pp [6] Uchaipichat, A. (2012)." Variation of Pile Capacity in Unsaturated Clay Layer with Suction". Electronic Journal of Geotechnical Engineering, Vol.17, pp [7] ASTM D3080/D3080M (2012) Standard Test Method for direct shear test under drained and undrained Conditions [8] ASTM-D , Standard Test Method for Measurement of soil potential (Suction) Using Filter Paper", Annual Book of ASTM Standards, Vol , Soil and Rock, pp 1-6. [9] Lemos, L.J., and P.R. Vaughan,(2000), '' Clay-Interface shear resistance'', Géotechnique,Vol.50, No.1, pp [10] Fredlund, D.G., and Rahardjo, H.(1993), "Soil Mechanics for Unsaturated Soils" John Wiley & Sone Inc. New York, United States of America. [11] Vanapalli SK, Eigenbrod KD, Taylan ZN, Catana C, Oh WT, Garven E, A technique for estimating the shaft resistance of test piles in unsaturated soils, in Proc. 5th Int. Conf. on UNSAT, 2010, pp editor@iaeme.com