Assessment of load-carrying capacity of bored pile in clay soil using different methods

It is very difficult to predict the load carrying capacity of bored piles because of the Complications that may arise such as difficult ground conditions , presence of ground water, method of boring, method of concreting, quality of concrete, expertise of the construction staff, the ground conditions and the pile geometry. Therefore the Pile design must be accompanied by in situ load testing. Many geotechnical codes emphasizes that pile design must be based on static load tests or on calculations that have been validated by three tests. This paper describe the simulation of four piles with different lengths carrying different loads embedded in the clay soil in Khartoum townSudan using BS 8004, Monte Carlo simulation using Matlab software and Finite Element Code -Plaxis software. The results from these methods were compared together and recommendation are made to estimate the pile capacity in these soils. The design parameters of pile are estimated, and back calculation of safety factors are made. Keyword: BS code, Monte Carlo simulation, Finite element method Introduction Bored pile is type of reinforced concrete pile which is used to support high building that has heavy vertical load. Normally bored piling has be to carried on those tall buildings or massive industrial complexes, which require foundations which can bear the load of thousands of tons, most probably in unstable or difficult soil conditions. Bored piles resist the uplift load by skin friction forces, and the formulae used to determine the magnitude of these forces seems to be an area where various codes and standards produce completely different results, the German code DIN 1054 (DIN 1996), which is widely used in Germany and other European countries, the uplift capacities strongly depends on the strength of the soil, i.e., the angle of friction normally depend determined by indirect methods such as standard penetration tests (Krabbenhoft et al. 2008); whereas the methods proposed by Fleming et al.(1992), mainly used in UK, take strength of the soil into account. whereas the method proposed by Reese and O'Neill(1994), mainly used in USA, almost ignore the strength of the soil as long as the soil can be characterized as being frictional soil (Krabbenhoft. 2008). Geological setting The study area is the part of Khartoum basin, which is the one of the major central Sudan rift basin. The sedimentary sub-basin is elongated in NW_SE trend, where the Pan-African Basement complex bounds it on the northeast and southwest, and forms its bottom limit at a depth 500m. The subsurface geology belongs to three Formations, which are regionally interconnected. These Formations are the (upper recent) superficial deposits and river alluvium, which rest unconformable on the Gezira Formation (quaternary-tertiary) and the upper part of Omdurman Formation (upper cretaceous) (Awad , 1994). Most of the surface is cover by clay soils, which varies in thickness. consists of unconsolidated clay, silts, sand and gravel. Its rests unconformable on the Cretaceous sandstone formation and is overlain by blown sand and other superficial. Awad (1994) suggested that it comprise the area between white and blue Nile. Abdelsalam (1966) divided Gezira formation into three members lower Mungata Member, Lower sandy Member and upper clay Member. Awad (1994) considered Wad Medani Member as part of Gezira formation. Engineering properties of subsurface soil The boreholes revealed existence of alternating layers of very stiff low to high plasticity silty clays (CL to CH) and very stiff low to high plasticity silts (ML to MH) in the upper 10 meters. This is underlain by medium dense sand ( SM or SP-SM) layer extended down to 20 meter and this layer overlain a very dense sand layer extend 25 depth . The alternating layers of weak mud-stone and weak sand stone extended down to the bottom of the boreholes at about 35 meters. These weak mud-stone and weak sandstone are belong to Omdurman formation which are extended to deepest depth. Data from the various exploration methods were used as a basis for typical sections to illustrate the more significant geological conditions. The objectives is to illustrate clearly the problems of the geologic environment influencing design and Abdelazim Makki Ibrahim, Ibrahim Malik, Omar Ataj Omar / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 4, July-August 2012, pp.1243-1253 1244 | P a g e construction. Three dimensional sections and fence diagram were also plotted to help for sites with complex geology, Fig 1. Fig 1 Three dimension and fence diagram models for the subsurface soil in the study area Axial response of deep foundation The axial load-displacement response of driven piles and drilled shafts may be expressed in terms of elastic continuum theory. Solutions have been developed using boundary element formulations (Poulos and Davis, 1980; Poulos, 1994), finite elements (Jardine et al. 1986), and approximate closed form solutions by Randolph ( 1979). The generalized method characterizes the soil by two elastic parameters: soil modulus (Es) and Poisson’s ratio (υs). Soil modulus may be either uniform with depth (constant Es) or a Gibson-profile (linearly increasing Es with depth). The pile may either be a floating-type or end-bearing type where the tip is underlain by a stratum of stiffer material. The elastic theory solution for the vertical displacement (δ) of a pile foundation subjected to axial compression loading is expressed by Pile axial settlement δ = Q ∗ Ip /EsL ∗ d where Q = applied axial load at the top of the shaft, EsL = soil modulus at the top the pile tip or foundation base, d = foundation diameter, and Ip = influence factor. Solution for Ip depend on the pile slenderness ratio (L/d), pile modulus, and soil modulus (Randolph & Wroth, 1979; Poulos & Davis, 1980; Poulos, 1989). The modified form of the expression to account for nonlinear modulus degradation is: δ = Q ∗ Ip /Emax ∗ d ∗ (1 − Q/Qu)0.3 Settlement of a single pile The settlement of a single rigid pile in homogeneous elastic half space can be determined based on theory of elasticity (Randolph & Worth 1978) (in(El-Mossallary & Lutz 2006) as follows: Q/ G r0 S single = 4/ E(1 − υ) + 2π L/ξ ro (8. 30) ξ = li(rm/ro) where: Q = Applied load S single= Settlement of single pile G = shear modulus of the soil r0 = pile radius L = length of the pile rm = influence radius at which shear stresses become negligible. Randolph (1977) has suggested r0 = 2.5 L(1-υ) based on a parametric study using an axi-symmetrical finite element analysis. The above-mentioned equation may be modified to consider approximately the pile stiffens, the soil in homogeneity in vertical direction , the thickness of the compressible layer (El-Mossallamy et. al 2006) and nonlinear soil stress/strain behavior adjacent to the pile shaft (Randolph 1977 and Randolph & Worth 1978). Another possibility to determine the settlement of a single pile is to use the chart of Pouls (Poulos & Davis 1980) or to apply the recommendation values as give by standards (DIN 1054-100) Pile Design Parameters Abdelazim Makki Ibrahim, Ibrahim Malik, Omar Ataj Omar / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 4, July-August 2012, pp.1243-1253 1245 | P a g e Fieldwork and laboratory tests have been carried out during the geotechnical investigations to determine pile parameters and to obtained a full overview of subsurface soils to minimize uncertainties. Determination of undrained cohesion was carried out by unconfined compression tests on undisturbed samples, Figure 2 shows the variation of undrained cohesion (Cu) with no trend with depths, This large scatter of the undrained cohesion values at different depths is due to the variation of seasonal depositional materials. Adhesion factors were calculated according to different sources. The data used to calculate soil parameters and bearing capacities are shown in Table 1. Table 1 data used in calculations Depth (meter) Dry density (kN/m3) Friction angle (φ) degree Cohesion (C) kN/m2 Undrained shear strength Cu(kN/m2) 2 12.56 3 44.9 46.216