Geotechnical Desk Study-Foundation - Report Example

Geotechnical desk study-Foundation Geotechnical desk study-Foundation Introduction The part of any structure that is responsible fortransmitting load directly to the ground is referred to as a foundation. The appropriate design of a foundation is fundamental since it determines the stability of any given structure. The stability of a foundation has been known to greatly depend on the underlying soil. There are several parameters used to describe the stability of the foundation of a structure basing on the soil onto which the foundation is it be build. These parameters include the basic soil properties, net bearing capacity of the soil, the bearing capacity of the soil, and the lateral soil properties. There are two distinct class of foundation, a shallow foundation and a deep foundation. A shallow foundation is built on a soil stratum located near the surface of the earth and has the capability of supporting the structural loads of the structure adequately. The foundation can either be a raft foundation or a footing foundation. The design of any foundation should be such that it is located within an adequate load bearing stratum and also, it should designed such that it is within a depth that is below the depth of the frost action (which is approximately 500 cm in the United Kingdom). The design of foundation follows two approaches, the permissible stress method that is described in BS8008:1986. The method aims at making sure that the load being applied to the foundation has been reduced such that it is significantly less than the design value the foundation can support. This is achieved by the application of a lumped factor of safety (Craig, 2004). Another approach used in the design of foundation is the limit state method. This method is described in Euro code 7. In this method, all the performance requirements of a given foundation must be able to satisfy all possible circumstances that may be encountered by the foundation. Foundation of a structure is faced by several damages. Settlement of the foundation is one of the major damage experienced on many foundation. These settlement can either be structural, functional or architectural. There are various classes of the settlement of the foundation which include the immediate settlement that occurs just after the construction process of a building has is completed, primary consolidation that is attributed to the gradual dissipation of pore pressure that is caused by external loading which results in the removal of water from the soil thus altering the volume of the soil. And finally, there is secondary consolidation that takes place as a result of constant volume and effective stress change. This report is aimed at the analysis of the geotechnical requirements of a foundation to determine the likely geotechnical parameters that may be encountered in the study area. After which the design of one aspect of the foundation relevant to the project is done. The location of the project is along Ripley road near St Mary’s mission church. In the study, there is the description of the study conditions, which is accompanied by the related sub-surface conditions. The report is structured into three parts. Settlement of the foundation, lateral earth pressure acting on the foundation and the risk of the bearing capacity failure of the foundation or the retaining structure. All the analysis in the report is done using the recommended and accepted standard codes, practices and criteria used in the design and construction of foundation process. Settlement foundation The total settlement of a foundation is the sum of the immediate settlement, primary settlement and secondary settlement. The calculation of the settlement of any given structure is based on the Terzaghi’s theory of consolidation. In this determination, the resultant compression due to the building load is determined by the consolidation curves obtained from the laboratory. The settlement of a building is dependent on the consolidation’s time and rate and this also depends on the rate of water expulsion from the voids in the soil. The rate of settlement is dependent on the type of soil, and this properties being relatively fast in sands and slow in clays. Determination of the soil consolidation is paramount since is the major determinant of the primary settlement of a foundation. There are various ways that are used in the determination of the soil consolidation. These consolidation tests are either field consolidation tests or laboratory consolidation tests. Consolidation is simply defined as the gradual decrease in volume of a saturated soils as a result of water expulsion from the pores. As it has been stated earlier, consolidation is the major cause of consolidation settlement. The consolidation can be determined by the installation piezometers that aid in the measuring and recording pore water pressure change with time. Consolidation Settlement In the determination of one dimensional consolidation, the odometer test is carried out. The test is done using the standardized procedure found in BS 1377. The Standards specifies fixed ring type of the odometer. Using the results of the odometer test, consolidation settlement is determined using the following equation Where Sc represents the consolidation settlement, C is the compression index, thickness of the soil layer, e0 is the initial void ration represents the initial over burden pressure and represents the extra pressure resulting from the new construction. The consolidation is carried out at specific times and this time is calculated by Where it represents the time taken and d represents the drainage pattern. Immediate settlement This is also referred to as the elastic settlement and in its determination, the elastic theory is utilized. The occurrence of immediate settlement is not soil type specific like the consolidation settlement but occurs in all types of soils. The elastic settlement occurs immediately the load is applied on the foundation and it greatly depends on the shape of the foundation, rigidity of the foundation, and the size of the foundation. To calculate immediate settlement, the following equation is used. Where Ip is the influence factor, B is the foundation’s width, is the foundation’s poisons ratio, Si is the immediate settlement, q is the contact pressure at the base of the foundation, and E is the Young’s modulus of elasticity. There are predetermined values for the modulus of elasticity, influence factors. Secondary Compression The initial stage of secondary settlement occurs after the completion of primary settlement. There is zero excess pore water pressure. The settlement occurs in loose sands, organic soils and very sensitive clays. The equation below is used in the calculation of secondary settlement. Lateral earth pressure acting on a retaining wall The design of such structures must be in such that it can withstands external soil pressures. Retaining walls can be described as structures that are built specifically to support backfill and concurrently allowing change of grade. There are various kinds of retaining walls. They can either be semi-gravity type structures, gravity structures, counterfort type structures, gravity type structures etc. Lateral earth pressures re divided into three categories. This are active earth pressure, at rest earth pressure, and passive earth pressure. When the retaining wall is not experiencing any load, it has at rest earth pressure... The resultant of third is due to the wall been exposed to movement restriction. On the other, the active lateral earth pressure is a resultant of the outwardly free movement of the retaining wall. This is to mobilize the retaini8ng walls’ shear strength. Finally, the passive earth pressure develops as a result of the wall moving into the soil. The illustration of the active earth pressure, passive earth pressure and at rest earth pressure are shown in the figure below. Calculation of the coefficients of earth pressures. At rest coefficient At rest coefficient is depended on the condition of the soil. The at rest coefficient is calculated using the equation below. Passive and active earth pressure coefficient. To determine these, several theories are determined. The most used theories are coulomb earth pressure theory and rankine earth pressure theory. In using the rankine earth pressure, it is assumed that there does not exist friction or adhesion between the walls, the lateral pressure is exclusively limited to vertical walls among other assumptions. The Rankine active and passive pressure is calculated using the equations below; The above values have predetermined values are as shown; Design of a foundation in the project area Generalized steps followed in the design of the foundation are as highlighted below. The first step involved the calculation of the passive pressures, surcharge pressures, active pressures and loads of the structure to be built on the structure. The second step involves the classification of the soil to determine the existing soil properties. This is then followed by the determination of the foundation’s location and depth. The soil bearing capacity is then evaluated and the while considering the soil bearing factor. The foundation size is then determined basing on the loads and the allowable soil pressure. The contact pressure is then calculated and the then the stability of the foundation checked. The calculation of the contact pressure is then followed by the estimating the settlements and the design of the foundation basing on the applicable structural design codes. Loads and stresses The bearing load conspired in this case is distributed to the soil material but since its variable, the resultant stress isn’t unified. Thus, it is assumed to be uniform in the design, Design stresses The underside pressure on the foundation is assumed to be uniformly distributed. The foundation is designed such that it is able to withstand one – way shear, two – way shear and bending. Foundation Design Consider a square foundation to support a18 inch square column with tied interior column that is reinforced with 8 bars. The column is to carry a factored axial dead load and axial live load of 245K and 200K respectively. The foundation base footing is 4 feet below the final grade. The allowable soil pressure is 5K/ft2. Assume a foundation depth of 2 ft. with the values fee = 60Ksi and fc = 60Ksi.The weight of soil and concrete are given as: Effective soil pressure is given by: Calculation of foundation sizing: Therefore, the size of foundation foot is taken as 10 feet. Calculation of the net upward pressure: Hence, the net upward pressure = = 6.83K/ ft2 Calculation of reinforcement depth.( 8 bars with cris-crossing layer) Calculation of the parameter for punch out shear. ( 18 inc square column) References Craig, R. F. (2004). Craig’s Soil Mechanics. New York: E & FN Spon. Read More