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Note for Foundation Engineering - FE by Engineering Kings

  • Foundation Engineering - FE
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Downloaded CE 2305 FOUNDATION ENGINEERING (SEMESTER V) UNIT 1 SITE INVESTIGATION AND SOIL EXPLORATION Site investigations and sub-surface explorations are done to obtain the information about subsurface conditions at the site of proposed construction. Information about the surface and subsurface feature is essential for the design of structures and for planning construction techniques. Site investigations consist of determining the profile of the natural soil deposits at the site, taking the soil samples and determining the engineering properties of the soil. It also includes in-situ testing of the soils. OBJECTIVES OF SITE INVESTIGATIONS 1. To select the type and depth of foundation for a given structure 2. To determine the bearing capacity of the soil 3. To determine the probable maximum and differential settlements 4. To establish the ground water table and to determine the properties of water 5. To predict the lateral earth pressure against retaining walls and abutments 6. To select suitable construction techniques 7. To predict and to solve potential foundation problems 8. To ascertain the suitability of the soil as a construction material 9. To investigate the safety of the existing structures and to suggest the remedial measures PLANNING A SUBSURFACE EXPLORATION PROGRAMME A subsurface exploration programme depends upon the type of the structure to be built and upon the variability of the strata at the proposed site. The extent of sub surface exploration is closely related to the relative cost of the investigation and that of the entire project for which is undertaken. In general the more detailed investigations are done, the more is known about the subsurface conditions. As a result a greater economy can be achieved in the construction of the project because the element of uncertainty is considerably reduced. 2 Downloaded

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Downloaded The extentof investigatiql1WQuld also depend upon the location of the "_~. -e'''.-' .".,.-.-.- .. -•.,._. .-_.,..•;.~,.,"- '- .. , --- pr()j~£t. . A small house in an already built up area would not require much exploration on the other hand if the house is to be built in a newly developed area, detailed investigation is necessary. If a multistoried building is to be constructed, extensive sub surface exploration is necessary. These buildings impose very heavy loads and the zone of influence is also very deep. It would be therefore more desirable to invest some amount on sub surface exploration than to overdesign the buildings and make it costlier. STAGES IN SUB SURFACE EXPLORATION (1) Reconnaissance Site reconnaissance is the first step in a subsurface exploration programme. It includes a visit to the site and to study the maps and other relevant details. The informations about the following features are obtained during reconnaissance, • The general topography of the site, the existence of drainage ditches and dumps of debris and sanitary fills. • Existence of settlement cracks in the structure already built near the site. • The evidence of landslides, creep of slopes and shrinkage cracks. • The stratification of soil as observed from deep cuts near the site. • The location of high flood marks on the nearby buildings and bridges. • The depth of ground water table as observed in the wells. • Existence of springs swamps etc. at the site. • The drainage pattern existing at the site. • Type of vegetation existing at the site. • Existence of underground water mains, power conduit etc at site The informations obtained during reconnaissance is helpful in evolving a suitable sub surface investigation programme. (2) Preliminary Explorations The aim of this is to determine the depth, thickness, extent and composition of each soil stratum at the site. The depth of the bed rock and ground water table is also 3 Downloaded

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Downloaded determined. The preliminary explorations are generally in the form of a few borings or test pits. (3) Detailed Explorations The purpose of the detailed exploration is to determine the engineering properties of the soils in different strata. It includes an extensive boring programme, sampling and testing of samples in the laboratory. DEPTH OF EXPLORAnON Depth of exploration required, depends on the type of the proposed structure, its total weight, the size, shape and disposition of the loaded area, the physical properties of the soil that constitute the different strata e site. Exploration, in general, should be carried out to a depth up to which the increase in pressure due to structural loading is likely to cause foundation failure. Such a minimum depth is known as critical depth or significant depth. The net loading intensity at any level below the foundation is obtained by approximately assuming a spread of load of, two vertical to one horizontal, from all sides of the foundation. Due allowance should be made for the overlapping effects of the load from closely spaced footings. It is rally safe to assume the significant depth up to a level at which the net increase in vertical pressure becomes less than 10% of the initial overburden pressure. The following guide rules may also be followed to decide the depth of exploration. For isolated spread footing or raft 1.5 times the width. Adjacent footings with clear spacing less 1.5 times the length. than twice the width For pile foundations 10 to 30 m and more or at least 1.5 times the width of the structure Base of retaining walls One and a half times the base width or the exposed height, whichever is greater. For black cotton areas, from the consideration of weathering. the exploration should be carried to a minimum depth of 4 m. t·· 1Ii ! /) 1, r t' ~ Downloaded t. (~~ I ~ l I, . / (\ 4

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Downloaded METHODS OF SOIL EXPLORATION Mainly there are three methods. They are 1. Direct methods 2. Semi-direct methods 3. Indirect methods (1) Direct methods It includes open excavation methods of exploration. They are, • Pits and Trenches Pits and trenches are excavated at the site to inspect the strata. The size of the pits should be sufficient to provide necessary working space.IS:4453-1967 recommends a clear working space of 1.2m x 102m at the bottom of the pit. The depth of the pit depends upon the requirement of the investigation. Shallow pits up to a depth 3m can be made without providing any lateral support. For deeper pits, especially below water table, the lateral support should be provided in the form of sheeting and bracing system. For depths greater than 6m, bore holesa re economical than pits. Deep pits should be properly ventilated to prevent accumulation of dead air. If water is encountered in the pit, it should be properly dewatered. Trenches are long shallow pits. A trench is continuous over a considerable length; it provides exposure along a line. The trenches are more suitable than pits for exploration on slopes. • Drifts and Shafts Drifts are horizontal tunnels made in the hill side to determine the nature and structure of geological formation. A drift should have minimum clear dimensions of 1.5m width and 2m height in hard rock. Shafts are large size vertical holes made in the geological formation. These may be rectangular or circular in section. The minimum width of rectangular shaft is 204m and for circular, the minimum diameter is 204m. (2) Semi-direct methods When the depth of exploration is large, borings are used for the explorations. A vertical hole is drilled in the ground to get the information about the sub soil strata. Samples are taken from the bore hole and tested in the laboratory. 5 Downloaded ~

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