Chapter -1 SUBSURFACE EXPLORATION 1.1 Introduction Earthwork forms the largest activity of a Civil Engineer. It is well understood that irrespective of the type of civil engineering structure on earth – • It has to be rested either in soil (e.g., foundations) • Rested on soil (e.g., pavements) or • The structure is itself constructed making use of soil (e.g., Earthen dams). This implies that a better knowledge of the spatial variation of the soils encountered is essential. Therefore, before construction of any civil engineering work a thorough investigation of the site is essential. Site investigations constitute an essential and important engineering program which, while guiding in assessing the general suitability of the site for the proposed works, enables the engineer to prepare an adequate and economic design and to foresee and provide against difficulties that may arise during the construction phase. Site investigations are equally necessary in reporting upon the safety or causes of failures of existing works or in examining the suitability and availability of construction materials. Site investigation refers to the methodology of determining surface and subsurface features of the proposed area. Information on surface conditions is necessary for planning the accessibility of site, for deciding the disposal of removed material (particularly in urban areas), for removal of surface water in water logged areas, for movement of construction equipment, and other factors that could affect construction procedures. Information on subsurface conditions is more critical requirement in planning and designing the foundations of structures, dewatering systems, shoring or bracing of excavations, the materials of construction and site improvement methods.
1.2 Soil Exploration The knowledge of subsoil conditions at a site is a prerequisite for safe and economical design of substructure elements. The field and laboratory studies carried out for obtaining the necessary information about the surface and subsurface features of the proposed area including the position of the ground water table, are termed as soil exploration or site investigation. 1.3 Objectives of soil exploration program The information from soil investigations will enable a Civil engineer to plan, decide, design, and execute a construction project. Soil investigations are done to obtain the information that is useful for one or more of the following purposes. 1. To know the geological condition of rock and soil formation. 2. To establish the groundwater levels and determine the properties of water. 3. To select the type and depth of foundation for proposed structure 4. To determine the bearing capacity of the site. 5. To estimate the probable maximum and differential settlements. 6. To predict the lateral earth pressure against retaining walls and abutments. 7. To select suitable construction techniques 8. To predict and to solve potential foundation problems 9. To ascertain the suitability of the soil as a construction material. 10. To determine soil properties required for design 11. Establish procedures for soil improvement to suit design purpose 12. To investigate the safety of existing structures and to suggest the remedial measures. 13. To observe the soil the soil performance after construction. 14. To locate suitable transportation routes. The objectives of soil investigations from various requirements point of view is summarized in Table 1.1
Table 1.1 Objectives of soil investigations Design requirements • define stratigraphy/geology • to determine soil properties required for design • aid material selection • to determine the type and depth of foundation Construction requirements • to select suitable construction techniques • define equipment and techniques needed Auditing • to locate suitable transportation routes • checking a site prior to sale/purchase • to establish procedures for soil improvement to suit design purpose Monitoring • to observe the soil performance after construction • determine reasons for poor behaviour • document performance for future reference 1.4 Scope of soil investigation The scope of a soils investigation depends on the type, size, and importance of the structure, the client, the engineer's familiarity with the soils at the site, and local building codes. Structures that are sensitive to settlement such as machine foundations and high-use buildings usually require a thorough soils investigation compared to a foundation for a house. A client may wish to take a greater risk than normal to save money and set limits on the type and extent of the site investigation. If the geotechnical engineer is familiar with a site, he/she may undertake a very simple soils investigation to confirm his/her experience. Some local
building codes have provisions that set out the extent of a site investigation. It is mandatory that a visit be made to the proposed site. In the early stages of a project, the available information is often inadequate to allow a detailed plan to be made. A site investigation must be developed in phases. 1.5 Phases of a Soils Investigation The soil investigation is conducted in phases. Each preceding phase affects the extent of the next phase. The various phases of a soil investigation are given below: Phase I. Collection of available information such as a site plan, type, size, and importance of the structure, loading conditions, previous geotechnical reports, topographic maps, air photographs, geologic maps, hydrological information and newspaper clippings. Phase II. Preliminary reconnaissance or a site visit to provide a general picture of the topography and geology of the site. It is necessary that you take with you on the site visit all the information gathered in Phase I to compare with the current conditions of the site. Here visual inspection is done to gather information on topography, soil stratification, vegetation, water marks, ground water level, and type of construction nearby. Phase II. Detailed soils exploration. Here we make a detailed planning for soil exploration in the form trial pits or borings, their spacing and depth. Accordingly, the soil exploration is carried out. The details of the soils encountered, the type of field tests adopted and the type of sampling done, presence of water table if met with are recorded in the form of bore log. The soil samples are properly labeled and sent to laboratory for evaluation of their physical and engineering properties. Phase IV. Write a report. The report must contain a clear description of the soils at the site, methods of exploration, soil profile, test methods and results, and the location of the groundwater. This should include information and/or explanations of any unusual soil, waterbearing stratum, and soil and groundwater condition that may be troublesome during construction.