Reading Assignment: Chapter 3 Section 03- Subsurface Exploration Know about the material below the ground surface before constructing buildings on it! Know before you trust??? 3.1 Introduction For practical design problems, structural engineers normally find the necessary material properties referring to handbooks/manufacturer’s data sheet etc. Example: For A36 steel: Compressive strength = 250 MPa (yield strength); Young’s modulus = 200 GPa Conversely, geotechnical engineers deal with natural materials (various types of rocks and soils) whose engineering properties are heterogeneous and unknown. Therefore, geotechnical engineer has to identify and test the materials (subsurface exploration) at each project site before designing foundations/structures Subsurface exploration Subsurface exploration is the process of identifying the soil deposits that underlie the proposed structure and their physical characteristics Cont. Sample soil profile from site investigation for a bridge construction (Organic clay/silt) (Clayey sand) (Silty sand) (Well graded sand) (Poorly graded gravel) (Well graded gravel) Purpose of Subsurface Exploration The purpose of subsurface exploration is to obtain information that will aid the geotechnical engineer in Selecting foundation type and depth for a given structure Evaluating load-bearing capacity of the foundation Estimating probable settlement of a structure Determining potential foundation problems (for example, expansive soil, collapsible soil, sanitary landfill, and so on) Determining the location of the ground water table Predicting lateral earth pressure for structures like retaining walls, sheet pile bulkheads, and braced cuts Establishing construction methods for changing subsoil conditions Factors that Affect the Scope of Subsurface Exploration Type, size and importance of the structure Structures that are sensitive to settlement such as machine foundations and high-use buildings require thorough soils investigation compared to foundation for a house Client 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 Engineer’s familiarity with the soils at the site If the geotechnical engineer is familiar with a site, he/she may undertake a very simple soils investigation to conform his/her experience Local building codes Some building codes have provisions that set out the extent of a site investigation Cost-Effectiveness of Extensive Site Characterization Programs Extensive investigation and testing program: In- situ testing and sampling at many locations and depths Gives better/reliable subsurface information Subsurface Exploration Program Subsurface exploration comprises several steps, including the collection of preliminary information, reconnaissance, and site investigation. Collection of preliminary information Available information related to the proposed site and project is collected before starting any work. This may include: Topographical maps Areal photographs Geologic maps Type of structure, structure layout, importance of the structure and the magnitude of the loads Previous geotechnical reports around the proposed project site Site plan and existing structures adjacent to the proposed project Cont. Reconnaissance or site visit Reconnaissance provides general picture of the topography and geology of the site. It is important to take the collected preliminary information to the field visit. The field reconnaissance may give answers to: Offsite conditions and their impact Any evidence of previous development in the site Any evidence of previous landslides or other stability problems Performance of nearby structures Surface drainage conditions and high-water marks on nearby buildings Soil and rock type at the surface Accessibility of the site for large equipment Possibility of impacting existing structures due to the proposed construction Cont. Site investigation The objectives of site investigation are to determine the geological structure, which should include the thickness, sequence and the extent of the soil strata determine the ground water conditions obtain soil samples (disturbed and/or undisturbed) at desired intervals for subsequent observation and laboratory tests conduct in-situ tests Note: Number of borings and depths are predetermined but may vary during drilling operating depending on the subsoil encountered. Read HO#01 .