Geotechnical Site Investigation
Geotechnical site investigation is the systematic process of collecting information about the subsurface soil and rock conditions at a proposed construction site. The investigation program must determine the soil stratification, groundwater conditions, and the engineering properties of the materials that will affect the design and construction of foundations, earthworks, and pavements. The scope of the investigation depends on the size and complexity of the project, the variability of the subsurface conditions, and the risks associated with the anticipated construction. The investigation typically proceeds in phases starting with a desk study of existing information, followed by a field exploration program, laboratory testing, and finally the preparation of a geotechnical report with design recommendations.
The number and depth of borings required for a site investigation depends on the project type, the soil conditions, and the foundation type. A typical investigation for a building foundation includes borings at each corner of the building and at locations of concentrated loads such as columns with heavy loads. The boring depth must extend through all unsuitable materials and into competent bearing strata. The minimum boring depth for spread footings is typically 10 to 20 feet below the footing elevation, depending on the anticipated stress bulb depth. For pile foundations, at least one boring should extend to a depth below the pile tip equal to the pile cap width to investigate the bearing stratum and underlying materials. The spacing between borings typically ranges from 50 to 200 feet depending on the subsurface variability.
Soil sampling during the field exploration program recovers representative samples for laboratory testing and visual classification. Disturbed samples collected from the split-spoon sampler during the Standard Penetration Test provide material for classification tests and moisture content determination. Undisturbed samples collected using thin-walled Shelby tube samplers preserve the natural soil structure and are required for strength and consolidation testing. The quality of undisturbed samples depends on the sampler design, the sampling procedure, and the care taken in handling and transporting the samples to the laboratory. The sample disturbance caused by the sampling process must be minimized to obtain test results that accurately represent the in-situ soil behavior.
Foundation Bearing Capacity
The bearing capacity of a soil determines the maximum pressure that can be applied to the soil without causing shear failure or excessive settlement. The ultimate bearing capacity is the pressure at which the soil fails in shear beneath the foundation. The allowable bearing capacity is the ultimate bearing capacity divided by a factor of safety, typically 2.5 to 3.0 for spread footings. The Terzaghi bearing capacity equation provides the theoretical framework for calculating the ultimate bearing capacity of shallow foundations based on the soil shear strength, the foundation width and depth, and the soil unit weight. standard penetration test procedures for soil investigation. terzaghi bearing capacity equation for shallow foundations. consolidation settlement analysis for clay soils. The equation includes terms for cohesion, surcharge, and soil weight contributions to bearing capacity, with bearing capacity factors that depend on the soil friction angle.
The bearing capacity of granular soils such as sands and gravels is primarily a function of the soil density and the effective stress at the foundation level. Standard Penetration Test blow counts provide a direct correlation with the relative density and bearing capacity of granular soils. The Meyerhof method relates the allowable bearing capacity for a given settlement to the SPT blow count and the foundation width. For a 3-foot wide footing on medium dense sand with an SPT blow count of 20, the allowable bearing capacity for 1 inch of settlement is approximately 4,000 psf. The bearing capacity of cohesive soils such as clays is controlled by the undrained shear strength and is independent of the foundation width for saturated clays under undrained loading conditions.
The effect of groundwater on bearing capacity must be considered when the water table is within the influence zone beneath the foundation. The effective unit weight of the soil below the water table is reduced by buoyancy, decreasing the bearing capacity. The factor of safety against bearing capacity failure must be evaluated for the worst-case groundwater conditions that could occur during the life of the structure. The eccentricity of the applied load from columns subjected to overturning moments reduces the effective bearing area and must be considered in the bearing capacity analysis. The footing dimensions must be selected so that the maximum edge pressure under the combined vertical load and moment does not exceed the allowable bearing capacity.
Settlement Analysis
Settlement analysis predicts the total and differential movements of foundations under the applied loads. The total settlement of a foundation on granular soils occurs primarily during construction as the sand particles densify under the applied load. The Schmertmann method calculates the settlement of footings on granular soils using strain influence factors that account for the vertical strain distribution beneath the footing. The method requires the soil modulus values determined from SPT blow counts or cone penetration test data. The settlement calculated using the Schmertmann method is typically within 25 percent of the measured settlement for well-characterized sites.
The consolidation settlement of clay soils occurs over time as pore water is squeezed out of the soil under the increased stress from the foundation load. The rate of consolidation depends on the coefficient of consolidation of the clay and the drainage path length. The total consolidation settlement is calculated from the compression index, the initial void ratio, and the stress increase in the clay layer. The compression index characterizes the compressibility of the clay and is determined from the results of one-dimensional consolidation tests on undisturbed samples. The stress increase from the foundation load at the mid-depth of each clay sublayer is calculated using elastic theory or the 2:1 stress distribution method.
Differential settlement between adjacent foundations can cause structural damage even when the total settlement is within acceptable limits. The differential settlement is influenced by variations in soil conditions across the site, differences in foundation loading, and differences in foundation size and depth. The angular distortion defined as the differential settlement divided by the distance between foundations is the most commonly used criterion for evaluating the potential for structural damage. Framed buildings with continuous footings can typically tolerate angular distortions up to 1/300 without significant cracking. Buildings with sensitive finishes or rigid structural systems may require angular distortion limits as low as 1/1000.
Geotechnical Report Preparation
The geotechnical report communicates the findings of the site investigation and provides recommendations for foundation design and construction. The report must present the subsurface conditions in a clear and understandable format, including soil profiles showing the stratification and groundwater conditions at each boring location. The laboratory test results should be presented in tables and graphs that support the design recommendations. The report should discuss the engineering properties of each soil stratum including the strength, compressibility, and permeability characteristics that affect the foundation design. The variability of the subsurface conditions across the site should be discussed, and the limitations of the investigation should be clearly stated.