may require high quality samples to accurately predict the amount and rate of settlement but if the same structure is on soil consisting of pockets and lenses of clay silt and sand information of greater value may be obtained by taking a greater number of less expensive disturbed samples to determine the location and size of these pockets and lenses these views of alluvial sand and gravel deposits are shown to emphasize that even relatively homogeneous deposits can have complex inter layer incurring and heterogeneity on a local scale and appreciation of the in thereat complexity of geological deposits in needed for planning or interpreting subsurface explorations to perform an effective subsurface investigation the project engineer must be familiar with tool and procedures understand the effects that drilling and sampling cam have on lab and field tests and appreciate the inherent uncertainties involved with characterizing subsurface conditions most subsurface investigations are performed using drill rigs numerous types and sizes drill rigs can be classified in to four basic categories highway off road over water and portable drill rigs high way drill rigs the most common are mounted on the back of an industrial truck allowing high mobility and minimal setup time a high way rig is used for sites that are relatively level and easy to access off-road drill rigs are used when access to a site is unusually difficult typically these drill rigs are either mounted on large rubber wield trucks or on tracks if an off-road drill rig cannot directly access the site a helicopter mey be used to move it into position for soil investigations beneath bedies