This manual is the reference text used for the FHWA NHI course No. 132031 on Subsurface Investigations and reflects current practice for such. The planning, execution, and interpretation of geotechnical site explorations in natural soil and rock are presented with regard to the design and construction of transportation facilities. The role of the geotechnical engineer in subsurface investigation, exploration methods, equipment types and their suitability are discussed. suitability are discussed. Various in-situ tests are presented, including cone penetration, dilatometer, pressuremeter, vane, and standard penetration. Rotary drilling and rock coring are reviewed in terms of the proper handling, transportation, and storage of soil and rock samples for laboratory testing. Geophysical wave and electromagnetic methods are covered. Laboratory index, strength, and stiffness testing are reviewed in complement to the field testing program. Geomaterial characterization requires the interpretation and correlation of engineering properties from the acquired field and lab measurements.