Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
Subsurface Exploration Techniques
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- Figure 3-1: Schematic of various in-situ tests (Mayne et al. 2002).
| 3.3.2 Subsurface Exploration Techniques Several in-situ techniques, including the standard penetration test (SPT) and the cone penetration test (CPT) are routinely used infield investigations to establish the stratigraphy at a site and obtain geotechnical parameters. Other techniques are also used including the pressuremeter test (PMT), the vane shear test (VST), and the dilatometer Marchetti test (DMT). These various types of in-situ tests are illustrated in Figure 3-1. Details regarding the specifics of the investigation types are presented in Mayne et al., 2002. 35 Figure 3-1: Schematic of various in-situ tests (Mayne et al. 2002). The use of geophysics methods is becoming more common in site investigations. However, due to the need for samples for soil and rock classification and laboratory testing, it is recommended that techniques that provide soil or rock specimens always be used whenever a geophysical investigation is performed. Geophysics can be used in preliminary or planning investigations, or for investigating specific issues for foundation design such as karst features or areas with highly variable rock surface. These methods maybe beneficial for general site characterization and planning subsurface investigations, but are generally of limited value for development of design parameters for this application. For soils, soil borings and SPT tests, or similar drive tests appropriate for site soils, should always be conducted on projects involving deep foundations in order to retrieve samples for proper subsurface characterization and laboratory testing. Similarly, where deep foundations are expected to bear on or in rock, borings with rock coring should always be performed. Additional explorations methods, such as CPT and DMT, maybe useful and maybe more economical by allowing a reduction in the number of exploratory borings, and may provide data that SPT and rock coring do not. However, the CPT and DMT do not recover soil samples (and do not penetrate rock, but could be used in conjunction with SPT borings in order to correlate the in-situ testing results with actual soil classification and laboratory test data, as well as with SPT N-values. Some types of in-situ tests, such as the PMT and VST, can be performed within an SPT boring if the casing size is large enough to accommodate the testing device. This may offer opportunities for combined types of data acquisition (recovered samples and in-situ test measurements) at a single exploration point. |