Geotechnical: Subsurface Exploration – Testing – Sampling

The properties of most construction materials (e.g. steel, concrete, wood, etc.) are consistent & can be readily defined, specified, & tested. The properties of soil & rock are highly variable & require site specific sampling & testing involving extremely limited data typically accounting for less than 1 part per million of the subsurface materials.  The sampling & testing results can be misleading without proper interpretation. Tight budgets often provide only limited sampling & laboratory testing for many projects. Inadequate or incorrect information may not disclose important subsurface conditions. Unexpected subsurface conditions often lead to site preparation & foundation construction problems, delayed construction schedules, & extra costs. Accordingly, an exploration of the subsurface conditions is necessary to characterize the arrangement & relevant physical properties of the soil & rock strata underling a site. The services of a qualified geotechnical engineer who will effectively plan & execute a cost-effective subsurface exploration are essential for every project. The cost of a well-planned & executed subsurface exploration, sampling & testing program from GEM is an investment in project performance.

A great deal can be learned by researching existing data. Valuable information includes: air photographs, topographic and geologic maps, soil surveys, prior boring and test data. This information aids formulation of a preliminary subsurface model used when planning the site-specific subsurface exploration.

Subsurface conditions can impact topography, vegetation, & other surface features. A site reconnaissance is essential for developing a more complete view of the subsurface conditions. Depressions may indicate sinkholes. Leaning trees may indicate slope movements. Lush vegetation in an otherwise sparsely overgrown area may indicate a hidden spring. Lack of vegetation may indicate past filling. Rock outcrops may be related to erosion or past excavation of soil. The knowledge, experience & observation skills of GEM personnel allow us to identify surface features that may be related to the underlying materials and conditions.

The most widely used form of subsurface exploration involves drilling holes, extracting samples for identification and laboratory testing, and conducting tests in the borehole. Drilling methods include: hand auger borings, augered soil borings with powered drilling equipment; and rotary core drilling. Backhoe excavations are frequently utilized to evaluate the near surface soil conditions. Sampling methods include: auger cuttings, driven tubes, pushed tubes, and cores. Field testing primarily involves a variety of soil penetration devices including the Standard Penetration Test split-spoon sampler, and the Dutch cone penetrometer. Field permeability tests are used to evaluate the flow rate of water through the soil or rock under actual site conditions. Preliminary groundwater levels are measured in the open boreholes. More accurate and long term groundwater monitoring requires piezometers or slotted well casing installed in the boreholes. Geophysical methods are used to indirectly gauge soil properties using sophisticated technology including: subsurface radar, seismic refraction, resistivity, micro-gravity, radiation, and others. Instrumentation can be installed to monitor subsurface behavior including: slope movement, settlement, subsidence, groundwater levels.