In order to design a building, pavement, or other structure, the capability of the soil or rock lying below the ground surface to support the structure needs to be determined or assumed.
A geotechnical engineer (soil engineer, soils engineer) commonly provides this service unless the design engineer chooses to make an assumption regarding this support. This service is sometimes referred to as soil borings, soil cores, or soil bores, but terms commonly used by geotechnical engineers to identify this service are geotechnical investigation, soils investigation, soil exploration, and geotechnical exploration.
Most of the time, the purpose of the geotechnical exploration is to provide information about the soil, rock, and groundwater that lie below the ground surface for the use of the team that is designing a project. A geotechnical exploration commonly involves drilling vertical holes, or borings, into the ground, followed by testing of soil or rock samples collected from the borings. It normally concludes with a report containing information obtained from the geotechnical exploration along with geotechnical recommendations.
The geotechnical exploration should be performed early in the design stage of the project. It should not be confused with construction testing and observation (construction "inspection"), which is performed during construction. Also, it should not be confused with an exploration to obtain information related to environmental contamination, such as that for a Phase II environmental site assessment.
The drilling is commonly performed by either a truck-mounted drill rig or a drill rig mounted on an all-terrain vehicle, depending on the site conditions. The drill crew usually consists of the driller and a helper. Drilling in soil and softer rock is frequently performed using a continuous flight auger, which is a drill having an auger, assembled in sections, extending the full depth of the bore hole. The auger is extended into the ground by the drill rig, creating a hole that is commonly about one-half foot in diameter. Samples of the soil or softer rock are usually obtained from the hole at selected depths using samplers that are either driven into the bottom of the hole or pushed into the bottom of the hole by the drill rig, prior to extending the hole deeper. Other sizes and types of drilling and sampling equipment are available, such as smaller rigs for confined spaces and different samplers and drilling equipment for various soil and rock conditions.
Samples are usually saved in appropriate containers and taken to a lab or office where they are studied and where specimens are selected for further testing. There are a number of tests that can be performed on the specimens to obtain additional information. Tests to assist in evaluating the strength and compressibility of soils are performed most often.
The geotechnical engineer then considers the site and project information, the results of the drilling and laboratory testing, and other pertinent information obtained during the geotechnical exploration and performs any calculations necessary to form geotechnical recommendations. A report is then prepared which may include a description of the site conditions, a description of the proposed project, a description of the field and laboratory testing procedures, a summary of the subsurface conditions encountered, and geotechnical recommendations related to the foundations and earthwork (for foundation, floor slab, and pavement areas). Recommendations frequently include suggested foundation type, depth, and allowable bearing pressure, if the encountered conditions indicate that conventional shallow foundations are appropriate.
Boring spacings may be in the range of 100 to 200 ft for buildings, and boring spacings for pavements may be somewhat larger. Smaller spacings may be used for smaller projects or on projects where more variability is anticipated in the subsurface conditions. Depths may be a minimum of 5 to 10 ft for pavements and 15 ft for buildings and other structures. However, these are minimums and boring depths may be significantly deeper depending on a number of factors, such as the elevation of the lowest floor (is a basement planned?) or lowest cut grade, the depth and lateral extent of planned fills, maximum foundation loads, and the depth to suitable soil or rock. For example, the boring depths for a single-family home with no basement may be about 15 ft, those for a three-story office building with a basement may be 30 ft, and those for a coal-fired power plant on softer soils may range up to 100 ft or more.
The information contained in this article is intended to give the reader an introductory exposure to the geotechnical exploration. This is not intended, in any way, to provide guidelines for the development of the scope of a geotechnical exploration. The scope of the geotechnical exploration for a specific project should be developed by an experienced geotechnical engineer, preferably the one performing the geotechnical exploration. The details of a geotechnical exploration will vary for each set of project and site conditions. In addition, it is likely that each geotechnical engineer will have his or her own unique way of approaching the geotechnical exploration for a specific project, and the reader may find a significant variation between scopes proposed by different geotechnical engineers for the same project.
The author, Jerry B. Givens, P.E., is the owner of Givens Geotech. This article, with minor edits, was first published in Properties Magazine (June 2005 and May 2008). Other articles by this author related to geotechnical exploration (and geotechnical engineering) appeared in the May 2009 (building on fill) and May 2010 (settlement) issues. Go to www.propertiesmag.com to view current and past issues of the magazine.