American Society of Civil Engineers

Ground Modification by Sub-Excavation for Subdivisions on Expansive Soils

by T. Hart, P.E., (Project Manager, CTL | Thompson, Inc. 1971 West 12th Avenue, Denver, CO 80204 E-mail:, R. W. Thompson, P.E., F.ASCE, (Chairman, Emeritus, CTL | Thompson, Inc. 1971 West 12th Avenue, Denver, CO 80204 E-mail:, and R. M. McOmber, P.E., (Chairman and CEO, CTL | Thompson, Inc. 1971 West 12th Avenue, Denver, CO 80204 E-mail:
Section: Solving Complex Geotechnical Challenges, pp. 22-37, (doi:

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Document type: Conference Proceeding Paper
Part of: GEO-Velopment: The Role of Geological and Geotechnical Engineering in New and Redevelopment Projects
Abstract: Deep over-excavation and recompaction at high moisture content is a process currently used to address swelling soil conditions for residential subdivisions and light commercial construction. Sub-excavation to relatively deep depths was initially conceived based on observations of performance of structures on steeply dipping bedrock. CTL | Thompson Inc. first used this approach in 1994/95 to address swelling issues in a large subdivision north of Golden which was underlain by steeply dipping expansive bedrock. The process was then used in other areas underlain by steeply dipping bedrock adjacent to the Front Range. Subsequently, as developers gained confidence and experience, the method spread to areas where the bedrock is relatively flat lying. The paper presents results, in summary form, of experience gained during the last 10 years throughout the Denver metro area, specifically at three sites: one underlain by dipping bedrock and two in flat lying bedrock locations. Graphs of swell potential before and after treatment and comparison of moisture content, dry density and swell potential as affected by sampling procedure are included. Typical quality control issues are identified. The process has been successfully used on several thousand single-family residential lots in the Denver metro area. Data developed over the last 10 years should give confidence to others as they discuss this approach with their clients.

ASCE Subject Headings:
Expansive soils