OF-02-08 Soil and Bedrock Conditions and Construction Considerations, North-Central Douglas County, Colorado


The purpose of this report is to describe and characterize critical construction-related problems that occur in soil and bedrock in the north-central part of Douglas County, and to evaluate currently used site-assessment and mitigation methods. Contents include an introduction, purpose and objectives, location, geology, methodology, engineering classifications and tests, map investigation results, statistical investigation results, discussion of results, recommendations for land use, conclusions, and references. 25 pages. 1 figure. 13 tables. 3 plates (1:24,000). Digital PDF download. OF-02-08D

From the Author’s Notes:

Douglas County has been one of the fastest growing counties in the United States in the 1990s and early 2000s. Much of the growth has occurred in the north central part of the county, on the fringe of the Denver Metropolitan Area. The unincorporated Highlands Ranch subdivision is the major growth center in this area. Additional population growth has occurred in newer subdivisions constructed in or near the towns of Lone Tree and Parker.

This area has been the scene of widespread and, sometimes severe damage to residences, commercial and government buildings, and county-owned roads. The damage may be largely attributed to two geologic phenomena: swelling soil and bedrock and collapsible soil. Since 1990, several class-action lawsuits have been filed over swelling-soil damage in the metropolitan Denver area, including several significant lawsuits in north-central Douglas County. These lawsuits pitted more than fifteen thousand homeowners versus the builders of those houses, and the resulting judgements were decided in favor of the homeowners.

This study by the CGS is in an effort to understand the geologic and soil conditions that affect construction in north-central Douglas County, and to evaluate currently used methods of site investigation, earth-materials testing, engineering design, and construction inspection for assessing and mitigating these conditions.

In order to evaluate this problem, data were obtained from geotechnical reports for 185 subdivisions located within the study area. The data set contains 4,900 samples from 4,200 test borings. The data were segregated into groups according to engineering properties and the underlying soil type and geologic unit.

The behavior of each soil type and geologic unit was characterized using the data obtained from test borings and soil surveys. Windblown surficial deposits and soils derived from these deposits appear to be prone to collapse or significant settlement when wetted and/or loaded. Bedrock containing claystone or interbedded claystone and the soils derived from these geologic units contain highly swelling clays, and are prone to expansion and heave when wetted.

The test-hole data shows that the engineering properties of soil, surficial deposits, and bedrock vary significantly, both laterally and vertically, across the study area. Due to this variability, it is important that adequate geologic and geotechnical investigations be done during all phases of the planning and construction process. In particular, site-specific geologic and geotechnical investigation should be done for each lot or building pad. The investigations for each lot should include a test boring and swell-consolidation testing at foundation and floor levels.