Full spectrum detention ponds are typically designed and constructed with embankments and control structures to store stormwater above the natural grade of the land. Our investigation included two Soil Test Borings in this region to characterize the subsurface soils pertinent to embankment construction, and to provide recommendations regarding embankment construction. These recommendations have been prepared in accordance with the requirements outlined in the El Paso County Land Development Code (LDC), the Engineering Criteria Manual (ECM) Section 2.2.6 and Appendix C.3.2.B, and the El Paso County (EPC) Drainage Criteria Manual, Volume 1 Section 11.3.3.
Detention Storage Criteria
Detention pond embankments that impound water above the natural grade of the land are considered dams under rules and regulation promulgated by the State of Colorado Department of Natural Resources. Rules and Regulations for Dam Safety and Dam Construction have been developed to provide guidance to design engineers and constructors. Dams are regulated as jurisdictional dams or non-jurisdictional dams. In accordance with El Paso County Drainage Criteria Manual, Volume 1, Section 6.6, embankments in this development will most likely qualify as non-jurisdictional, minor dams, with a Class III hazard rating.
The purpose of our recommendations is to provide information to comply with the referenced guidelines and provide pertinent geotechnical information upon which to base the design and construction of pond embankments. This section presents the findings of the investigation performed by RMG and our recommendations regarding detention pond construction.
General Physiographic Setting
The site is located within the western flank of the Colorado Piedmont section of the Great Plains physiographic province. The Colorado Piedmont which formed during Late Tertiary and Early Quaternary time (approximately 2,000,000 years ago) is a broad, erosional trench which separates the Southern Rocky Mountains from the High Plains. During the Late Mesozoic and Early Cenozoic Periods (approximately 70,000,000 years ago), intense tectonic activity occurred, causing the uplifting
of the Front Range and associated downwarping of the Denver Basin to the east. Relatively flat uplands
and broad valleys characterize the present-day topography of the Colorado Piedmont in this region.
More particularly, the site is located on alluvial deposits with bedrock intrusions above Fountain Creek.
Topography
The ground surface generally slopes gently down to the south and southwest across the entire site and drops precipitously into Fountain Creek at the south end. Fountain Creek is adjacent to and forms the western property line.
Vegetation
Vegetation across the site generally consists of native grasses, shrubs, and weeds.
General Soil Types
The general geology of the area is typically stream terrace deposits and alluvium soils overlying the Pierre Shale. Samples from each Soil Test Boring exhibited characteristics of the general geology. The subsurface conditions can be characterized by describing two geologic units that were mapped in the vicinity of the site identified (Morgan, et al., 2003) as:
- al: alluvium is loose, unconsolidated (not cemented together into a solid rock) soil or sediments, which has been eroded, reshaped by water in some form, and redeposited in a non-marine setting. Alluvium is typically made up of a variety of materials, including fine particles of silt and clay and larger particles of sand and gravel.
- Kp: Pierre Shale – (Upper Cretaceous) Underlain by the Piney Creek Alluvium. Permeability is generally low, excavation and compaction generally easy. Foundation stability is less than fair. The majority of the formation has low to high swell potential. Slope stability is generally poor and slopes steeper than 5 degrees may slide, if the toe of the slope is removed.
Subsurface Materials
The subsurface materials encountered in Test Borings R-09 and R-10 were classified using the Unified Soils Classification System (USCS) and the materials were grouped into the general categories of silty sand and shale. These soils classify as SW-SM, well-graded silty sand, and CH, sandy fat clay. It is anticipated that subgrade foundations for embankments will be in alluvial material, and that the embankments themselves will be constructed from on-site alluvial material. Embankments are not anticipated to be constructed directly upon or built up from shale bedrock.
Groundwater
Groundwater was not encountered in R-09. Groundwater was encountered in R-10 at 6-feet below the existing ground surface. Groundwater may influence detention pond embankment design and construction.