Soil and Rock Design Parameters
RMG has performed numerous laboratory tests of soil similar to the soils encountered in the Soil Test Borings. Based upon field and laboratory testing, the following soil and rock parameters are typical for the soils likely to be encountered, and are recommended for use in detention pond embankment design.
| Soil Description | Unite Weight (lb/ft3) | Friction Angle (degree) | Active Earth Pressure, Ka | Passive Earth Pressure, Kp | At Rest Earth Pressure, Ko | Unconfined compressive Strength (kip/ft2) |
|---|---|---|---|---|---|---|
| Alluvial Soil SW-SM | 115 | 30 | 0.33 | 3.0 | 0.50 | – |
| Shale Bedrock CH | 124 | – | – | – | 72 |
6.2 Seismic Design
In accordance with the International Building Code, 2012/2015, seismic design parameters have been determined for this site. The Seismic Site Class has been interpreted from the results of the soil test borings drilled within the project site. The USGS seismic design tool has been used to determine the seismic response acceleration parameters. USGS output is presented in Appendix B. The soil on this site is not considered susceptible to liquefaction. The following recommended Seismic Design Parameters are based upon Seismic Site Class D, and a 2 percent probability of exceedance in 50 years. The Seismic Design Category is “B”.
| Period (sec) | Mapped MCE Spectral Response Acceleration (g) | Site Coefficients | Adjusted MCE Spectral Response Acceleration (g) | Design Spectral Response Acceleration (g) | ||||
| 0.2 | S3 | 0.175 | Fa | 1.6 | Sms | 0.280 | Sds | 0.187 |
| 1.0 | S1 | 0.060 | Fv | 2.4 | Sml | 0.145 | Ddl | 0.097 |
Notes:
- MCE = Maximum Considered Earthquake
- g = acceleration due to gravity
6.3 Embankment Recommendations
Development plans providing detention pond details were not available. In general, embankments should be constructed with 4:1 slopes. Embankments should be constructed in accordance with applicable sections of the El Paso County Engineering Criteria Manual, the El Paso County Drainage Criteria Manual, and the El Paso County Land Development Manual. The following recommendations are in accordance with the El Paso county DCM Volume 2, Extended Detention Basin (EDB), Design Procedure and Criteria, paragraph 8.
The ground area to receive embankments should be cleared and grubbed to a minimum depth of two-feet to remove grass, shrubs, trees, roots, stumps, and other organic material. The exposed soil should be moisture conditioned to facilitate compaction (usually within 2 percent of the optimum moisture content) and compacted to a minimum of 95 percent of the maximum dry density as determined by the Modified Proctor test (ASTM D-1557). The prepared surface should present a firm and stable condition.
Embankment should be constructed as structural fill on a prepared stable base. On-site native soil when screened of all deleterious material and cobbles greater than 6-inches in any dimension is suitable for embankment construction. Structural fill should be placed in 10-inch loose lifts and moisture conditioned to facilitate compaction (usually within 2 percent of the optimum moisture content) and compacted to a minimum of 95 percent of the maximum dry density as determined by the Modified
Proctor test (ASTM D-1557).
Structural fill placed on slopes should be benched into the slope. Maximum bench heights should not exceed 4 feet, and bench widths should be wide enough to accommodate compaction equipment. Structural fill should not be placed on frozen subgrade or allowed to freeze during moisture conditioning and placement. To verify the condition of the compacted soils, density tests should be performed during placement. The first density tests should be conducted when 24 inches of fill have been placed.