Dealing with Tricky Soils at the SDS Water Treatment Plant

The Southern Delivery System's (SDS) 189,300 m³/d (50-mgd) water treatment plant and finished water pump station in southeast Colorado Springs is the largest and most complex component of the regional project. The project is also being built on some of Colorado's most complex geology. Foundation components of the project are constructed and construction continues on the various facilities. The geology of the water treatment plant site consists of eolian deposits over alluvium. Two large site structures were founded within the eolian deposits at depths ranging from 9.1 m (30 ft) below to 3 m (10 ft) above existing grade. The eolian soils are windblown soils that were deposited in a loose condition and later the individual grains were fused with weak water-soluble "cement." When these soils become wetted, particularly under conditions of increased loading, they collapse. Design, construction, and operation measures were necessary to mitigate the potential for water to collect below grade or to pond above grade, as well as minimizing the potential for pipe or tank leakage. The design-build project team mitigated the collapse potential of the site soils. Ground improvement included deep dynamic compaction (DDC) for areas of the site. A test pilot program was successful in demonstrating that DDC could be used as a cost effective method to mitigate the soils across large areas of the site. Using DDC saved 1,704,957.3 m³ (2,230,000 yd³) of over-excavation that would have otherwise been required.