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A close-up showing the scale of one of the blocks of Castle Rock Conglomerate that is already displaced, Castle Rock, Colorado, January 1981. Photo credit: Colorado Geological Survey.

Case Study: Rockfall – St. Francis of Assisi, Castle Rock

2009-07-10 | Dr. John Hopkins

The CGS studied the site of St. Francis of Assisi Church in Castle Rock extensively after a block detached from the upper cliff face in January 1981. The block presented a risk to homes at the base of the slope south of the church, and was subsequently broken up using passive demolition methods. Other detached blocks continued to present a rockfall hazard to six homes located at the base of the bluff. No consideration was made to address rockfall hazards at the base of the slope when the homes were originally built.

The church was planning a major expansion, and in 2005 the CGS was asked by Douglas County to review the church’s development plans. The church sits atop a bluff that is composed of hard, blocky Castle Rock Conglomerate overlying soft, erodible Dawson Arkose (a type of sandstone). Tension fractures in the cap rock conglomerate indicate that large blocks are actively detaching from the cliff face, and large fallen blocks are present on the slope below. Some of these large rocks have even been incorporated into the landscaping of homes below the bluff.

However, since the homes pre-date the proposed expansion, the church was required to make every effort to ensure that the expansion will not further destabilize the bluff. The CGS and Douglas County were concerned that the proposed expansion would impose construction-related disturbances and vibrations that could increase the rockfall hazard. Post-construction runoff from the planned large roof and pavement areas could result in increased infiltration and seepage, further destabilizing the precarious blocks along the cliff.

A rockfall mitigation plan was developed for the site. The mitigation plan included (1) constructing a rockfall catchment trench, (2) cable-lashing a large pillar, (3) scaling unstable rocks, and (4) using rock bolts with wire mesh and shotcrete to anchor the larger areas of unstable rocks. The mitigation was completed in September 2008.

For more on rockfall issues around the state, see the original RockTalk: Rockfall in Colorado.

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