WSP11325

• File Number: 8443.400
• Description: Narrows Unit - Reports
• State: CO
• Basin: South Platte
• Water Division: 1
• Date: 3/1/1978
• Author: Woodward-Clyde Consu
• Title: Review Of Data And Studies Developed By The Bureau Of Reclamation For The Feasibility Stage Design Of Narrows Dam Colorado
• Water Supply Pro - Doc Type: Report/Study

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Woodward.CIyde Consultants

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IV. STABILITY OF DOWNSTREAM SLOPE

We reviewed the Bureau's evaluation of the stability of

the downstream slope at the maximum section of the dam for

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steady state conditions by conducting independent stability

analyses. Several failure surfaces with varying strength

parameters were studied utilizing computerized programs for

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circular arc and composite failure surfaces. In our analyses

we used the parameters given in Table 2 and as shown on

Figure 5. Further, we investigated stability conditions

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utilizing horizontal force earthquake factors of zero, 0.025

and 0.05g. A steady state flow net similar to that used by the

Bureau was considered to be reasonable and was used in our

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analyses.
The results of our stability analyses are recorded on

Figure 5. In all cases, not including assumed earthquake

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forces, the ratios of theoretical resisting forces divided by

theoretical driving forces were greater than 1.5. In the

analysis of failure surface "1" (see Figure 5) we took into

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consideration the two seismic coefficients in conventional

pseudostatic analyses. The document "Earthquake Design and

Analysis for Corps of Engineers Dams," previously discussed on

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p. 10 utilizes a seismic coefficient of 0.025 for dams in

Seismic Zone 1. For this case the ratio of theoretical

resisting forces to theoretical resisting forces was 1.46. We

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believe the risk of earthquake induced ground motions adversely

affecting the stability of the dam is very low.

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