2006-P10446 - retaining wall - Laserfiche WebLink

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because they are not connected to the wall, as shown in Photos 7 and 8. Degradation of the joints at the corners was also visible, as shown in Photos 9 and 10. Walls of this type will likely creep forward without any effective reinforcement in the form of tie-backs or geosynthetics and eventually will topple. We do not, however, believe that this is imminent. No other failure mechanism appears to be responsible for the movement of the wall other than the degradation of the timbers and its impact on the connections. According to Mr. Sundby, no seepage has been observed flowing from the wall. This is likely because no permanent source of water is apparent as the driveway and house are located atop a "ridge". Given that high pore water pressures are not present behind the wall, the only role that water has played in the wall movement appears to be its role in the timber decay process. Computations Hand computations, using the Rankine method of active earth pressures, were performed to quantify the stability of the existing timber retaining wall and the proposed boulder wall. The computations are attached. Only the southeast-facing portion of the main wall was analyzed in the computations. We believe that the lower overall height of the second wall on the west side of the driveway makes it less critical for stability. The upper tier was analyzed for sliding and overturning, and shown to have a factor of safety below one. This indicates that failure is occurring for both failure mechanisms analyzed, which would explain the movement observed in the existing timber wall. Although the factor of safety shown in the computations is far below one, the actual factor of safety is likely near one because the benefit achieved by the pinned corners is not accounted for in the analysis and the shear strength of the back�ll is likely higher than the assumeci friction angle of 30 degrees (at low confining stress). Computations were also performed for the proposed boulder wall. Although the design of a replacement wall was outside the scope of this evaluation, the calculations were performed to show the feasibility of a replacement wall at the site. In this case, the lowest tier of a three-tier, un-reinforced boulder wall was analyzed. To account for the weight of the upper two tiers (each measuring 3 1/z feet high and set back 7 feet), a 2H:1 V angle of sloping backfill was assumed, though this approach is slightly conservative. The assumed width of the wall was 2 feet, with the wall comprising 24- to 30-inch diameter fieldstone boulders. Moreover, a granular backfill material was assumed. The calculations for the proposed boulder wall show that the wall would be stable against sliding (FS = 2.45). The factor of safety against overturning is acceptable according to some recommendations and unacceptable according to others (FS = 2.46). Local building codes may have a recommended factor of safety against overturning,.and this value should be used. In any case, reinforcement of the backfill soil using geosynthetics will raise the factor of safety to an acceptable value. Also, stability of the wall against bearing capacity failure was not analyzed because the present soil loads, which will remain