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because they aze not connected to the wall;as shown in Photos 7 and 8. Degradation of <br /> the joints at the comers was also visible, as shown in Photos 9 and 10.Walls of this type <br /> will likely creep forwazd without any effecrive reinforcement in the form of tie-backs or <br /> geosynthetics and eventually will topple.We do not,however,helieve that this is <br /> inun.inent. <br /> No other failure mechanism appeazs to be responsible for tlie moveinent of the wall other <br /> than the degradation of the timbers and its impact on the connections. According to Mr. <br /> Sundby,no seepage has been observed flowing from the wall.This is likely because no <br /> permanent source of water is apparent as the driveway and honse are located atop a ' <br /> "ridge". Given that high pore water pressures are not present behind the wall,the only <br /> role that water bas played in the wall movement appears to be its role in the timber decay <br /> process. <br /> Computations <br /> Hand computations,using the Rankine method of active earth pressures, were performed <br /> • to quantify the stability of the existing timber retaining wall and the proposed boulder <br /> wall. The computations are attached. Only the southeast-facing portion of the main wall <br /> was analyzed in the computatians. We believe that the lawer overall height of the sec�nd <br /> wall on the west side of the driveway makes it less critical for stability. <br /> The upper tier was analyzed for sliding and overtuming, and shown to have a factor of <br /> safety below one.This indicates that failure is occurring for both failure mechanisms � <br /> analyzed, which would explai.n the movement observed in the existing timber wall. <br /> Although the factor of safety shown in the computations is far below one,the actual <br /> factor of safety is likely near one because the benefit achieved by the pinned corners is <br /> not accounted for in the analysis and the shear strength of the backfill is likely higher <br /> than the assumed friction angle of 30 degrees (at low confining stress). <br /> C:omputations were also performed for the proposed boulder wall.Although the design of <br /> a replacement wall was outside the scope of this evaluation,the calculations were <br /> performed to show the feasibility of a replacement wall at the site.In this case,the lowest <br /> tier of a three-tier,un-reinforced boulder wall was analyzed.To account for the weight of <br /> the upper two tiers (each measuring 3 %z feet high and set back 7 feet), a 2H:1V angle of <br /> sloping backfill was assumed,though this approach is slightly conservative. The assunled <br /> width of the wall was 2 feet,with the wall comprising 24-to 30-inch diameter fieldstone <br /> boulders.Moreover, a granular backfill material was assumed. <br /> The calculations for the proposed boulder wall show that the wall would be stable against <br /> sliding (FS =�.45).The factor of safety against overturning is acceptable according to <br /> some recommendations and unacceptable according to others (FS =2.46). Local building <br /> codes may have a zecommended factor of safety against overtuming, and this value <br /> should be used. In any case,reinforcement of the backfill soil using geosynthetics will <br /> raise the factor of safety to an accepta.ble value. Also, stability of the wall against bearing <br /> capacity failure was not analyzed because the present soil loads, which will remain <br />