2006-P10446 - retaining wall - Laserfiche WebLink

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Services Performed A visit to the Sundby residence was made by a Barr geotechnical engineer on May 17`�', 2006. The puipose of the site visit was to collect information regarding the general condition of the walls, perform soil borings with a hand auger, and evaluate the feasibility of various alternatives for replacing the existing wall. These alternatives include cast-in- place concrete, modular block, boulders, and another timber wall. Mr. Sundby's. preference is a boulder wall, so this option was evaluated particularly. Soil borings with a 5-foot hand auger were performed at three locations at the main wall. The first, referred to as HA-1, was performed about three feet away from the top of the upper tier wall. This hole was placed in line with one of the shallow wooden tie-backs to evaluate its integrity. The second hole, called HA-2, was placed about 3 feet southwest (parallel to the wall) of HA-1. The purpose of this hole was to collect information about the type of backfill soil and the amount of water present in the soil. The third boring, referred to as HA-3, was performed in the backfill soil above the lower tier wall about 2 feet from the timbers af the lower wall. Soils encountered in the hand auger borings were classified according to ASTM D2488. Results Site Visit Logs of the hand auger borings are attached. As seen in the boring logs, HA-1 went to a depth of 1.8 feet before hitting refusal on wood and the other two terminated at a depth of 5 feet(the maximum reach of the hand auger). Photographs of the final hand auger depths are shown in Photos 3, 4, and 5. The borings indicate that the backfill soils comprise a 6- inch layer of topsoil overlying silty lean clay with varying amounts of sand and gravel. The material becomes coarser with depth, especially at HA-3 where the soil is classified as sand with silt from a depth of 4 to 5 feet. No saturated soil was encountered, but was classified as moist. The wood encountered in HA-1 was likely the timber tie-back. The wood of the tie-back appeared to be in good condition because the samples that were recovered from the auger were relatively well-preserved and the auger could not penetrate all the way through it. If the tie-back had been rotted, the auger should have been able to pass through without significant effort. The condition of the individual timbers on the face of the wall is poor. Although the timbers are treated, the wood has undergone moderate degradation due to wood rot. This is probably due in part to the backfill soil behind the wall holding moisture next to the timbers. With the wood also having access to oxygen from the face of the wall, the timbers have been subjected to decay. Additionally, there is normal decay that occurs in timbers that are 20 or more years o1d. One result oi the wood deterioration is thal some of the pins connecting the face timbers and especially those joining the tie-backs to the face timbers have had their effectiveness decreased significantly. In other words, the pins no longer have any woo.d material left to hold onto in some locations to tie the timbers to�ether, as shown in Photo 6. The most observable evidence of this phenomenon is that the wall has moved forward relative to the tie-backs, which have remained stationary