2001 Geotechnical Evaluation Report

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RVC Homes Project BABX-01-0124 April 12, 2001 Page 4 not be changed by more than a few feet, however, recommendations are provided if large grade changes are anticipated. At this time we estimate the lowest floor slab elevation will be about 935 C.Z. Discussion Based on the results of our soil borings and our understanding of the proposed construction, it is our opinion the near-surface soils are not suitable to support typical spread footing foundations. The old fill, slopewash soils and organic soils will likely settle in time,which would lead to foundation and structural damage. It is our experience that driven piling is generally a more economical foundation solution, as compared to an excavation/backfill approach when excavation depths exceed 15 feet. Based on the borings,the excavation depth would likely exceed 30 feet. The high water table, coupled with deep excavations, makes the excavation/backfill soil correction approach difficult, expensive and, in our experience, not feasible. We recommend the proposed house, including the lowest floor slab, be supported by a deep foundation system. A deep foundation system will bypass the soft or unsuitable soils,transferring the building loads to competent bearing soils at depth. Several deep foundation systems are available,however, it is our opinion driven closed-end pipe-pile would be the most economical. A helical anchor system can be looked at, however the length of the anchor and the poor lateral support for the anchor shafts may preclude use of anchors. A specialty contractor could be contacted to address the feasibility and cost of helical anchors at this site. C.3. Driven Piling Building loads, or a desired pile capacity,were not available for this report. Building loads for single-family homes are generally light. Typical net working pile capacities for residential construction are in the range of 20 to 35 tons per pile. For the purpose of this report, we have generated pile length estimates that provide net working capacities of 25 and 50 tons per pile. The purpose of evaluating the 50-ton pile design is that the additional capacity can be achieved with a relatively small increase of pile length over that of the 25-ton pile design. Using a 50-ton pile design could decrease the total number of pile required resulting in a cost savings. If desired additional pile length estimates can be evaluated for other pile capacities or other pile types and sizes. If the assumed loads exceed these values, if the building grades differ by more than 1 foot or if the location of the proposed house additions changes, we should be informed. Additional analyses and revised recommendations may be necessary.