3A Public Hearing 2006 1204 - Laserfiche WebLink

Home

GEOTECNIA Project Number: 060805 2888 Darius Way, San Leandro October 27,2006 Page 14 Retaining walls should be fully backdrained. The backdrains should consist of a 4-inch- diameter, rigid perforated pipe surrounded by a drainage blanket. The pipe should be sloped to drain by gravity to appropriate outlets. The drainage blanket should consist of Caltrans Class 2 "Permeable Material." Alternately, the drainage blanket could consist of clean, free-draining crushed rock or gravel, wrapped in a filter fabric such as Mirafi l40N. The pipe should be placed with the perforations pointing down, and the top of the pipe should be at least 6 inches below the lowest adjacent finished grade (typically the ground surface/top of slab in front of the wall or the crawl space). The drainage blanket should be at least one foot wide and extend to within one foot of the surface. The uppermost one-foot should be backfilled with compacted in- situ soils to exclude surface water. Alternately, a prefabricated drainage structure may be used provided our firm is given the opportunity to review the manufacturer's details for the drain to check that it would perform similarly to a conventional backdrain as described above. French drains should be installed along the outside of the perimeter of the buildings except where there are retaining walls with backdrains. French drains should consist of trenches, at least 12 inches wide and extending at least 12 inches below the elevation of the crawl space, backfilled with Caltrans Class 2 "Permeable Material" or drain rock/gravel encapsulated in filter fabric, with a 4-inch-diameter perforated pipe near the bottom of the trench. The collected water should flow by gravity through solid collector pipes to appropriate outlets near the front of the lot. Gutters, downspouts, collector systems, retaining wall backdrains, and surface and subsurface drains should be checked periodically for breaks, leaks, or obstructions. Accessible subdrain cleanouts should be provided and maintained on a routine basis. The drainage facilities should be cleaned and maintained as necessary so that they continue to function properly. We also recommend that all the existing drainage facilities--including the concrete-lined V-ditches, the drain inlets, and the subsurface drainpipe connecting the drain inlets--be checked or tested, and that any necessary repairs be made so they continue to perform as intended. SUDDlemental Services We recommend that GEOTECNIA be retained to review the geotechnical aspects of the project plans (grading and foundation plans), structural calculations, and specifications, as appropriate, to evaluate if they are in general conformance with the intent of our geotechnical recommendations. In addition, we should be retained to observe the geotechnical aspects of construction, particularly drilled pier construction (drilling and concrete placement), grade beam and slab subgrade preparation, footing excavations (prior to placing reinforcement or forms), installation of retaining wall backdrain and subdrain components, placement and compaction of backfill materials and pavement section materials, and to perform appropriate field and laboratory testing. If, during construction, subsurface conditions different from those encountered in the exploratory borings are observed, or appear to be present beneath excavations, we should be advised at once so that these conditions may be reviewed and our recommendations reconsidered. The