518 51ST ST SW 2023-06-23 - Laserfiche WebLink

• • April 6,2021 Page 5 of 7 Limited Geotechnical Evaluation 6. The proposal mitigates all impacts identified in the geotechnical letter or geotechnical report; 7. All utilities and access roads or driveways to and within the site are located so as to require the minimum amount of modification to slopes,vegetation or geologically hazardous areas; and 8. The improvements are certified as safe as designed and under anticipated conditions by a geologist. The proposed landscaping work is considered`certified' as safe provided the following conditions are utilized/implemented as part of development: • Excavations are limited to the areas for pier installation near the foundation elements • Piers are driven to refusal per design • Areas are fully backfilled and landscaped to a like condition • All runoff is controlled on site during construction(not anticipated) Conclusions and Recommendation Based on our observations, it appears likely that the observed settlement is likely the result of consolidation of fill placed below the site during area construction. There could be areas of underlying weathered till that was loose and not fully stripped prior to fill/foundation placement. The underlying glacial till or outwash is typically dense within 15 feet of the ground surface and is suitable for support of new foundation support elements. The proposed mitigation utilizing helical anchors attached to the foundation system with mechanical connections appears suitable to mitigate/reduce ongoing soil settlement. Based on our observations,we anticipate that foundation support elements may extend 10 to 20 feet below existing foundation elements with an average depth of 15 feet. Note that this is only a rough estimate as we achieved refusal in fill or drift. A deeper boring would be required to determine likely depths to refusal. This does not appear warranted provided the piers are driven to adequate pressures with suitable embedments. Helical Piers@ Helical Piers®may be used to support the existing single-family residence. The Helical Piers® could be installed using portable rotary tools, truck mounted rotary tools, backhoe mounted rotary tools,caisson drills,or skid-steer loaders. It is important that the torque output,rotational speed, down pressure capability, and angle control of the installation equipment is compatible with the required foundation system. The pile installation equipment should have adequate torque capacity to prevent refusal conditions at relatively shallower depths that are well above recommended bearing depths or layers. A Helical Pier®consists of an anchor(lead section)with 1, 2, 3 or more helical flights on a shaft. The number and diameter of the helices on the anchor are dependent on the soil characteristics of the site and the design loads to be applied to the pier. Based on these parameters the anchor helix configuration is chosen to best fit the site conditions. As the anchor is advanced into the soil extension sections (shaft) are placed on the lead section. The shaft configuration is based on the design loads and anticipated installation torque. www.cobaltgeo.com (206)331-1097