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HomeMy WebLinkAbout Earthwork Observation Summary Final ReportAugust 28, 2025 Project No. T-8566-1 Mr. Tony Brandolini D.R. Horton 11241 Slater Avenue Northeast, Suite 200 Kirkland, Washington 98033 Subject: Final Report Earthwork Observation Summary Towns on 12th 3916 Northeast 12th Street Renton, Washington References: 1. Geotechnical Report, Towns on 12th, 3916 Northeast 12th Street, Renton, Washington, Project No. T-8566, prepared by Terra Associates, Inc., dated August 5, 2021, revised February 15, 2022 2. Field Reports, prepared by Terra Associates, Inc., dated April 23, 2024, through March 5, 2025 Dear Mr. Brandolini: As requested, representatives of Terra Associates, Inc. have been providing geotechnical engineering observation services for the subject project. We began our work April 23, 2024, and as of March 5, 2025, we have completed our observations. The purpose of our work was to verify geotechnical elements of the construction were completed in accordance with the referenced report, approved construction drawings and recommendations presented by our onsite personnel. Our services included geotechnical observation of the following:  Observation and verification of site stripping to remove topsoil.  Observation and compaction testing of structural fill placed for site fills.  Observation and compaction testing of utility trench backfill.  Observation and verification of detention vault excavation, drainage, and foundation subgrade. 12220 – 113th Avenue NE, Ste. 130, Kirkland, Washington 98034 │ Phone (425) 821‐7777   Mr. Tony Brandolini August 28, 2025 Project No. T-8566-1 Page No. 2  Observation and compaction testing of detention vault backfill.  Observation and verification of suitable Keystone Block foundation subgrade.  Observation and verification of Keystone Block wall construction including drainage and reinforcement.  Observation and compaction testing of Keystone Block wall backfill.  Subgrade for support of curb, gutter, and sidewalk.  Subgrade for support of pavement. Summaries of our observations and testing are provided in the referenced field reports prepared following each site visit. Copies of these field reports were provided to your representative, the City of Renton, and the contractor. Compaction tests were conducted at the site as structural fill for wall, lot, roadway, vault, and utility backfill was placed and compacted. Tests were performed in accordance with ASTM Test Designation D-6938 (Nuclear Gauge Method). Compaction control was based on the soil’s maximum dry density and optimum moisture content as determined by ASTM Test Designation D-1557 (Modified Proctor). Results of our compaction testing indicated structural fill placed for site fill, lot fill, roadway fill, utility trench backfill, detention vault backfill, and wall backfill were compacted to project specifications in the areas tested. Based on the results of our observation and testing, it is our opinion that the geotechnical engineering elements of project construction were completed in accordance with our recommendations and the grading plans approved by the City of Renton. The structural fill placed on the lots was adequately compacted in accordance with project requirements. Where residential foundations are supported on structural fill or firm native soils, they can be designed for an allowable soil bearing pressure of 2,500 pounds per square foot. It should be noted that the structural lot fills as well as the native soils, while firm and compact, will be subject to disturbance from normal construction activities, including foot traffic, when the soils are wet. Because of the flat lot grades, wet subgrade should be expected immediately below the topsoil layer that has been placed over the structural lot grade. The depth of the wet soils will be dependent upon the duration of rainfall and how well the surface of the lot drains. Care must be exercised when excavating and constructing building foundations during wet weather to avoid disturbing the foundation subgrade. Construction vehicle traffic across the lots should be limited during wet weather. Foundation excavations should also be completed using an excavator with a smooth blade bucket. The excavator should not travel across the exposed footing grade. While constructing form work and placing steel, to avoid disturbance from worker foot traffic, consideration should be given to over excavating the footing grade four inches and restoring grade using clean one-half to two-inch size crushed rock.