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HomeMy WebLinkAboutObservation of Excavation Copyright © 2024 GeoStrata 1 Excavation Observation - 6094 County Road 320 Residence Addtion October 8, 2024 Mr. Jim Leemon Trunnel and Brace Timberframes 185 Francis Street Longmont, Colorado 80501 718-753-9606 GeoStrata Project No. 1934-001 RE: Foundation Soils Observation for 6094 CR 320 Residence Addition Rifle, Colorado Mr. Leemon: As requested and in accordance with our Master Service Agreement dated September 27, 2024 and executed on September 30, 2024, on September 30, 2024, GeoStrata performed an observation of the foundation excavation for the residence located at 6094 County Road 320 in Rifle, Colorado. A geotechnical report prepared by GeoStrata titled “Geotechnical Engineering Report 6094 County Road 320 Proposed Residential Addition and Hay Barn, Garfield County, Colorado” and dated April 30, 2024 (GeoStrata Job No. 1869-001) was reviewed as part of our foundation observation. During our foundation excavation observation at the residence we were looking for compliance with the recommendations contained in the geotechnical report which are summarized as follows: • Any unsuitable soils such as vegetation, topsoil, undocumented fill, debris, and soft, loose, or disturbed soils should be removed prior to the placement of the footings. • As over-excavation was required, the excavation should extend one foot laterally for every one foot of depth required of over-excavation. • Footings should be placed entirely on 2-feet of properly placed and compacted structural fill for a maximum net allowable bearing capacity of 2,000 psf. • All structural fill should be free of vegetation, debris, frozen material, and should contain no materials larger than 4-inches nominal size. Structural fill should be placed in lifts not exceeding 6-inch loose lifts if compacted by small hand operated equipment, 8-inch loose lifts if compacted by light-duty rollers, and 10-inch loose lifts if compacted by heavy-duty compaction equipment. Structural fill should be compacted to at least 95% of the maximum dry density, and at or slightly above the optimum moisture content at the time of placement and compaction, as determined by ASTM D-1557. Observations At the time of our site visit on September 30, 2024, the foundation excavation had been completed and ranged from approximately 10 to 11 feet in depth below the existing site grade. The soils exposed in the excavation sidewall comprised of slightly moist, medium dense to dense sandy, silty/clayey gravel with cobble and boulders GC/GM with isolated clay pockets and lenses. This material was observed to underlie the entire foundation excavation bottom and to comprise undisturbed native soil. 2487 Industrial Boulevard, Grand Junction, Colorado 81505 T: (801) 501-0583 ~ info@geostrata-llc.com Copyright © 2024 GeoStrata 2 Excavation Observation - 6094 County Road 320 Residence Addtion It should be noted that at the time our previously mentioned geotechnical engineering report was issued, there were no below grade areas (such as basements) planned for the residence. Between the issuance of the geotechnical engineering report and our site visit on April 30, 2024, the Owner had decided to construct the residence addition with a basement. Part of on-site observations were to confirm that the deeper bearing materials were consistent with what was encountered at shallower elevations at the specific site, and that our recommendations provided in our geotechnical engineering report were valid for existing conditions at the bottom of the foundation excavation. GeoStrata observed that all unsuitable soils such as vegetation, topsoil, undocumented fill, debris, and soft, loose, or disturbed soils were removed from the excavation bottom. We recommend the footings be placed upon 2-feet of properly placed and compacted structural fill. Based on our observations and the soils encountered during our previously mentioned geotechnical engineering report, the exposed soils at the bottom of the excavation are suitable native soils for the placement of 2-feet of properly placed and compacted structural fill beneath the footings for a maximum net allowable bearing capacity of 2,000 psf. Conclusions and Recommendations Based on our observations, the recommendations in the soils report pertaining to foundation excavation, over-excavation, and excavation bottom soil suitability have been complied with and the site is suitable for the placement of 2-feet of properly placed and compacted structural fill beneath the footings for a maximum net allowable bearing capacity of 2,000 psf. Exposed foundation soils should be protected against freezing prior to and during footing and foundation construction. Freezing of foundation soils could result in excessive total and differential settlements causing structural damage. If soils in the foundation excavation are allowed to freeze, the soil which were frozen should be over - excavated and removed. The over-excavation should be backfilled with properly placed, compacted and tested suitable structural fill. Soils loosened during preparation of the foundation should be re- compacted prior to footing placement. The footings should not be placed on soft or pumping soil, or in areas of ponded water. Perched groundwater elevation can be altered by flood irrigation and/or other surges of water introduced to the subsurface. As an added precaution the following moisture protection mitigation recommendations should be implemented to help reduce the risk of potential settlement from occurring. These items are summarized as follows: • Minimum embedment of the footings for frost protection should be 36-inches below ground surface. • Moisture should not be allowed to infiltrate into the soils in the vicinity of the foundations. As such, design strategies to minimize ponding and infiltration near the home should be implemented. • The ground surface within 10 feet of the entire perimeter of the building should slope a minimum of five percent away from the structure. Alternatively, a slope of 2% is acceptable if the water is conveyed to a concrete ditch that will convey the water to a point of discharge that is at least 10 feet from the structures. Copyright © 2024 GeoStrata 3 Excavation Observation - 6094 County Road 320 Residence Addtion • Roof runoff devices (rain gutters) should be installed to direct all runoff a minimum of 10 feet away from the structure. Rain gutters discharging roof runoff adjacent to or within the near vicinity of the structure may result in excessive differential settlement. • We recommend irrigation around foundations be minimized by selective landscaping and that irrigation valves be constructed at least 5 feet away from foundations. • Jetting (injecting water beneath the surface) to compact backfill against foundation soils may result in excessive settlement beneath the building and is not allowed. • Backfill against foundations walls should be placed in lifts and compacted to 90% modified proctor to create a moisture barrier. In areas where collapsible soils are present, backfill should consist of fine-grained soils. • Any additional precautions which may become evident during construction. • If undocumented fill or collapsible soils were identified at the site, the contractor and owner should be aware that if driveways, sidewalks, or other flatwork is constructed on the undocumented fill or collapsible soils, these improvements have a high potential to settle. All recommendations in the geotechnical report should be complied with during construction of these improvements. Over-wetting of the native or fill soils beneath the footings by natural or man-made means prior to, during or after construction may result in soil stability problems and potential excessive settlement of the soils. Failure to comply with these recommendations could result in excessive total and differential settlements causing structural damage. As previously mentioned, it was our understanding that the residence addition would not have any below grade areas (such as basements) at the time of our geotechnical engineering report. As it was later decided by the Owner to construct the addition with a basement, it is our recommendation that a foundation drain be installed. A foundation drain detail is attached to this letter. It should be noted that our boreholes extended from depths of 4.5’ to 60’ at the subject site. Copyright © 2024 GeoStrata 4 Excavation Observation - 6094 County Road 320 Residence Addtion Limitations The observations for the above referenced building lot are limited to the issues specifically identified in this letter. This letter addresses the soils observed in the foundation excavation for the foundation of the proposed residence only. Soils underlying sidewalks, driveways, and other flatwork were not observed and are outside our scope of services. The GeoStrata 2024 geotechnical report should be read in its entirety and all recommendations followed. This letter only contains portions of the GeoStrata 2024 geotechnical recommendations relevant to foundation footings. GeoStrata has relied on the geotechnical engineering report in performing its services. All services were completed in accordance with the current standard of care, no other warranty expressed or implied is provided. No laboratory testing or engineering analysis was performed as a part of this observation. All services were performed for the exclusive use and benefit of the above addressee. No other person is entitled to rely on GeoStrata’s services or use the information contained in this letter without the express written consent of GeoStrata. We did not supervise, direct or have any control over any of the Contractor’s or Subcontractor’s work. We were not responsible for the contractor’s or Subcontractor’s scheduling of our services, means, methods, procedures, techniques, or sequences of construction, nor responsible for ensuring that any Contractor or Subcontractor performed or completed the work in accordance with the contract documents or construction industry standards. Ω We appreciate the opportunity to provide these services. If you would like to discuss any of the issues contained in this letter in more detail or have additional questions, please contact us at your convenience (970) 307-4191. Respectfully, GeoStrata Ryan Hogge Project Engineer REFERENCES GeoStrata, April 30, 2024, Geotechnical Engineering Report 6094 County Road 320 Proposed Residential Addition and Hay Barn, Garfield County, Colorado, GeoStrata Job No. 1869-001. Foundation Drain Detail Free Draining Gravel Separated From Native Soils with Geotextile 4-inch Diameter Perforated Pipe covered with 12 inches of gravel Drawing Not To Scale Copyright GeoStrata 2024 2 Feet Prefabricated Drain Panel Trunnel and Brace Timberframes 6094 County Road 320 Residence Addition Rifle, Colorado Project Number: 1394-001 Attachment N/A