The following discussion is based on the subsurface conditions encountered in the test borings and on the project characteristics previously described. If conditions are different from those described in this report or the project characteristics change, RMG should be retained to review our recommendations and adjust them, if necessary.
Geotechnical Considerations
Overall, the subsurface soil conditions are favorable for residential development on shallow foundation systems. The well-graded silty sand can be prepared to provide adequate bearing capacity. Claystone and shale are not considered suitable for direct foundation bearing. In those locations where claystone and shale are present overexcavation and replacement with compacted structural fill will be necessary to provide for a minimum of 4-feet of separation between unsuitable soil and foundation elements. The area of the site that may require overexcavation and replacement can be described as the southwest portion of the site roughly defined by Soil Test Borings R-05, R-06, R-08, R-10, and R-11.
The preliminary site plan provided to RMG shows the far southern end of the site reserved for a full spectrum stormwater detention area. Soil Test Borings R-09 and R-10 were performed in this region. Detention area considerations are discussed in Section 2 – Full Spectrum Detention Area. The site plan also shows proposed lift station at the extreme southwestern part of the site. Soil Test Boring R-11 was performed in this location.
Site Preparation
Final grading plans were not available for review. In general, the following site preparation procedures are recommended.
Standard Penetration Test blow counts vary across the site and with depth. Due to this variability we recommend improving the soil under foundations by overexcavating the foundation areas and backfilling with compacted structural fill. The on-site silty sand soil is suitable as structural backfill. The clay and claystone is not recommended as structural backfill. Site preparation should include clearing and grubbing the site of all vegetation, topsoil, and any other deleterious material within the construction area and disposing this material appropriately. Following clearing and grubbing, the area within the foundation footprint, under basements, and a 2-foot perimeter beyond should be overexcavated two (2) feet below the bottom of footing elevation. Excavated sand soil may be stockpiled for reuse as structural backfill. An Open Excavation Observation should be made at this point to verify soil conditions are as reported in the soil boring logs herein.
Upon verification, the upper 6 inches of the exposed subsurface soils should then be scarified and moisture conditioned to facilitate compaction (usually within 2 percent of the optimum moisture content) and compacted to a minimum of 95 percent of the maximum dry density as determined by the Modified Proctor test (ASTM D-1557) prior to placing structural fill.
After compaction of the subgrade, the native material previously removed may be used as structural backfill to bring the site to bottom of footing grade. The material should not be excessively wet, should be free of organic matter and construction debris, and contain no rock fragments greater than 3-inches in any dimension. Fill material should be placed in ten-inch loose lifts with moisture content within 2 percent of optimum as determined by ASTM D-1557. Each loose lift should be compacted o a minimum of 95 percent of Modified Proctor maximum dry density as determined by ASTM D-1557. Backfill soil should be density tested to verify compaction meets these requirements.
Foundation Recommendations
Structures may be supported on shallow foundations bearing on the onsite soils when prepared in accordance with the recommendations above. When so prepared, a maximum allowable bearing pressure of 2,500 psf with no minimum dead load requirement may be used for design. The foundation design should be prepared by a qualified Colorado Registered Professional Engineer using the recommendations presented in this report. This foundation system should be designed to span a minimum of 10 feet under the design loads. The bottoms of exterior foundations should be at least 30 inches below finished grade for frost protection. When prepared and properly compacted, total settlement of 1-inch or less with differential settlement on the order of ½ inch or less is estimated. Settlement in granular material will occur relatively rapidly with construction loads. Long term consolidation settlement should not be an issue in the site material if prepared as recommended above.
All foundation and site preparation recommendations contained herein apply equally to the proposed sanitary lift station.
Open Excavation Observations
As referenced above, foundation excavations should be observed by RMG prior to placing structural fill, forms, or concrete to verify the foundation bearing conditions for each structure. Based on the conditions observed in the foundation excavation, the recommendations made at the time of construction may vary from those contained herein. In particular if claystone is encountered within 4-feet of foundation elements, additional overexcavation will be recommended. In the case of differences, the Open Excavation Observation report shall be considered to be the governing document to be used to modify the site preparation recommendations as necessary.
Floor Slabs
The in-situ silty sand soil is non-plastic and should be stable at its natural moisture content. The onsite soil is suitable as backfill material. Any fill material from outside sources used to bring the site to grade should be non-expansive granular material to control slab movement.
Soil for interior floor slabs should be prepared in a manner similar to foundations above. Areas under floor slabs should be overexcavated a minimum of 1-foot and the upper 6 inches of the exposed subsurface soils should then be scarified and moisture conditioned to facilitate compaction (usually within 2 percent of the optimum moisture content) and compacted to a minimum of 95 percent of the maximum dry density as determined by the Modified Proctor test (ASTM D-1557) prior to placing structural fill. Floor slabs should bear upon a minimum of 1-foot of structural backfill compacted to a minimum of 95 percent of Modified Proctor maximum dry density as determined by ASTM D-1557. Non-structural slabs should be isolated from foundation members with expansion material. To reduce the possibility of capillary rise of groundwater into the floor slab, and to reduce the potential for concrete curling, a minimum 3-inch layer of ¾-inch crushed stone may be placed atop the compacted structural fill. A 6-mil vapor retarder may be installed above the crushed stone.
Exterior Concrete Flatwork
Reinforced concrete exterior slabs should be constructed similarly to floor slabs on compacted structural fill, with the additional caveat they be isolated from the building with expansion material, and have a downturned reinforced thickened edge.
Lateral Earth Pressures
Foundation and basement walls should be designed to resist lateral pressures. For non-expansive backfill materials, we recommend an equivalent fluid pressure of 40 pcf for design. Expansive soils or bedrock should not be used as backfill against walls. The above lateral pressure applies to level, drained backfill conditions. Equivalent Fluid Pressures for sloping/undrained conditions should be determined on an individual basis.