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330 Land Company, LLC Arroyo Lago <br />9785.004.001 Design-Level Geotechnical Report <br /> <br /> <br /> Page | 21 April 16, 2024 <br />Latest Revision October 31, 2025 <br />• preventing water from draining toward or ponding near building foundations, slabs-on-grade, <br />or pavements. <br />• avoiding open planting areas within 3 feet of the building perimeter. <br /> <br />We recommend that these items be incorporated into the landscaping plans. <br /> <br />6.0 FOUNDATION RECOMMENDATIONS <br /> <br />6.1 POST-TENSIONED MAT FOUNDATIONS <br /> <br />After completion of a surcharge program, the proposed lightly to moderately loaded residential <br />structures may be supported on post-tensioned (PT) mat foundations bearing on prepared <br />engineered fill. PT mats may be designed for an average allowable bearing pressure of up to <br />1,500 pounds per square foot (psf) for dead-plus-live loads with maximum localized bearing <br />pressures of 2,000 psf at column or wall loads. The allowable bearing pressures can be increased <br />by one-third for wind or seismic loads. PT mats should be designed using the criteria presented <br />in Table 6.1-1 below. <br /> <br />TABLE 6.1-1: Post-Tensioned Mat Design Recommendations <br />CONDITION CENTER LIFT EDGE LIFT <br />Edge Moisture Variation Distance, em (feet) 8.9 4.9 <br />Differential Soil Movement, ym (inches) 1.1 1.7 <br /> <br />The above values are based on the procedure presented by the Post-Tensioning Institute “Design <br />of Post-Tensioned Slabs-on-Ground” Third Edition, including appropriate addenda (2013). <br />PT mats should be underlain with a moisture reduction system, as recommended in Section 6.2. <br />In addition, moisture conditioning of the building foundation subgrade should be to a moisture <br />content at least 4 percentage points above optimum immediately prior to foundation construction. <br />The subgrade should not be allowed to dry prior to concrete placement. We also recommend that <br />we be retained to observe the pre-pour moisture conditions to check that our report <br />recommendations have been followed. <br /> <br />6.2 FOUNDATION SUBGRADE MOISTURE VAPOR REDUCTION <br /> <br />When buildings are constructed with concrete slab-on-grade floors, including PT mats, water <br />vapor from beneath the slab will migrate through the slab and into the building. This water vapor <br />can be reduced but not stopped. Vapor transmission can negatively affect floor coverings and <br />lead to increased moisture within a building. When water vapor migrating through the slab would <br />be undesirable, we recommend the following to reduce, but not stop, water vapor transmission <br />upward through the slab-on-grade. <br /> <br />1. A moisture retarder system should be constructed directly beneath the slab-on-grade or PT <br />mat that consists of the following. <br />a. Vapor retarder membrane sealed at all seams and pipe penetrations and connected to all <br />footings. Vapor retarders should conform to Class A vapor retarder in accordance with <br />ASTM E 1745, latest edition, “Standard Specification for Plastic Water Vapor Retarders <br />used in Contact with Soil or Granular Fill under Concrete Slabs.” The vapor retarder should <br />be underlain by