5.0 SITE CONDITIONS. 5.1 Surface Conditions

Transcription

1 5.0 SITE CONDITIONS 5.1 Surface Conditions The project site is located at approximately along Ashdown Forest Road and Nichols Canyon Road in the Fiddlers Canyon Area of Cedar City, Utah. At the time of our field investigation, the study area was undeveloped with some rough cut roads passing through the site. The site was sloped slightly down to the northwest within the study area. Vegetation consisted of weeds, grasses, trees and brush typical of the area. The site was bounded by subdivisions to the northwest and by vacant land in all other directions. 5.2 Subsurface Conditions During the explorations performed for this investigation the subsurface conditions were observed to generally consist of approximately, one foot to two feet of soft clayey sand with some organics, overlying loose to medium dense silty sand which extended to the bottom of the test holes at a depth of approximately 10 feet below the existing site grade. Within exploration T-1 through T-5 medium dense sandy gravel with cobbles was encountered at a depth of four feet below the existing site grade. The subsurface soils were generally slightly moist to moist condition throughout the depths explored. The subsurface conditions encountered in our explorations are described in detail on the enclosed trench logs. Groundwater was not encountered within the explorations. Numerous factors contribute to fluctuations in groundwater levels. The evaluation of these factors was beyond the scope of this study. The stratification lines shown on the enclosed trench represent the approximate boundaries between soil types. The actual in-situ transitions may be gradual. Due to the nature and depositional characteristics of the native soils, care should be taken m interpolating subsurface conditions between and beyond the exploration locations. The laboratory test results indicated that the on-site soils exhibited a low degree of solubility and a high collapse potential. RG063 Copyright GC:in ENGINEERING, INC.

2 6.0 ENGINEERING ANALYSES AND RECOMMENDATIONS 6.1 General Based on the subsurface conditions encountered at the site, it is our opinion that the subject site is suitable for construction provided that the recommendations contained in this report are complied with. Specifically, it is our opinion that the loose clayey sand with organic materials encountered within the top one to two feet of the explorations are not considered suitable for support of the proposed structures or pavements in their present condition. Additionally, the hydro-collapsible soils which exist over the entire site are not considered suitable for the support of footings, slabs-on-grade or pavements. The engineering properties of these soils can be improved by overexcavation and recompaction. The majority of the on-site soils free of organics should be suitable for reuse as structural fill. However, care should be taken to remove all debris from the soils exist at the site. The proposed structures should then receive adequate support from conventional foundations established on a zone of structural fill. Overexcavation will not be required in areas where medium dense sandy gravel soils are encountered which are at least 2 feet in thickness. The anticipated zone of overexcavations should extend a minimum of 2~ feet below footings or 2~ feet below the existing site grade whichever is greater. Overexcavations may be terminated on competent medium dense granular soils if encountered. Competent medium dense native soils were encountered within most of the explorations. Slabs-on-grade, exterior concrete flatwork and pavements should be supported by a zone of properly placed and compacted structural fill. Overexcavations on the order of 18 inches below the supportive gravel layer or 18 inches below the existing site grade site grade, whichever is greater, will be required. The majority of the on-site soils should be suitable for use as compacted structural fill. The following sections of this report present our recommendations, to reduce the potential for structural damage due to the presence of compressible soils and contains specific opinions and recommendations concerning construction considerations, site preparation and grading, structural fill, foundation design, sub grade and retaining walls, RG063 Copyright Gi::ill ENGINEERING, INC.

3 concrete slabs-on-grade, soil corrosion, moisture protection, and structural pavement sections. 6.2 CONSTRUCTION CONSIDERATIONS Foundation Systems After overexcavation and recompaction are completed, the structure can be supported by conventional strip and/or spread footing founded on properly placed and compacted structural fill. 6.3 EARTHWORK Site Preparation and Grading Within the areas to be graded, existing vegetation, and debris should be removed and hauled off the site. Any undocumented fill soils, soft, loose, and/or disturbed-loose native soils should be excavated. The proposed structure should then receive support from conventional foundations established upon on a zone of structural fill. The anticipated zone of overexcavations should extend a minimum of 2V2 feet below footings or 2V2 feet below the existing site grade whichever is greater. Overexcavations may be terminated on competent medium dense granular soils if encountered. Slabs-on-grade, exterior concrete flatwork and pavements should be supported by a zone of properly placed and compacted structural fill. Overexcavations on the order of 18 inches below the supportive gravel layer or 18 inches below the existing site grade site grade, whichever is greater, will be required. The majority of the on-site soils should be suitable for use as compacted structural fill. Overexcavations may be terminated on competent medium dense native soils if encountered. Site preparation operations should extend laterally a minimum of 5 feet beyond the perimeter of the structure and/or foundations and 2 feet beyond exterior flatwork and RG063 Copyright i3c:in ENGINEERING, INC.

4 pavement areas, or to a distance equal to the depth of overexcavation, which ever is greater. A GEM Engineering representative should be present during grading operations to observe that the recommended removals have been accomplished. If soft soils are exposed in the excavation bottoms, further excavation or stabilization may be required prior to the placement of structural fill. A GEM Engineering representative should be present during grading operations to observe that the recommended removals have been accomplished. Native soils exposed after overexcavation has been accomplished should be scarified to a depth of at least 6 inches, moisture conditioned to within 2 percent of the optimum moisture content for granular soils, and slightly above optimum for fine-grained soils, and recompacted to at least 90 percent of the maximum dry density as determined by ASTM D The site should then be brought to rough pad grade with structural fill as described in the following section. Sub grade materials supporting slabs-on-grade, exterior concrete flatwork, and pavements should be kept moist and undisturbed. If the sub grade has been disturbed prior to pouring the concrete slabs, the exposed soils should be moisture-conditioned and recompacted as outlined in the Structural fill section of this report. We recommend that the Geotechnical Engineer be allowed to review the grading plans when prepared, to evaluate the compatibility of these recommendations Excavations The majority of the soils encountered in our explorations should be readily excavatable with conventional earthwork equipment. It is also possible that soft pumping soils may be encountered. Pumping soils will need to be stabilized prior to placing of structural fill. Safety of construction personnel is the responsibility of the Contractor Material Volume Changes There will be shrinkage losses when excavating and compacting the on-site soils. An estimated average shrinkage factor of 20 to 25 percent is applicable for the soft, loose and RG063 Copyright iic:m ENGINEERING, INC.

5 medium stiff near surface native soils. A subsidence factor of 0.1 feet should be used in all areas where the surficial soils are scarified and recompacted to a depth of 6 inches Structural Fill All fill placed for the support of slabs-on-grade, exterior concrete flatwork, and pavements should consist of structural fill. Structural fill may consist of approved excavated on-site soils or imported fill materials. Structural fill should have a swell potential less than 4 percent under a 60 psf surcharge, have solubility less than 3 percent, and be free of organics, salts, or inert materials larger than 4 inches nominal size, and be similar in gradation to the on-site soils. Structural fill should be placed in maximum eight-inch loose lifts and compacted on a horizontal plane, unless otherwise approved by the Geotechnical Engineer. Soils in compacted fills should be compacted to at least 90 percent of the maximum dry density as determined by ASTM-D1557 for fine grained soils and 95 percent for granular soils. The moisture content should be within 2 percent of optimum for granular soils and at least 2 percent above optimum for fine-grained soils. Any imported fill materials should be approved prior to importing. Also, prior to placing any fill, the excavations should be observed by a GEM Engineering representative to observe that unsuitable materials have been removed. 6.4 FOUNDATIONS Conventional Foundations General: Conventional shallow foundations consisting of strip and/or spread footings can be utilized for the proposed building provided that overexcavation of the surficial loose, soft and/of collapsible soils is accomplished. Foundation areas should be prepared in accordance with site preparation recommendations previously provided. Exterior conventional spread foundations should be established at least 30 inches below the lowest adjacent final compacted sub grade for frost protection. RG063 Copyright ui::in ENGINEERING, INC.