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HomeMy WebLinkAboutGeotech Report Final 4-29-16 Adapt Engineering 615 8th Avenue South Seattle, Washington 98104 Tel (206) 654-7045 Fax (206) 654-7048 www.adaptengr.com April 29, 2016 Adapt Project No. WA16-20531-GEO AT&T Mobility c/o Mastec Network Solutions 1203 114th Avenue SE Bellevue, Washington 98004 Attention: Dan Kelly Subject: Geotechnical Engineering Evaluation SD24 Cedar River 2439 SE Maple Valley Hwy Renton, Washington 98056 Dear Mr Kelly: Adapt Engineering (Adapt) is pleased to submit this report describing our recent geotechnical engineering evaluation for the above referenced tower site. The purpose of this study was to interpret general surface and subsurface site conditions, from which we could evaluate the feasibility of the project and formulate design recommendations concerning site preparation, equipment pad and tower foundations, access road, structural fill, and other considerations. Our scope of services consisted of a surface reconnaissance, a subsurface exploration, geotechnical analyses, and report preparation. Authorization to proceed with our study was given in the form of Mastec Network Solutions (Mastec) Purchase Order Number 832303 prior to our performing the work. This report has been prepared in accordance with general accepted geotechnical engineering practices for the exclusive use of AT&T Mobility (AT&T), Mastec, and their agents, for specific application to this project. Use or reliance upon this report by a third party is at their own risk. Adapt does not make any representation or warranty, express or implied, to such other parties as to the accuracy or completeness of this report or the suitability of its use by such other parties for any purpose whatever, known or unknown, to Adapt. Adapt Engineering 615 8th Avenue South Seattle, Washington 98104 Tel (206) 654-7045 Fax (206) 654-7048 www.adaptengr.com AT&T Mobility c/o Mastec Network Solutions Geotechnical Engineering Evaluation SD24 Cedar River Renton, Washington WA16-20531-GEO April 2016 Adapt Engineering AT&T Mobility c/o Mastec Network Solutions April 29, 2016 Adapt Project No. WA16-20531-GEO Page 1 PROJECT DESCRIPTION We understand that current development plans call for construction of a new 50-foot wood laminated telecommunication tower and equipment modifications to an existing associated cellular equipment cabinet pad. The site is located at 2439 SE Maple Valley Hwy in Renton, Washington; as shown on the attached Location/Topographic Map (Figure 1). The site may be accessed south off of Maple Valley Hwy through an existing asphalt parking lot. The existing and proposed site features, in relation to our exploration, are shown on the attached Site & Exploration Plan (Figure 2). It should be emphasized that the conclusions and recommendations contained in this report are based on our understanding of the currently proposed utilization of the project site, as derived from written and verbal information supplied to us by Ryka Consulting (Ryka). Consequently, if any changes are made to the project, we recommend that we review the changes and modify our recommendations, if appropriate, to reflect those changes. DOCUMENT REVIEW As a part of our study, we reviewed the following maps and documents pertaining to the subject property and vicinity: United States Department of Agriculture, Natural Resources Conservation Service (Formerly SCS), King County, Washington Washington State Department of Natural Resources, 2008, Tacoma Quadrant, King County, Washington, Washington State Geology Index. In addition, Adapt has reviewed the results of previous explorations accomplished in the immediate vicinity of the project. Our conclusions and recommendations are based in part or wholly on the information contained in these documents. Our geotechnical recommendations are based in part on the accuracy of these documents; Adapt assumes no responsibility for errors or omissions resulting from possible inaccuracies on these documents prepared by others. EXPLORATORY METHODS We explored surface and subsurface conditions at the project site on April 21, 2016. Our surface exploration consisted of a visual site reconnaissance. Our subsurface exploration consisted of advancing one test boring (designated B-1) to a maximum depth of approximately 41.5-feet below existing ground surface (bgs). The procedures used for subsurface exploration during our site visit are presented in the subsequent sections of this report. The location of the exploration advanced for this study is shown on the attached Figure 2. The specific location and depth of the exploration performed was selected in relation to the proposed site features, under the constraints of budget and site access. The boring location and other features shown on Figure 2 were obtained by hand taping from existing site features; as such, the exploration location shown should be considered accurate only to the degree implied by the measuring methods used. Adapt Engineering AT&T Mobility c/o Mastec Network Solutions April 29, 2016 Adapt Project No. WA16-20531-GEO Page 2 It should be noted that the exploration performed for this evaluation revealed subsurface conditions only at a discrete location across the project site and that actual conditions in other areas could vary. Furthermore, the nature and extent of any such variations would not become evident until additional explorations are performed or until construction activities have commenced. If significant variations are observed at the time of construction, we may need to modify our conclusions and recommendations contained in this report to reflect the actual site conditions. Auger Boring Procedures The boring was advanced using a track-mounted, hollow-stem auger drill rig operated by an independent company working under subcontract to Adapt. A geotechnical representative of Adapt was on-site to observe the boring, obtain representative soil samples, and log the subsurface conditions. After the boring was completed, the borehole was backfilled with a mixture of soil cuttings and bentonite chips. During drilling, soil samples were obtained on 5-foot depth intervals using the Standard Penetration Test (SPT) procedure (ASTM: D 1586). This test and sampling method consists of driving a standard 2-inch outside diameter (OD) split-barrel sampler a distance of 18 inches into the soil with a 140-pound hammer, free-falling a distance of 30 inches. The number of blows required to drive the sampler through each of the three, 6-inch intervals is noted. The total number of blows struck during the final 12 inches of penetration is considered the Standard Penetration Resistance, or “blow count”. If 50 or more blows are struck within one 6-inch interval, the driving is ceased and the blow count is recorded as 50 blows for the actual number of inches of penetration. The resulting Standard Penetration Resistance values provide a measure of the relative density of granular soils or the relative consistency of cohesive soils. The Boring Log attached to this report describes the various types of soils encountered in the boring, based primarily on visual interpretations made in the field and supported by our subsequent laboratory examination and testing. The log indicates the approximate depth of the contacts between different soil layers, although these contacts may be gradational or undulating. Where a change in soil type occurred between sampling intervals, we inferred the depth of contact. Our log also graphically indicates the blow count, sample type, sample number, and approximate depth of each soil sample obtained from the boring, along with any laboratory tests performed on the soil samples. If any groundwater was encountered in the boreholes, the approximate groundwater depths are depicted on the boring log. Groundwater depth estimates are typically based on the moisture content of soil samples, the wetted height on the drilling rods, and the water level measured in the borehole after the auger has been extracted. Subsurface materials encountered were logged and classified in general accordance with the Manual Visual Classification Method (ASTM D 2488) by the geotechnical representative. SITE CONDITIONS The following sections describe our observations, measurements, and interpretations concerning surface, soil, groundwater, and seismic conditions at the project site: Adapt Engineering AT&T Mobility c/o Mastec Network Solutions April 29, 2016 Adapt Project No. WA16-20531-GEO Page 3 Surface Conditions Our surface exploration consisted of a visual site reconnaissance. The proposed tower location is located approximately 20-feet northwest of an existing telecommunication equipment facility, in an approximately 14.5-foot wide grass median that separates an upper paved parking lot to the north with a lower paved parking lot to the south. The grass median has a concrete retaining wall on the southern boundary that is approximately 8-feet tall in the vicinity of the proposed tower location. The proposed tower, according to the “Proposed Enlarged Site Plans” provided by Ryka, is to be placed approximately 12-feet north of the concrete retaining wall. The grass median is slightly sloped to the south, towards the existing concrete retaining wall. Subsurface Conditions At the exploration location designated B-1, the test boring encountered approximately 1 to 2-inches of asphalt pavement overlying roughly 6-inches of silty sand with gravel and approximately 2-inches of relic concrete. Below the relic concrete, the test boring encountered very loose, silty sand until an approximate depth of 7.5-feet bgs, which we interpret to be possible fill associated with construction of the concrete retaining wall. Below these surficial soils, the test boring encountered very loose, poorly graded sand which showed silt laminations at approximately 10-feet bgs. At an approximate depth of 11-feet bgs, medium dense, poorly graded sand with gravel was encountered. Very dense, poorly graded gravel with silt and sand was encountered at approximately 20-feet bgs, overlying 10-feet of dense, poorly graded sand with silt and gravel. Below an approximate depth of 35-feet, dense, poorly graded gravel with silt and sand was encountered, which became very dense with depth and then extended to the full exploration depth of 41.5-feet bgs. Groundwater was encountered at an approximate depth of 25.5-feet bgs at the time of drilling. It should be noted that throughout the year, groundwater levels will likely fluctuate in response to changing precipitation patterns, off-site construction activities, and changes in site utilization. Seismic Conditions Based on our analysis of subsurface exploration logs and a review of published geologic maps, we interpret the on-site soil conditions to correspond to Site Class D, as defined by Table 20.3-1 within Chapter 20 of ASCE 7 in accordance with the 2012 International Building Code (IBC). The soil profile type for this site classification is characterized by stiff soils with an average blowcount between 15 and 50 within the upper 100 feet bgs. Current (2003) National Seismic Hazard Maps prepared by the U.S. Geological Survey indicate that peak bedrock site acceleration coefficients of about 0.321 and 0.625 are appropriate for an earthquake having a 10-percent and 2-percent probability of exceedance in 50 years (corresponding to return intervals of 475 and 2,475 years, respectively). The IBC mapped spectral accelerations for short periods at the subject site (SS and S1; Site Class B) are 140.8 and 48.1 (expressed in percent of gravity) at 0.2 and 1.0-second periods, respectively with 2 percent probability of exceedance in 50 years. In accordance with Tables 1613.5.3(1) and 1613.5.3(2), Site Coefficients, Fa and Fv, are 1.000 and 1.519, respectively for a Site Class C. Therefore the adjusted MCE ground motions are SMS=1.408g and SM1=0.731g. For purposes of seismic site characterization, the observed soil conditions were Adapt Engineering AT&T Mobility c/o Mastec Network Solutions April 29, 2016 Adapt Project No. WA16-20531-GEO Page 4 extrapolated below the exploration termination depth, based on a review of geologic maps and our knowledge of regional geology. CONCLUSIONS AND RECOMMENDATIONS Current development plans call for the construction of a new 50-foot wood laminated telecommunication tower and equipment modifications to an existing associated equipment cabinet pad within the proposed lease area. Based on the subsurface conditions revealed by our field exploration, we recommend that the proposed tower be supported on a drilled pier. A drilled pier can provide a cost-effective foundation for communication tower structures, provided that adequate embedment depths can be achieved with the drilled pier augering equipment, that the site is accessible to the drill rig, and that drilled pier contractors are available within a reasonable distance from the site. Alternatively, a reinforced concrete mat foundation may be selected if difficult drilling conditions are anticipated due to the presence of shallow bedrock or boulders, provided that the proposed lease area can accommodate the generally larger excavation plan area required for a mat foundation. Given the size and orientation of the tower and lease area, it does not appear likely that a mat foundation could be used due to the site constraints. For planning purposes, we have therefore provided design criteria for compressive, uplift and lateral support of a drilled pier foundation option below. Our specific recommendations concerning site preparation, equipment building or cabinet foundations, tower foundation, access driveway, and structural fill are presented in the subsequent sections. If further consideration of a mat foundation is warranted, Adapt may be contacted for design criteria Site Preparation Preparation of the lease area for construction should involve clearing, grubbing, stripping, cutting, filling, dewatering, and subgrade preparation. We provide the following comments and recommendations relative to site preparation. Temporary Drainage: We recommend intercepting and diverting any potential sources of surface or near-surface water within the construction zones before stripping begins. Because the selection of an appropriate drainage system will depend on the water quantity, season, weather conditions, construction sequence, and contractor's methods, final decisions regarding drainage systems are best made in the field at the time of construction. Nonetheless, we anticipate that curbs, berms, or ditches placed along the uphill side of the work areas will adequately intercept or divert surface water runoff away from the work area. Clearing and Stripping: After surface and near-surface water sources have been controlled, the construction areas should be cleared and stripped of all vegetation, topsoil, and debris. Any miscellaneous materials stored in this area should be relocated. Our site exploration indicated surface soil conditions consist of roughly 1 to 2-inches of asphalt overlying silty sand and relic concrete; all mantling silty sand which may be possible fill material, but significant variations could exist. It should also be realized that if the stripping operation proceeds during wet weather, a generally greater stripping depth might be necessary to remove disturbed, surficial, moisture-sensitive soils; therefore, stripping is best performed Adapt Engineering AT&T Mobility c/o Mastec Network Solutions April 29, 2016 Adapt Project No. WA16-20531-GEO Page 5 during a period of dry weather. Backfill materials, where required, should be placed and compacted according to the recommendations presented in the Structural Fill section of this report. Excavations: Site excavations ranging up to 2-feet deep are anticipated to accommodate the proposed equipment pad footings. Based on our exploration, we anticipate that these excavations will encounter roughly 1 to 2-inches of asphalt overlying silty sand and relic concrete; all mantling silty sand which may be possible fill material. We anticipate these surficial soils can be cut with conventional earth working equipment such as small dozers and trackhoes. Backfill materials, where required, should be placed and compacted according to recommendations presented in the Structural Fill section of this report. Temporary Cut Slopes: All temporary soil cuts (greater than 4-feet in height) associated with site excavations or regrading activities should be adequately sloped back to prevent sloughing and collapse, unless a shoring box or other suitable excavation side wall bracing is provided. We tentatively recommend a maximum cut slope inclination of 1.5H:1V (Horizontal:Vertical) within the surficial soils that will likely be exposed within the upper 4-feet below the ground surface across the site. If groundwater seepage is encountered within the excavation slopes, the cut slope inclination may need to be on the order of 2H:1V, or flatter. However, appropriate inclinations will ultimately depend on the actual soil, rock and groundwater seepage conditions exposed in the cuts at the time of construction. It is the responsibility of the contractor to ensure that the excavation is properly sloped or braced for worker safety protection, in accordance with OSHA safety guidelines. In addition to proper sloping, the excavation cuts should be draped with plastic sheeting for the duration of the excavation to minimize surface erosion and ravelling. Dewatering: Based on our site reconnaissance investigation, we do not anticipate significant groundwater seepage within the upper 2-feet. However, perched groundwater may be encountered depending on the actual excavation depth and the time of year that construction proceeds. If groundwater is encountered, we anticipate that an internal system of ditches, sump holes, and pumps will be adequate to temporarily dewater the excavations. Subgrade Preparation: Exposed subgrades for shallow footings, slabs-on-grade, roadway sections and other structures should be compacted to a firm, unyielding state, if required to achieve adequate density and warranted by soil moisture conditions. Any localized zones of loose, granular soils observed within a subgrade area should be compacted to a density commensurate with the surrounding soils. In contrast, any uncontrolled fill material or organic, soft, or pumping soils observed within a subgrade should be overexcavated and replaced with a suitable structural fill material. Frozen Subgrades: If earthwork takes place during freezing conditions, we recommend that all exposed subgrades be allowed to thaw and be recompacted prior to placing foundations or subsequent lifts of structural fill. Adapt Engineering AT&T Mobility c/o Mastec Network Solutions April 29, 2016 Adapt Project No. WA16-20531-GEO Page 6 Equipment Foundations Based on available site plans, it is our understanding that the existing equipment pad will accommodate the proposed equipment upgrades associated with the project. Therefore, it does not appear necessary to construct a new equipment pad. However, should it be necessary to provide an extension or improvement to the existing equipment pad, we recommend that, support for the equipment cabinet pad will consist of a poured-in-place, concrete slab-on-grade with thickened edges; we recommend that these thickened slab edges be designed as spread footings. Alternatively, the equipment support pad may be designed as a structural slab-on-grade with a uniform thickness and a reduced bearing pressure. In either case, we anticipate that the support pad bearing pressure will be relatively light. The following sections provide our recommendations and comments for equipment pad design and construction. Consequently, if any changes are made to the project, we recommend that we review the changes and modify our recommendations, if appropriate, to reflect those changes. Subgrade Conditions: The prepared bearing subgrade soils should consist of firm and unyielding, silty sand. Exposed slab-on-grade, footing or overexcavation subgrades should be compacted to a firm, unyielding state, in accordance with the recommendations provided in the Site Preparation section of this report. Subgrade Verification: Footings or slabs-on-grade should never be cast atop soft, loose, organic, or frozen soils; nor atop subgrades covered by standing water. A representative from Adapt should be retained to observe the condition of footing subgrades before concrete is poured to verify that they have been adequately prepared. Bearing Subgrades: The proposed shallow spread footing system is expected to be founded on silty sand. Before concrete is placed, any localized zones of loose soils encountered in the footing subgrades should be compacted to a firm, unyielding condition, if warranted by soil moisture conditions. Any uncontrolled fill material containing a significant amount of organic or debris/deleterious materials within the basement footprint area will need to be overexcavated and replaced with structural fill, as discussed below. Footing Dimensions: For a poured-in-place, concrete slab-on-grade with thickened-edge footings, we recommend that the spread footing elements be constructed to have a minimum width of 12-inches. For frost protection, we recommend that the footings exposed to frost at this site penetrate at least 18-inches below the lowest adjacent exterior grades, or deeper, according to local jurisdictional code. Bearing Pressure and Lateral Resistance: Owing to the presence of loose, silty sand at shallow depth, we recommend limiting the maximum allowable static soil bearing pressure of 1,750 pounds per-square-foot (psf) for thickened-edge pad footings designed as described. For the alternate equipment support pad design using a uniform thickness, structural slab-on-grade, we recommend a maximum allowable static soil bearing pressure of 300 psf across the pad area. These bearing pressure values can be increased by one-third to accommodate transient wind or seismic loads. An allowable base friction coefficient of 0.30 Adapt Engineering AT&T Mobility c/o Mastec Network Solutions April 29, 2016 Adapt Project No. WA16-20531-GEO Page 7 and an allowable passive earth pressure of 150 pounds per cubic foot (pcf), expressed as an equivalent fluid unit weight, may be used for that portion of the foundation embedded more than 1-foot below finished exterior subgrade elevation. These lateral resistance values incorporate a minimum safety factor of 1.5. Grading and Capping: Final site grades should slope downward away from the structure so that runoff water will flow by gravity to suitable collection points, rather than ponding near the structure. Ideally, the area surrounding the structure would be capped with concrete, asphalt, or compacted, low-permeability (silty) soils to reduce surface-water infiltration into the subsoils adjacent to/below the foundation. Settlements: We estimate that total post-construction settlements of properly designed thickened-edge footings bearing on properly prepared subgrades will be less than 1-inch, with differential settlements approaching one-half of the total. For a structural slab-on-grade equipment pad with a uniform thickness (without thickened edges), somewhat greater movements may be experienced. Tower Drilled Pier Foundations The subsurface soil and groundwater conditions observed in our site exploration are considered to be generally suitable for the use of a drilled pier foundation to support the proposed tower. The following recommendations and comments are provided for purposes of drilled pier design and construction. End Bearing Capacities: We recommend that the drilled pier be founded below approximately 15-feet below the ground surface. For vertical compressive soil bearing capacity, we recommend using the unit end bearing capacity presented in Table 1 below, where B is the diameter of the pier in feet and D is the depth into the bearing layer in feet, in accordance with the EIA/TIA G-code. This ultimate end bearing capacity does not include a safety factor. Table 1 Ultimate End Bearing Capacity Depth (feet) Ultimate Bearing Capacity (tsf) Limiting Point Resistance (tsf) 15-25 4.0 D/B 3.5 Frictional Capacities: For frictional resistance along the shaft of the drilled piers, acting both downward and in uplift, we recommend using the ultimate skin friction value listed in Table 2. We recommend that frictional resistance be neglected in the uppermost 2-feet below the ground surface. The ultimate skin friction values presented do not include a safety factor, in accordance with the provisions of the EIA/TIA 222-G code. Adapt Engineering AT&T Mobility c/o Mastec Network Solutions April 29, 2016 Adapt Project No. WA16-20531-GEO Page 8 Table 2 Ultimate Skin Friction Capacities Depth (feet) Ultimate Skin Friction (tsf) 0-2 2-10 10-20 20-30 0.00 0.05 0.35 0.65 Lateral Capacities: Drilled pier foundations for communication towers are typically rigid and act as a pole, which rotates around a fixed point at depth. Although more complex and detailed analyses are available, either the simplified passive earth pressure method or the subgrade reaction method is typically used to determine the pier diameter and depth required to resist groundline reaction forces and moments. Due to the proximity of the adjacent retaining wall, we have reduced our recommended passive pressure above the wall foundation to reduce the potential for additional lateral pressure caused by the pole foundation. These methods are described below.  Passive Earth Pressure Method: The passive earth pressure method is a simplified approach that is generally used to estimate an allowable lateral load capacity based on soil wedge failure theory. Although the lateral deflection associated with the soil wedge failure may be estimated, design lateral deflections using the passive earth pressure method should be considered approximate, due to the simplified nature of the method. According to the NAVFAC Design Manual 7.02 (1986), a lateral deflection equal to about 0.001 times the pier length would be required to mobilize the allowable passive pressure presented below; higher deflections would mobilize higher passive pressures. The ultimate passive pressure may be taken as the product of the allowable pressure and factor of safety. Our recommended passive earth pressures for the soil layers encountered at this site are presented in Table 3 and do not incorporate a safety factor. These values are expressed as equivalent fluid unit weights, which are to be multiplied by the depth (bgs) to reflect the linear increase within the depth interval of the corresponding soil layer. The passive earth pressures may be assumed to act over an area measuring two pier diameters wide by up to eight pier diameters deep. Table 3 Ultimate Passive Pressures Depth (feet) Ultimate Passive Pressure (pcf) 0-2 2-10 10-20 20-30 0 250 350 450 Adapt Engineering AT&T Mobility c/o Mastec Network Solutions April 29, 2016 Adapt Project No. WA16-20531-GEO Page 9  Subgrade Reaction Method: The subgrade reaction method is typically used to compute lateral design loads based on allowable lateral deflections. Using this method, the soil reaction pressure (p) on the face of the pier is related to the lateral displacement (y) of the pier by the horizontal subgrade modulus (kh); this relationship is expressed as p=khy. Because soil modulus values are based on small scale, beam load test data, and are usually reported as a vertical subgrade modulus (kv), they must be converted to horizontal subgrade modulus values representative for larger scale applications (such as large pier diameters) by means of various scaling factors, as discussed below. In addition to the scaling and loading orientation, the soil-pier interaction governing kh is also affected by the soil type, as follows:  SAND and Soft CLAY: For cohesion-less soils (sand, non-plastic silt) and soft cohesive soils (clay, cohesive silt), the horizontal subgrade modulus (kh) increases linearly with depth (z). This relationship is expressed as kh = nhz(1/B), where nh is the coefficient of horizontal subgrade reaction and (1/B) is the scaling factor.  Stiff or Hard CLAY: For stiff or hard cohesive soils (clay, cohesive silts), the horizontal subgrade modulus (kh) is essentially the same as the vertical subgrade modulus (kv) and is considered constant with depth. This relationship is expressed as kh=kv[1(ft)/1.5B], where [1(ft)/1.5B] is the scaling factor (B is expressed in feet). Our recommended values for the coefficient of horizontal subgrade reaction (nh) and the vertical subgrade modulus (kv) for the soil layers encountered at this site are presented in Table 4 below. These values do not include a factor of safety since they model the relationship between contact pressure and displacement and are ultimate values. We have reduced our recommended horizontal subgrade reaction coefficient design value to limit the potential for higher lateral pressures on the nearby retaining wall. Therefore, the structural engineer or monopole manufacturer should select an appropriate allowable displacement for design, based on the specific requirements of the communication equipment mounted on the tower. Adapt Engineering AT&T Mobility c/o Mastec Network Solutions April 29, 2016 Adapt Project No. WA16-20531-GEO Page 10 Table 4 Recommended Horizontal Subgrade Reaction Values Depth Interval (feet) nh (pci) kv (pci) 0-2 2-10 10-20 20-30 0 3 20 40 N/A N/A N/A N/A Coefficient of Horizontal Subgrade Reaction (pci) kh= nh(z/B) (Sand & Soft Clay) kh=kv/(1.5B) (Stiff Clay) Construction Considerations: At the exploration location designated B-1, approximately 1 to 2-inches of asphalt pavement overlying roughly 6-inches of silty sand with gravel and approximately 2-inches of relic concrete. Below the relic concrete, the test boring encountered very loose silty sand to an approximate depth of 7.5-feet bgs, which we interpret to be possible fill associated with construction of the concrete retaining wall. Below these surficial soils, the test boring encountered very loose, poorly graded sand which showed some silty laminations near 10-feet bgs. At an approximate depth of 11-feet bgs, medium dense, poorly graded sand with gravel was encountered. Very dense, poorly graded gravel with silt and sand was encountered at approximately 20-feet bgs, overlying 10-feet of dense, poorly graded sand with silt and gravel. Below an approximate depth of 35-feet, dense, poorly graded gravel with silt and sand was encountered, which became very dense with depth and then extended to the full exploration depth of 41.5- feet bgs. The presence of large gravel was indicated during advancement of our test boring; therefore, the drilled pier contractor should anticipate the possibility of difficult drilling conditions and presence of large particles. Groundwater was encountered at an approximate depth of 25.5-feet bgs at the time of drilling. Dewatering may be required depending on the actual depth and time of year of drilled pier construction. The foundation-drilling contractor should be prepared to case the excavation to prevent caving and raveling of the pier shaft sidewall, if necessary due to unexpected soil or excessive groundwater seepage conditions. Should heavy groundwater inflow be encountered in the drilled pier excavation, it may be necessary to pump out the accumulated groundwater prior to concrete placement, or to use a tremie tube to place the concrete from the bottom of the drilled pier excavation, thereby displacing the accumulated water during concrete placement. Alternatively, the use of bentonite slurry could be utilized to stabilize the drilled pier excavation. Drilled Pier Excavation Conditions: The drilling contractor should be prepared to clean out the bottom of the pier excavation if loose soil is observed or suspected, with or without the presence of slurry or groundwater. As a minimum, we recommend that the drilling contractor have a cleanout bucket on site to remove loose soils and/or mud from the bottom of the pier. If groundwater is present and abundant within Adapt Engineering AT&T Mobility c/o Mastec Network Solutions April 29, 2016 Adapt Project No. WA16-20531-GEO Page 11 the pier hole, we recommend that the foundation concrete be tremied from the bottom of the hole to displace the water and minimize the risk of contaminating the concrete mix. The Drilled Shaft Manual published by the Federal Highway Administration recommends that concrete be placed by tremie methods if more than 3 inches of water has accumulated in the excavation. Access Driveway Based on available site plans and our site reconnaissance visit, it does not appear necessary to construct a new access road. Should it be necessary to provide an extension to the existing roadways or to improve the existing access roads, we recommend that the subgrade be prepared in accordance with the Site Preparation section of this report. For planning purposes, we anticipate that 6 to 12-inches of “clean” sand and gravel subbase material and a minimum 3-inches of crushed rock surfacing will be required to create a stable gravel roadway surface at this site. Adapt can provide additional subgrade stabilization or gravel road section recommendations based on observed field conditions at the time of construction. Where cuts and fills are required, they should be accomplished in accordance with the recommendations provided in the Site Preparation and Structural Fill sections of this report. Structural Fill The following comments, recommendations, and conclusions regarding structural fill are provided for design and construction purposes. Materials: Structural fill includes any fill materials placed under footings, pavements, driveways, and other such structures. Typical materials used for structural fill include: clean, well-graded sand and gravel (pit-run); clean sand; crushed rock; controlled-density fill (CDF); lean-mix concrete; and various soil mixtures of silt, sand, and gravel. Recycled concrete, asphalt, and glass, derived from pulverized parent materials may also be used as structural fill. Placement and Compaction: Generally, CDF, and lean-mix concrete do not require special placement and compaction procedures. In contrast, pit-run, sand, crushed rock, soil mixtures, and recycled materials should be placed in horizontal lifts not exceeding 8 inches in loose thickness, and each lift should be thoroughly compacted with a mechanical compactor. Using the modified Proctor maximum dry density (ASTM: D-1557) as a standard, we recommend that structural fill used for various on-site applications be compacted to the following minimum densities: Fill Application Minimum Compaction Slab/Footing subgrade 90 percent Gravel drive subgrade (upper 1 foot) 95 percent Gravel drive subgrade (below 1 foot) 90 percent Subgrades and Testing: Regardless of location or material, all structural fill should be placed over firm, unyielding subgrade soils. We recommend that a representative from Adapt be retained to observe the condition of subgrade soils before fill placement begins, and to perform a series of in-place density tests Adapt Engineering AT&T Mobility c/o Mastec Network Solutions April 29, 2016 Adapt Project No. WA16-20531-GEO Page 12 during soil fill placement. In this way, the adequacy of soil compaction efforts may be evaluated as earthwork progresses. Fines Content: Soils used for structural fill should not contain individual particles greater than about 6 inches in diameter and should be free of organics, debris, and other deleterious materials. Given these prerequisites, the suitability of soils used for structural fill depends primarily on the grain-size distribution and moisture content of the soils when they are placed. When the “fines” content (that soil fraction passing the U.S. No. 200 Sieve) increases, soils become more sensitive to small changes in moisture content. Soils containing more than about 5 percent fines (by weight) cannot be consistently compacted to a firm, unyielding condition when the moisture content is more than about 2 percentage points above optimum. The near surface silty sand encountered by the test boring should be considered to be highly moisture sensitive. The poorly graded sands and gravels encountered at depth should be considered to be moderately moisture sensitive. The use of “clean” soil is necessary for fill placement during wet-weather site work, or if the in-situ moisture content of the sandy site soils is too high to allow adequate compaction. Clean soils are defined as granular soils that have a fines content of less than 5 percent (by weight) based on the soil fraction passing the U.S. 3/4-inch Sieve. CLOSURE We have prepared this report for use by the owner/developer and other members of the design and construction team for the proposed SD24 Cedar River tower site. The opinions and recommendations contained within this report are not intended to be, nor should they be, construed as a warranty of subsurface conditions, but are forwarded to assist in the planning and design process. We have made observations based on our explorations that indicate the soil conditions at only those specific locations and only to the depths penetrated. These observations do not necessarily reflect soil types, strata thickness, or water level variations that may exist in other locations. If subsurface conditions vary from those encountered in our site exploration, Adapt should be alerted to the change in conditions so that we may provide additional geotechnical recommendations, if necessary. The future performance and integrity of the improvements will depend largely on proper initial site preparation, drainage, and construction procedures. Observation by experienced geotechnical personnel should be considered an integral part of the construction process. The conclusions and recommendations contained in this report are based on our understanding of the currently proposed project, as derived from written and verbal information supplied to us by Ryka. When the design has been finalized, we recommend that we review the design and specifications to see that our recommendations have been interpreted and implemented as intended. If design changes are made, we request that we be retained to review our conclusions and recommendations and to provide a written modification or verification. The scope of our services does not include services related to construction safety precautions, and our recommendations are not intended to direct the contractor’s methods, techniques, sequences, or procedures, except as specifically described in our report for consideration in design. Adapt Engineering AT&T Mobility c/o Mastec Network Solutions April 29, 2016 Adapt Project No. WA16-20531-GEO Page 13 Within the limitations of scope, schedule, and budget, our services have been executed in accordance with the generally accepted practices in this area at the time this report was prepared. No warranty or other conditions, express or implied, should be understood.