WENNER RESISTIVITY TESTING

Transcription

1 June 21, 2013 Subject: Soil Resistivity Testing, Eagle Radio Tower, Eagle County, Colorado Mr. Rick Ullom Eagle County, Construction Manager Project Management Department PO Box 850 Eagle, Colorado Job No Dear Mr. Ullom: At your request, GROUND Engineering Consultants, Inc. (GROUND) performed 4-point (Wenner) method soil resistivity testing at an existing Radio Tower site located north of Hwy 6 between the Towns of Gypsum and Eagle in Eagle County, Colorado. The observations, methods utilized and data obtained from the field testing are summarized below. WENNER RESISTIVITY TESTING The Wenner 4-Pin Resistivity testing for the project was conducted at the radio tower site on June 18 th, Due to site constraints, an array of three tests were performed at various orientations in the general location of the proposed pre-fabricated equipment storage building in the open area just south of the radio tower. At the time of the testing, the air was mostly dry, it was cloudy and the ambient temperature was in the mid-70s ºF. There had been no significant precipitation events over the last 72 hours. The project site existed mainly as undeveloped, mountainous terrain. There is an unimproved access road that approaches from the south and terminates at the existing radio tower. The site is primarily atop a small mountain outcrop, steeply sloping on the north side and overlooking the Eagle River Valley. The on-site soils appeared to consist mainly of sandy silts and clays with occasional gravel and cobble with numerous protrusions of dense bedrock materials scattered PO Box 464, 101A Airpark Drive #9, Gypsum, CO Phone (970) Fax (970) Office Locations: Englewood Commerce City Loveland Granby Gypsum Grand Junction

2 August 15, 2013 Subject: Subsurface Exploration Program and Geotechnical Recommendations, Eagle Radio Tower Equipment Building, Eagle County, Colorado Job No Mr. Rick Ullom Eagle County, Construction Manager Project Management Department PO Box 850 Eagle, Colorado Dear Mr. Ullom: This letter presents the results of a subsurface exploration program to evaluate the bearing conditions of the proposed 10 foot by 16 foot, pre-fabricated equipment building for the existing radio tower site located north of Hwy 6 between the Towns of Gypsum and Eagle in Eagle County, Colorado. SUBSURFACE EXPLORATION The subsurface exploration for the project was conducted on July 24, A total of two (2) test pits were excavated with a rubber-tire Komatsu WB 150 AWS backhoe to evaluate the bearing conditions in two locations for the equipment building. The test pits were advanced to depths ranging from approximately 3 to 7 feet below existing grades at which point practical excavator refusal was encountered. A GROUND engineer directed the subsurface exploration, logged the test pits in the field, and prepared the soil samples for transport to our laboratory. Samples of the subsurface materials from the sides of the test pits were retrieved with hand driven 2-inch I.D. brass liners. Additionally, large disturbed samples were collected from the excavator spoils. Depths at which the samples were obtained are shown on the test hole logs. The approximate locations of the test pits are shown in Figure 1. Logs of the exploratory test pits are presented in Figures 2 and 3. Explanatory notes and a legend are provided in Figure 4. LABORATORY TESTING Samples retrieved from our test holes were examined and visually classified in the laboratory by the project engineer. Laboratory testing of soil samples obtained from the subject site included standard property tests, such as natural moisture contents, dry unit weights, grain size analyses, liquid and plastic limits, and water-soluble sulfate. Standard Proctor testing was also PO Box 464, 101A Airpark Drive #9, Gypsum, CO Phone (970) Fax (970) Office Locations: Englewood Commerce City Loveland Granby Gypsum Grand Junction

3 Radio Tower Equipment Building Eagle County, CO completed on a composite bulk sample. Laboratory tests were performed in general accordance with applicable ASTM protocols. Results of the laboratory testing program are summarized in Figures 5 through 8 and on Table 1. SUBSURFACE CONDITIONS The subsurface conditions encountered in the Test Pit 1 generally consisted of silt/clay, sand, gravel, and cobbles to practical drill rig refusal at depths of 3 and 7 feet. A layer containing what appeared to be fire altered silt was also encountered. This material generally consisted of black gravel to cobble sized rock in a silt/clay, sand, gravel and cobble matrix, but was otherwise similar in terms of engineering properties to the remainder of the materials encountered. The layer appeared to increase in thickness to the south and east, ranging from 2 feet below ground surface and 1.5 feet in thickness to 3 feet below ground surface and 4 feet in thickness. We understand there are anecdotal reports of various groups of local youth having periodic bonfires over the last several decades, which may be the source of possible alteration. The subsurface conditions in Test Pit 2 generally consisted of sand and silt with occasional gravels, and very hard bedrock at 3 feet and associated excavator refusal. Representative photos of the test pits are shown in Appendix A. Silt/Clay, Sand, Gravel, and Cobbles contained fine to coarse grained sand and fine to medium sized cobbles, were estimated to be loose to medium dense, slightly moist to moist, had medium to high plasticity, and were light brown in color. Possible Fire Altered Materials were generally the same as the silt/clay, sand, gravel, and cobbles, however the fire altered materials were somewhat friable to non-friable and generally broke down upon sieving in the laboratory, however some fragments were more durable. These materials were brown to dark brown to black in color. Sand and Silt were interlayered and had a fine to coarse sand fraction and occasional fine gravels, were estimated to be loose to medium dense or medium stiff to very stiff, slightly moist, had medium to high plasticity, and were light brown in color. SEISMIC CLASSIFICATION Based on extrapolation to depth of the subsurface data obtained for this study, and our experience in the project area, GROUND estimates that the site will likely meet the characteristics of a Site Class B site, according to the 2006/2009 IBC classification (Table ). To determine the site class quantitatively would require drilling and testing to a depth of at least 100 feet. Based on the site coordinates, the USGS s Earthquake Ground Motion Tool v.5.0.9a indicates an SD S value of g and an SD 1 value of g, for a Site Class B. Job No GROUND Engineering Consultants, Inc. Page 2

4 Radio Tower Equipment Building Eagle County, CO FOUNDATION We were provided with a copy of the pre-fabricated building plans by Thermobond Buildings, dated August 14, Based on the provided plans, we understand the pre-fabricated building will rest on a slab foundation with thickened edges. The slab foundation will be six inches thick near the center and 18 inches along the edges and rest on top of a six inch layer of gravel. The in-place site soils appear suitable in general to support the proposed pre-fabricated building. Based on our observations and the available geotechnical data, we estimate that the soils represent an allowable bearing capacity of 1,500 psf at Test Pit 1 and between 750 and 1,000 psf at Test Pit 2. Items of note at Test Pit 1 include the possible fire altered materials. Although the materials encountered in the test pit appeared adequate to support the building, if the materials are associated with fire alteration it would stand to reason that it may be possible for buried loose deposits of ash and burned debris to be located nearby. Items of note at Test Pit 2 include the very low maximum dry density, high optimum moisture content, and high plasticity of the silt soils as shown on a Proctor of the material (Figure 8). This material is likely acceptable at the in-place moisture content, however once it has been disturbed it will require significant amounts of construction water to be placed as properly compacted fill and will be difficult to work with. We recommend foundations placed on these materials be placed on firm, undisturbed soils. Loose soils exposed during foundation excavation should be removed and replaced with non-silty on-site soils or imported fill. Alternatively, a lean concrete mud mat can be placed to achieve foundation subgrade where loose soils need to be removed. Given the shallow depth to bedrock at the Test Pit 2 location, it may be more feasible to found the building on short, circular drilled footings for a building in that location. It should also be noted that the foundation will not be located below the local frost depth, and some seasonal frost heave should be anticipated from the silty soils. Frost heave magnitudes will likely not exceed 2 inches, and should typically be on the order of less than an inch. We recommend at least 2 inches of flex be provided for any connection to the building and ready access for periodic maintenance. Given the somewhat problematic nature of both locations, we generally recommend the bearing pressures be kept as low and uniform as possible. We also recommend that the building site be selected for positive drainage away from the foundation or be provided with positive drainage through grading. We anticipate a combination of low bearing pressures and adequate drainage will likely result in acceptable performance at either test pit location. WATER-SOLUBLE SULFATES The concentrations of water-soluble sulfates measured in a selected sample retrieved from the test pits was approximately 0.23 percent by weight (Table 1). Such concentrations of watersoluble sulfates represent a severe environment for sulfate attack on concrete exposed to these Job No GROUND Engineering Consultants, Inc. Page 3

5 Radio Tower Equipment Building Eagle County, CO materials. Degrees of attack are based on the scale of negligible, moderate, severe and very severe as described in the Design and Control of Concrete Mixtures, published by the Portland Cement Association (PCA). The Colorado Department of Transportation (CDOT) utilizes a corresponding scale with 4 classes of severity of sulfate exposure (Class 0 to Class 3) as described in the published table below. REQUIREMENTS TO PROTECT AGAINST DAMAGE TO CONCRETE BY SULFATE ATTACK FROM EXTERNAL SOURCES OF SULFATE Severity of Sulfate Exposure Water-Soluble Sulfate (SO 4 ) In Dry Soil (%) Sulfate (SO 4 ) In Water (ppm) Water Cementitious Ratio (maximum) Cementitious Material Requirements Class to to Class 0 Class to to Class 1 Class to to 10, Class 2 Class or greater 10,001 or greater 0.40 Class 3 Based on our test results and PCA and CDOT guidelines, GROUND recommends use of sulfate-resistant cement in all concrete exposed to site soils, conforming to one of the following Class 2 requirements: (1) ASTM C 150 Type V with a minimum of a 20 percent substitution of Class F fly ash by weight (2) ASTM C 150 Type II or III with a minimum of a 20 percent substitution of Class F fly ash by weight. The Type II or III cement shall have no more than percent expansion at 14 days when tested according ASTM C 452 (3) ASTM C 1157 Type HS; Class C fly ash shall not be substituted for cement. (4) ASTM C 1157 Type MS plus Class F fly ash where the blend has less than 0.05 percent expansion at 6 months or 0.10 percent expansion at 12 months when tested according to ASTM C (5) A blend of Portland cement meeting ASTM C 150 Type II or III with a minimum of 20 percent Class F fly ash by weight, where the blend has less than 0.05 percent expansion at 6 months or 0.10 percent expansion at 12 months when tested according to ASTM C Job No GROUND Engineering Consultants, Inc. Page 4

6 Radio Tower Equipment Building Eagle County, CO (6) ASTM C 595 Type IP(HS); Class C fly ash shall not be substituted for cement. When fly ash is used to enhance sulfate resistance, it shall be used in a proportion greater than or equal to the proportion tested in accordance to ASTM C 1012, shall be the same source, and it shall have a calcium oxide content no more than 2.0 percent greater than the fly ash tested according to ASTM C All concrete exposed to site soil and bedrock should have a minimum compressive strength of 4,500 psi. The contractor should be aware that certain concrete mix components affecting sulfate resistance including, but not limited to, the cement, entrained air, and fly ash, can affect workability, set time, and other characteristics during placement, finishing and curing. The contractor should develop mix(es) for use in project concrete which are suitable with regard to these construction factors, as well as sulfate resistance. A reduced, but still significant, sulfate resistance may be acceptable to the owner, in exchange for desired construction characteristics. SOIL CORROSIVITY Reference is made to the Eagle Radio Tower Soil Resistivity letter dated June 21, 2013 (Job No ) for Wenner resistivity values for use in the evaluation of corrosion potential to buried metals. CLOSURE Materials Testing: The client should consider retaining a Geotechnical Engineer to perform materials testing during construction. The performance of such testing or lack thereof, in no way alleviates the burden of the contractor or subcontractor from constructing in a manner that conforms to applicable project documents and industry standards. The contractor or pertinent subcontractor is ultimately responsible for managing the quality of their work; furthermore, testing by the geotechnical engineer does not preclude the contractor from obtaining or providing whatever services they deem necessary to complete the project in accordance with applicable documents. Limitations: This report has been prepared for Eagle County as it pertains to the proposed project as described herein. It may not contain sufficient information for other parties or other purposes. The owner or any prospective buyer relying upon this report must be made aware of and must agree to the terms, conditions, and liability limitations outlined in the proposal. The geotechnical conclusions and recommendations in this report relied upon subsurface exploration at a limited number of exploration points, as shown in Figure 1, as well as the means and methods described herein. Subsurface conditions were interpolated between and extrapolated beyond these locations. It is not possible to guarantee the subsurface conditions Job No GROUND Engineering Consultants, Inc. Page 5

7 Radio Tower Equipment Building Eagle County, CO are as indicated in this report. Actual conditions exposed during construction may differ from those encountered during site exploration. If during construction, surface, soil, bedrock, or groundwater conditions appear to be at variance with those described herein, a geotechnical engineer should be advised at once, so that reevaluation of the recommendations may be made in a timely manner. In addition, a contractor who relies upon this report for development of his scope of work or cost estimates may find the geotechnical information in this report to be inadequate for his purposes or find the geotechnical conditions described herein to be at variance with his experience in the greater project area. The contractor is responsible for obtaining the additional geotechnical information that is necessary to develop his workscope and cost estimates with sufficient precision. This includes current depths to groundwater, etc. The materials present on-site are stable at their natural moisture content, but may change volume or lose bearing capacity or stability with changes in moisture content. Performance of the proposed structure and pavement will depend on implementation of the recommendations in this report and on proper maintenance after construction is completed. Because water is a significant cause of volume change in soils and rock, allowing moisture infiltration may result in movements, some of which will exceed estimates provided herein and should therefore be expected by the owner. This report was prepared in accordance with generally accepted soil and foundation engineering practice in the project area at the date of preparation. GROUND makes no warranties, either expressed or implied, as to the professional data, opinions or recommendations contained herein. Because of numerous considerations that are beyond GROUND s control, the economic or technical performance of the project cannot be guaranteed in any respect. ALL DEVELOPMENT CONTAINS INHERENT RISKS. It is important that ALL aspects of this report, as well as the estimated performance (and limitations with any such estimations) of proposed project improvements are understood by the Client, Project Owner (if different), or properly conveyed to any future owner(s). Utilizing these recommendations for planning, design, and/or construction constitutes understanding and acceptance of recommendations or information provided herein, potential risks, associated improvement performance, as well as the limitations inherent within such estimations. If any information referred to herein is not well understood, it is imperative for the Owner or anyone using this report to contact the author or a company principal immediately. Sincerely, GROUND Engineering Consultants, Inc. Reviewed by Chris Hutto, C.E.T. Carl Henderson, P.E. Job No GROUND Engineering Consultants, Inc. Page 6

8 2 1 1 Indicates test pit number and approximate location. LOCATION OF TEST PITS 1 inch ~ 20 feet JOB NO.: FIGURE: 1 CADFILE NAME: 6019SITE.DWG

9 +1 Test Pit Depth - feet BOUNDARIES ARE APPROXIMATELY DRAWN TEST PIT PROFILES JOB NO.: FIGURE: 2 CADFILE NAME: 6019PIT1.DWG

10 +1 Test Pit Depth - feet BOUNDARIES ARE APPROXIMATELY DRAWN TEST PIT PROFILES JOB NO.: FIGURE: 3 CADFILE NAME: 6019PIT2.DWG

11 LEGEND: Silt, Sand, Gravel, and Cobbles: Contained fine to coarse grained sand and fine to medium sized cobbles, were estimated to be loose to medium dense, slightly moist to moist, had medium to high plasticity, and were light brown in color. Possible Fire Altered Materials: Were generally the same as the silt/clay, sand, gravel, and cobbles, however the fire altered materials were somewhat friable to non-friable and generally broke down upon sieving in the laboratory, however some fragments were more durable. These materials were brown to dark brown to black in color. Sand and Silt: Were interlayered and had a fine to coarse sand fraction and occasional fine gravels, were Wstimated to be loose to medium dense or medium stiff to very stiff, slightly moist, had medium to high plasticity, and was light brown in color. Hand Drive sample, 2-inch I.D. Brass liner sample Large disturbed sample. Practical Excavator Refusal NOTES: 1) Test pits were excavated on 07/24/13 with a rubber tire Komatsu WB 150 AWS back hoe. 2) Locations of the test pits were measured approximately by pacing from features shown on the site plan provided. 3) Elevations of the test pits were not measured and the logs of the test pits are drawn to depth. 4) The test pit locations and elevations should be considered accurate only to the degree implied by the method used. 5) The lines between materials shown on the test pit logs represent the approximate boundaries between material types and the transitions may be gradual. 6) Groundwater was not encountered during excavating. Groundwater levels can fluctuate seasonally and in response to landscape irrigation. LEGEND AND NOTES JOB NO.: FIGURE: 4 CADFILE NAME: 6019LEG01.DWG

12

13

14

15 COMPACTION TEST REPORT Project No.: Date: 7/25/13 Project: Eagle Radio Tower Client: Eagle County Location: TH-2 Depth: 0-3 feet Remarks: MATERIAL DESCRIPTION Description: Sl. Gravelly, Sandy Silt Classifications - USCS: (ML)s AASHTO: A-5 (9) Nat. Moist. = Sp.G. = 2.65 Liquid Limit = 48 Plasticity Index = 7 % < No.200 = 71 % ROCK CORRECTED TEST RESULTS Maximum dry density = 85.2 pcf Optimum moisture = 29.9 % UNCORRECTED 79.9 pcf 33.6 % 140 Test specification: ASTM D Method A Standard ASTM D Oversize Corr. Applied to Each Test Point 130 Dry density, pcf % SATURATION CURVES FOR SPEC. GRAV. EQUAL TO: Water content, % - Rock Corrected - Uncorrected GROUND ENGINEERING CONSULTANTS, INC. Figure 8

16 TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Sample Location Natural Natural Gradation Percent Atterberg Limits Water USCS AASHTO Test Moisture Dry Passing Liquid Plasticity Soluble Classifi- Classifi- Soil or Hole Depth Content Density Gravel Sand No. 200 Limit Index Sulfates cation cation Bedrock Type No. (feet) (%) (pcf) (%) (%) Sieve (%) (GI) TP (ML)s A-4(0) Sandy Silt TP SD ML A-4(0) Silt TP SD (SC)g A-4(1) Gravelly, v. Clayey Sand TP SD (GC)s A-6(1) Sandy, v. Clayey Gravel TP *29.9 * (ML)s A-5(9) sl. Gravelly, Sandy Silt *Opt WC and Max Dry Density ASTM D698; SD = Sample Disturbed Job No

17 APPENDIX A PHOTOGRAPHS OF TEST PITS

18 Test Pit 1 Test Pit 2

19 Soil Resistivity Testing Eagle County Radio Tower Eagle County, CO Job No Page 2 of 4 around the perimeter of the testing locations. According to geologic maps 1, the site is depicted as underlain by the Phanerozoic to Carboniferous Pennsylvanian Eagle Valley Formation consisting of gypsum, siltstone, and shale. The observed bedrock outcrops and site soils are interpreted to be part of, or derived from, the Eagle Valley Formation. In the specific testing area, we observed little to no topsoil on the surface which gently sloped away to the north, east and west sides. The approximate locations of the tests are shown below (Google Maps 2010). In general, the site drainage appeared to be good. 1 Tweto, Ogden 1979, Geologic Map of Colorado: U.S. Geological Survey Special Geologic Map, scale 1:500,000. GROUND Engineering Consultants, Inc.

20 Soil Resistivity Testing Eagle County Radio Tower Eagle County, CO Job No Page 3 of 4 A Nilsson Model 400 resistivity meter was used to perform testing. A 300-foot long tape measure was utilized to layout the test probes and a tape measure was used to determine probe depth. A probe depth of 9 inches was used for each line and set of measurements. Testing was performed in general accordance with the Soil Resistivity Measurements Appendix B of the document titled, Motorola R56 Issue B, 2005, provided by the Client, as well as ASTM G and Nilsson Model 400 manufacturer s recommendations. The observed resistance values and calculated resistivity test results are summarized on Table 1. We trust that this provides the information that you need at this time. If you have any questions, please contact this office. Sincerely, GROUND Engineering Consultants, Inc. Chris Hutto, C.E.T. Carl Henderson, P.E. GROUND Engineering Consultants, Inc.

21 Soil Resistivity Testing Eagle County Radio Tower Eagle County, CO Job No Page 4 of 4 Table 1- Observed Resistance Values and Calculated Resistivity GROUND Engineering Consultants, Inc.