AttachmentD-Geotech.pdf

Attachment D – Geotechnical Report

2390 South Lipan Street Denver, CO 80223 phone: (303) 742-9700 fax: (303) 742-9666 email: kadenver@kumarusa.com www.kumarusa.com

Office Locations: Denver (HQ), Colorado Springs, Fort Collins, and Frisco, Colorado

October 9, 2014 Revised October 14, 2014

Joel Deis, P.E.

Engineering Specialties Group 8501 Turnpike Drive, Suite 106 Westminster, Colorado 80031

Subject: Geotechnical Engineering Study, Proposed Communication Tower, West of La Veta, Huerfano County, Colorado

Project No. 14-6-148

Dear Mr. Deis:

This revised report presents the results of a geotechnical engineering study performed for the proposed communication tower to be constructed west of the Town of La Veta in Huerfano County, Colorado.

The study was conducted to obtain information on subsurface conditions at the project site and to provide geotechnical related recommendations for design and construction of the proposed tower. The location of the project site is shown on Fig. 1.

This report has been prepared to summarize the data obtained during this study and to present our conclusions and recommendations based on the proposed construction and the subsurface conditions encountered. Design parameters and a discussion of geotechnical engineering considerations related to construction of the proposed improvements are included in the report.

Kumar and Associates prepared a previous report for the tower structure under our Project No.

14-6-148, dated October 9, 2014. In that report we presented the results of a geotechnical engineering study and provided our recommendations for support of the tower using a mat foundation system. We understand that a mat foundation may not provide enough resistance to counter potential overturning moments. Therefore, recommendations for a deep foundation consisting of drilled piers are desired. Additionally, the field resistivity values presented in the original report were re-calculated based on reevaluated field data and are presented below.

Proposed Construction: We understand that a single, self-supporting communication tower measuring approximately 100 feet in height will be installed at the site. The tower will be fixed atop a concrete tower base, having dimensions of approximately 21 ft. x 21 ft. x 2 ft. (L x W x D). The tower will have a total shear load of approximately 46.9 kips, and an axial load of about

11.7 kips. We assume that overturning loads will govern the overall structural design.

Engineering Specialties Group

Kumar & Associates, Inc

If the proposed construction varies significantly from that described above or depicted in this report, we should be notified to reevaluate the recommendations provided in this report.

Site Conditions: The project site is located approximately 8.5 miles west of La Veta, Colorado in a remote area surrounded by coniferous tress. The tower site is located in a 100 ft. x 150 ft.

clearing. At the time of the field exploration, the cleared area was occupied by a small single-story building, a weather station and an existing 100 foot communications tower that is similar to the proposed construction. Propane tanks were also present at the site.

The topography in the area of the proposed construction was relatively flat to gently sloping in a west to east direction. Areas immediately east of the site steeply slope down from west to east.

Field Exploration: The field exploration was conducted on September 18, and September 29, 2014, and consisted of one (1) exploratory boring drilled at the approximate location shown on Fig. 1. Samples of the soil and bedrock obtained during the field exploration were tested in the laboratory to determine their classifications and engineering characteristics. The results of the field exploration and laboratory testing were analyzed to develop recommendations for foundation types, depths and allowable pressures for the proposed structure.

Subsurface Conditions: The boring was advanced through the overburden soils and into the underlying bedrock with 4-inch diameter continuous flight augers. The boring was logged by a representative of Kumar & Associates, Inc.

Samples of the bedrock materials were taken with a 2-inch I.D. California liner sampler. The sampler was driven into the various strata with blows from a 140-pound hammer falling 30 inches. This test is similar to the standard penetration test described by ASTM Method D 1586.

Penetration resistance values, when properly evaluated, indicate the relative density or consistency of the soils. Depths at which the samples were taken and the associated penetration resistance values are shown on the Logs of Exploratory Borings, Fig. 2, along with a legend and notes describing the sub-soils encountered...

The borings encountered a thin layer of topsoil and gravels directly on top of sandstone bedrock. The bedrock was fine to coarse-grained, slightly moist and tan-brown. The bedrock was partially cemented and was hard to very hard, based on sampler penetration resistance values. The boring was terminated in the sandstone bedrock at a depth of approximately 10 feet where practical auger refusal was encountered.

Groundwater was not encountered in the depth of the boring at the time of the field exploration.

Laboratory Testing: Samples obtained from the exploratory borings were visually classified in the laboratory by the project engineer and a selected sample was tested in our laboratory to determine the engineering characteristics and classification properties of the on-site bedrock.

Laboratory testing included moisture content and dry unit weight, liquid and plastic limits, and concentration of water soluble sulfates. Results of the laboratory testing are shown adjacent to the boring logs on Fig. 2 and are summarized in the attached Table I.

Foundation Recommendations: As indicated above, we assume that the overturning moments will govern the overall structural design of the foundation system. In addition to the mat recommendations presented below and in the original report, recommendations for a drilled pier foundation system are included in this report.

Mat Foundations: Considering the nature of the proposed construction regarding the communication tower, we believe that a mat foundation system is feasible for support of the structure.

The design and construction criteria presented below should be observed for a mat foundation system. Construction details should be considered when preparing project documents.

1. A mat foundation supported directly on undisturbed sandstone bedrock or compacted granular structural fill extending to undisturbed sandstone bedrock above should be designed for a net allowable contact pressure of 5,000 psf to resist uplift and the design overturning moment. The mat should be sized so that the factor of safety against overturning is at least 1.5.

2. For average sustained loads approaching the net allowable base contact pressure, and assuming a rigid mat with symmetrically distributed loads, we estimate total settlements beneath a rigid mat will be on the order of 1 inch or less. Post-construction differential settlements between the middle and the edges/corners of a rigid mat may range up to about one-half of the total settlement for mats of significant width and length. Non-uniformity of the subsurface conditions and deviation from the rigid mat assumption will contribute to total and differential settlements. These settlements are assumed to occur immediately after the application of loads.

If the mat cannot be considered rigid, the soil pressure distribution should be computed using a method that models the soil-structure interaction, such as the beam on an elastic foundation procedure. The rigidity of the mat foundation is dependent on its dimensions and the modulus of subgrade reaction of the supporting soils. A modulus of vertical subgrade reaction equal to 200 tcf may be used for the bedrock. The modulus value given is for a 1-foot square plate and must be corrected for mat shape and size.

3. The lateral resistance of a mat foundation will be a combination of the sliding resistance of the footing on the foundation materials and the passive earth pressure against the side of the footing. Resistance to sliding at the bottoms of the footings can be calculated based on a coefficient of friction of 0.35. Passive pressure against the sides of the mat can be calculated using an equivalent fluid unit weight of 195 pcf. The above values are working values with a factor of safety applied.

4. The bottom of the mat should be placed a minimum of 36 inches below exterior grade for frost protection.

5. Compacted fill placed against the sides of the mat to resist lateral loads should consist of a non-expansive material compacted to at least 95% of the standard Proctor (ASTM D

698) maximum dry density at a moisture content within 2 percentage points of optimum.

6. The ultimate overturning resistance can be calculated using the full weight of the mat plus the weight of any material enclosed in the soil prism above the mat. For estimating purposes, it can be assumed that the soil prism extends outwards and upwards from the top of the mat at a 30 degree angle from vertical.

7. A representative of the geotechnical engineer should observe all mat excavations prior to concrete placement.

Drilled Pier Recommendations: The design and construction criteria presented below should be observed for a straight-shaft drilled pier foundation system, if selected. The construction details should be considered when preparing project documents.

1. Piers should be designed for an allowable end bearing pressure of 40,000 psf and a skin friction of 4,000 psf for the portion of the pier in bedrock. Uplift due to structural loadings on the piers can be resisted by using 75% the allowable skin friction value plus an allowance for pier weight.

2. Piers should penetrate the bedrock the required depth from the design standpoint, but should be at least 10 feet deep to overcome the disturbance from drilling in the upper reaches of the shallow bedrock.

3. Closely spaced piers may require appropriate reductions of the lateral and axial capacities. Reduction in lateral load capacity may be avoided by spacing the piles at least 6 pier diameters (center to center) in the direction parallel to pile loading, and 2.5 diameters in the direction perpendicular to loading. For axial loading, the piers should be spaced a minimum of 3 diameters center to center. More closely spaced piers should be studied on an individual basis to determine the appropriate reduction in axial and lateral load design parameters.

If the recommended pier spacing cannot be achieved, we recommend the load-displacement curve (p-y curve) for an isolated pier be modified for closely spaced piers by using p-multipliers to reduce all the p values on the curve. With this approach, the computed load carrying capacity of the caisson in a group is reduced relative to the isolated pier capacity. The modified p-y curve should then be reentered into the LPILE software to calculate the pier deflection. It should be noted that the reduction for the leading pier, the pier leading the direction of movement of the group, is less than that for the trailing piers. We recommend p-multipliers of 0.6 and 1.0 for pier spacing of 3 and 6 diameters, respectively, for the leading pier, and 0.4 and 1.0 for 3 and 6 diameter spacing, respectively, for the trailing piers. Reduction factors for spacing between 3 and 6 diameters may be obtained by linear interpolation. It will be necessary to determine the load distribution between the piers that attains deflection compatibility because the leading pier carries a higher proportion of the group load and the pier cap prevents differential movement between the piers.

4. The following table presents the lateral design parameters for the LPILE computer program. These values should be used when analyzing lateral capacity of drilled piers with the LPILE software.

Material c (psf) Ø γ T ks kc Є 50

Soil Type

Sand above Water Level 0 32 120 90 90 - 1 Bedrock 8,000 0 125 2,000 800 0.004 2 c -Cohesion intercept (psf) ø -Angle of internal friction (degrees) γ T -Total unit weight (pcf) ks -Initial static modulus of horizontal subgrade reaction (pci) kc -Initial cyclic modulus of horizontal subgrade reaction (pci) Є 50 -Strain at 50 percent of peak shear strength

Soil Types:

1 – Sand (Reese) 2 – Stiff Clay above the Groundwater Table

Due to the weathered nature of the of the bedrock near the existing ground surface, the upper 2-3 feet may be treated as Soil Type 1.

5. Concrete used in the piers should be a fluid mix with sufficient slump so it will fill the void between reinforcing steel and the pier hole and reduce the potential for adhesion of the concrete to the casing during casing extraction. We recommend concrete with a slump ranging from 5 to 8 inches be used.

6. Based on the results of our field exploration, laboratory testing, and our experience with similar projects, we estimate pier settlement will be low. Generally, we estimate the settlement to be less than ¾-inch when designed and constructed according to the criteria presented herein. The settlement of closely spaced piers will be larger and should be studied on an individual basis.

7. Pier holes should be properly cleaned prior to the placement of concrete.

8. Concrete should be placed in piers the same day they are drilled. The presence of water or caving soils may require that concrete be placed immediately after the pier hole is completed. Failure to place concrete the day of drilling will normally result in a requirement for additional bedrock penetration.

9. The absence of water in the exploratory borings indicates the use of temporary casing or dewatering equipment in the pier holes will probably not be required to reduce water infiltration. However, if water infiltration does occur, the requirements for casing can sometimes be reduced by placing concrete immediately upon cleaning and observing the pier hole. In no case should concrete be placed in more than 3 inches of water.

10. Hard drilling conditions should be anticipated at the site. The potential presence of well-cemented to partially-cemented bedrock will require the use of a core barrel or pilot hole to advance the pier holes to the required depth. The drilled shaft contractor should mobilize equipment of sufficient size to achieve the required penetration in the bedrock.

11. A representative of a qualified geotechnical engineer should observe caisson drilling operations on a full-time basis to assist in identification of adequate bedrock strata and monitor caisson construction procedures.

Site Grading: Site grading information was not available at the time of report preparation. Site grading should provide positive drainage away from the tower base.

Material Suitability: Based on the results of the field and laboratory studies, it appears that the on-site natural soils are generally suitable for use as backfill and structural fill. Imported fill if required, should consist of non- to low swelling granular materials containing less than 50% passing the No. 200 sieve with a Liquid Limit of less than 30% and a Plasticity Index of less than 15%. On-site or imported structural fill material not meeting these criteria may be acceptable if the swell potential does not exceed 1% when a sample remolded to 98% of the standard Proctor (ASTM D 698) maximum dry density at the optimum moisture content is wetted under a 200 psf surcharge.

Structural fill should not contain gravel larger than 3 inches. All fill material should be free of any organic matter and other deleterious substances. The geotechnical engineer should evaluate the suitability of the proposed fill materials prior to placement.

Moisture Conditioning and Compaction: Fill placed at the site should be compacted to at least 98% of the standard Proctor (ASTM D 698) maximum dry density.

Fill material should be moisture conditioned to within 2 percentage points of the optimum moisture content for the on-site soils. In fill areas, the natural soils should be scarified to a depth of 8 inches, adjusted to a moisture content within 2 percentage points of optimum moisture content and compacted to 95% of the standard Proctor (ASTM D 698) maximum dry density.

Excavation Considerations: Excavation into partially cemented sandstone bedrock may be required for foundation installation of the mats, and/or general grading associated with site preparation. In our opinion, excavation of the bedrock should be possible with conventional heavy-duty equipment with rippers; however, due to the small area of the site the use of heavy equipment may be limited. Hydraulic chisels and/or jackhammers may be required to break down and excavate cemented areas of the bedrock. Some light blasting may be required for deeper excavations.

Temporary excavations should be constructed by excavating the slopes to a stable configuration. All excavations should be constructed in accordance with OSHA requirements, as well as state, local and other applicable requirements.

The sandstone bedrock soils generally classify as OSHA Type B soils. Slopes should either be adequately laid back depending on the soil type exposed.

Electrical Resistivity: The electrical resistivity of the near-surface soils at the site was measured in the field using the Wenner Four-Electrode test method. The tests were performed along two lines, one oriented generally N-S (Line 1) and another oriented generally E-W (Line 2) within the vicinity of the proposed tower footprint at the locations indicated on Fig. 1. The tests were performed at 5-, 10-, 20-foot probe spacing. The results ranged from approximately 3,713- to 27,640 ohm-cm, as summarized in the tables below.

Probe Spacing (ft.)

Calculated Electrical Resistivity (ohm-cm)

Line 1 Line 2

5.0 5,484 3,847

10.0 13,858 3,924

20.0 27,640 3,713

Based on the field resistivity test results, the sub-soils, would be classified as slightly – to moderately corrosive to buried metal at in situ moisture conditions. The corrosion classification is based on a range of slightly, mildly, moderately, to very corrosive as presented by the U.S.

Bureau of Reclamation.

It should be noted that soil electrical resistivity generally decreases with increased moisture content. The measured field electrical resistivity values represent average values for depths corresponding to the probe spacing, and the variation in measured values likely reflect material and moisture variations within the underlying natural soils.

Corrosion of buried metal is a complex process and requires an understanding of the combined effects of pH, ion content, electrical resistivity, soil moisture, and other conditions not evaluated as part of this study. We recommend a qualified corrosion engineer review the information presented herein to determine the need for and appropriate level of corrosion protection for buried metals at the site.

Water Soluble Sulfates: The concentration of water soluble sulfates measured in a sample obtained from the exploratory boring was determined to be 0.02%. This concentration of water soluble sulfates represents a Class 0 severity exposure of sulfate attack on concrete exposed to these materials. The degree of attack is based on a range of Class 0 through Class 3 severity exposure as presented in ACI 201.2R. Based on this information, we believe that special sulfate resistant cement will not be required for concrete exposed to these on-site soils.