New Wind Turbine USVI - Geotechnical Report.pdf

Report of Geotechnical Engineering Evaluation

New Wind Turbine – Air National Guard Station St. Croix, US Virgin Islands

Prepared for Pond & Company October 31, 2016

1000 Cobb Place Boulevard, Suite 290 • Kennesaw, Georgia 30144 o: 770.426.7100 • f: 770.426.5209 • www.geohydro.com

Mr. George Fragulis, PE, PMP, CEM, BEMP, LEED AP October 31, 2016 Pond & Company 3500 Parkway Lane Suite 600 Norcross, Georgia 30092

Report of Geotechnical Engineering Evaluation New Wind Turbine – Air National Guard Station

St. Croix, US Virgin Islands Geo-Hydro Project Number 160861.20

Dear Mr. Fragulis:

Geo-Hydro Engineers, Inc. has completed the authorized geotechnical engineering evaluation for the above referenced project. The scope of services for this project was outlined in our proposal number 19758.2 dated October 24, 2016.

Project Information

The US Virgin Islands Air National Guard Station is located just north of Henry E. Rohlsen Airport and is accessed via Melvin H. Evans Highway. Figure 1 in the Appendix shows the approximate site location.

The project consists of a new wind turbine to be constructed at the approximate location shown on the annotated aerial photograph below. The wind turbine will have a total height of 70 feet. Based on our conversations with the project structural engineer, Mr. Mark Hanselman, PE, SE, of Pond & Company, we understand that current plans call for supporting the wind turbine on a reinforced concrete mat foundation with plan dimensions of 28 feet by 28 feet. The foundation will be 6 feet thick and will bear about 7½ feet below the prevailing ground surface.

We understand that the wind turbine will have a dead weight of approximately 20 kips and a maximum overturning moment of about 780 kip-feet. Based on our conversations with Mr. Hanselman, we understand that the mat foundation has been designed based on an allowable bearing pressure of 2,000 psf for gravity loading conditions and a modulus of subgrade reaction of 100 pci for transient loading conditions such as wind and earthquake loading.

We have been provided soil test boring logs from an exploration performed in 1996 related to the design of a building that has since been constructed on site. The aerial photograph to the right shows the planned wind turbine location and the locations of the borings performed in 1996.

Proposed Wind Turbine Location

SC-4

SC-3

New Wind Turbine – Air National Guard Station • St. Croix, US Virgin Islands Project Number 160861.20

October 31, 2016 | 2

Test Boring Summary

Both of the boring logs indicate about 6 inches of topsoil. Beneath the topsoil, both logs show about 1½ feet of fill material. The fill was classified as sandy silt with trace clay. Standard penetration resistances of 19 and 25 blows per foot were recorded in the fill.

Beneath the fill, both borings encountered native soils classified as sandy silt with varying amounts of trace clay. Standard penetration resistances recorded in the native soils ranged from 18 to over 100 blows per foot. Boring SC-3 encountered a lens of weathered limestone between depths of about 3 and 4½ feet.

At the time of drilling, no groundwater was encountered in the soil test borings.

For more detailed descriptions of subsurface conditions, please refer to the test boring records included in the Appendix.

Evaluations and Recommendations

The following evaluations and recommendations are based on the information available on the proposed construction, the 1996 test boring records, which we have considered a reasonable representation of subsurface conditions at the proposed wind turbine site, and our experience with soils and subsurface conditions similar to those encountered at this site. The 1996 test borings were performed roughly 250 feet away from the proposed wind turbine site. Additionally, because the test borings represent a statistically small sampling of subsurface conditions, it is possible that conditions may be encountered during construction that are substantially different from those indicated by the test borings. In these instances, adjustments to the design and construction may be necessary.

Geotechnical Considerations

The following geotechnical characteristics of the site should be taken into account for planning and design:

• Based on the results of the test borings, it is our opinion that the new wind turbine can be supported using the planned reinforced concrete mat foundation as designed. An allowable bearing pressure of 2,000 psf is reasonable. Based on a bearing surface elevation about 7½ feet below the prevailing ground surface, this corresponds to a gross allowable bearing pressure of about 2,900 psf.

• According to the 2015 International Building Code (IBC) as adopted by UFC 3-3-1-01, the allowable bearing pressure may be increased by one-third under certain transient loading combinations. Please refer to sections 1605.3.2 and 1806.1 of IBC 2015 for more details.

• The borings performed in 1996 did not encounter groundwater. However, the boring logs do not include elevation data, and the elevation of the proposed wind mill is also unknown. Based on publically available elevation data (Google Earth), the wind turbine location may be as much as 20 feet lower in elevation that the borings performed in 1996. Although there is no indication that groundwater

October 31, 2016 | 3 will be encountered, the project documents should include a performance specification for dewatering.

If dewatering is necessary, the groundwater level should be maintained at least 3 feet below the prevailing excavation level. Please refer to the Groundwater section of this report for an example performance specification and other considerations for dewatering.

• Based on the results of the test borings, the soils on site have low potential for shrink/swell activity that may affect the proposed construction.

The following sections provide recommendations regarding these issues and other geotechnical aspects of the project.

Site Preparation

After foundation excavation has been completed, the resulting bearing surface should be evaluated using a combination of random hand probing with a small-diameter steel rod, visual observations, and hand auger and penetrometer testing (ASTM STP-399). Any loose material should be removed from the foundation excavation. Any loose, unstable material that cannot be densified in place should be removed and replaced with concrete.

Excavation Characteristics and Reuse of Excavated Material

The test boring indicates generally favorable excavation conditions for installation of the turbine mat foundation as well as any associated utilities. Fill materials and native soils should be readily excavatable using conventional soil excavation equipment such as loaders and backhoes.

Based on the results of test boring, the existing fill materials and native soils appear to be suitable for reuse as structural fill after routine moisture content adjustment. However, it is possible that some excavated fill may not be suitable for reuse. The project special inspections engineer should observe the excavation of existing fill materials to evaluate their suitability for reuse. Soft, unstable fill soils free of deleterious materials may be reusable after routine moisture adjustment.

It is important to establish as part of the construction contract whether soils having elevated moisture content will be considered suitable for reuse. We often find this issue to be a point of contention and a source of delays and change orders. From a technical standpoint, soils with moisture contents wet of optimum as determined by the standard Proctor test (ASTM D698) can be reused provided that the moisture is properly adjusted to within the workable range. From a practical standpoint, wet soils can be very difficult to dry in small or congested sites and such difficulties should be considered during planning and budgeting. A clear understanding by the general contractor and grading subcontractor regarding the reuse of excavated soils will be important to avoid delays and unexpected cost overruns.

Groundwater

The borings performed in 1996 did not encounter groundwater within 25 feet of the ground surface. The boring logs do not include elevation data, and it is unclear what the ground surface elevation is at the

October 31, 2016 | 4 proposed wind turbine location. Based on publically available elevation data (Google Earth), the proposed wind turbine location appears to be about 20 feet lower in elevation than the 1996 borings. Due to the elevation change and the lack of project-specific geotechnical data, the contractor should be prepared to dewater the mat foundation excavation as necessary. Additionally, the groundwater level will fluctuate depending on recent rainfall. Typically, dewatering should be performed to maintain the groundwater level at least 2 to 3 feet below the lowest prevailing excavation depth. We expect that small-scale dewatering efforts such as direct pumping from one or more sumps will suffice to manage groundwater. However, we recommend that the project specifications require the use of dewatering as necessary, and dictate the result of the dewatering operation (performance specification). The contractor may then implement a technique or combination of techniques appropriate for the actual field conditions encountered. Exhibit “A” as follows provides a minimum guide specification that may be used to develop a dewatering performance specification suitable for this project.

EXHIBIT "A"

Minimum Guide Specification for Dewatering

NOTE: The following specifications are for use as a guide for development of actual specifications. The guide is not intended for direct use as a construction specification without modifications to reflect specific project conditions.

Control of groundwater shall be accomplished in a manner that will preserve the strength of the foundation soils, will not cause instability of the excavation slopes, and will not result in damage to existing structures. Where necessary to these purposes, the water level shall be lowered in advance of excavation, utilizing trenches, sumps, wells, well points or similar methods. The water level, as measured in piezometers, shall be maintained a minimum of 3 feet below the prevailing excavation level. Open pumping from sumps and ditches, if it results in boils, loss of soil fines, softening of the ground or instability of slopes, will not be permitted. Wells and well points shall be installed with suitable screens and filters so that continuous pumping of soil fines does not occur. The discharge shall be arranged to facilitate collection of samples by the Engineer.

Adapted from Construction Dewatering - A Guide to Theory and Practice, John Wiley and Sons.

Regardless of the groundwater conditions encountered in the borings, waterproofing and subsurface drainage is required for all retaining walls and walls below grade.

An existing building is located about 75 feet south of the proposed wind turbine location. If dewatering operations also lower the groundwater level beneath the building, consolidation may occur as the soils at

October 31, 2016 | 5 depths between the long-term and temporary groundwater levels are stressed. The potential for settlement of the existing building should be evaluated prior to any dewatering operations on site. However, the following general guidelines will reduce the risk of settlement of the existing building:

• Temporary sumps or well points should be located on the north side of the excavation to reduce the amount of groundwater drawdown experienced at the existing building location.

• A monitoring well should be installed just north of the existing building so that the groundwater level at the edge of the building can be monitored.

• In general, the mat foundation should be constructed as quickly as possible. The groundwater drawdown experienced beneath the existing building will be greater over time.

Earth Slopes

Temporary construction slopes should be designed in strict compliance with OSHA regulations. Due to the variability of near-surface conditions and the lack of project-specific geotechnical data, we recommend assuming that the soils on site are Type C as defined in 29 CFR 1926.650 (1994 Edition). This dictates that temporary construction slopes be no steeper than 1.5H:1V for excavation depths of 20 feet or less.

Temporary construction slopes should be closely observed on a daily basis by the contractor’s “competent person” for signs of mass movement: tension cracks near the crest, bulging at the toe of the slope, etc. The responsibility for excavation safety and stability of construction slopes should lie solely with the contractor.

We recommend that extreme caution be observed in trench excavations. Numerous cases of loss of life due to trench collapses point out the lack of attention given to excavation safety on some projects. We recommend that applicable local and federal regulations regarding temporary slopes, and shoring and bracing of trench excavations be closely followed.

Formal analysis of slope stability was beyond the scope of work for this project. Based on our experience, permanent cut or fill slopes should be no steeper than 2.5H:1V to maintain long term stability and to provide ease of maintenance. The crest or toe of cut or fill slopes should be no closer than 10 feet to any foundation.

The crest or toe should be no closer than 5 feet to the edge of any pavements. Erosion protection of slopes during construction and during establishment of vegetation should be considered an essential part of construction.

Where foundations must be located in proximity to earth slopes, foundation capacity can be adversely affected. Such conditions should be examined on a case-by-case basis by Geo-Hydro.

Foundation Design

Based on the results of the 1996 test borings and our experience, it is our opinion that the wind turbine can be supported on the planned reinforced concrete mat. A maximum allowable bearing pressure of 2,000 psf is appropriate for design. Increasing the allowable bearing pressure by one-third is allowed for transient loading conditions such as earthquake or wind loading. We estimate that total settlement of the mat will

October 31, 2016 | 6 be less than 1 inch with minimal planar tilt. For modeling the soil response during transient loading conditions such as earthquakes and wind loading, a modulus of subgrade reaction of 100 pci is appropriate for design.

The foundation bearing surface should be inspected prior to placement of reinforcing steel. This will allow the identification of any deficient areas and the development of remedial measures, if necessary. Remedial measures will likely include removal of unstable soils and replacement with concrete. For budgeting purposes, we recommend assuming that 15 percent of the mat foundation footprint will require excavation and replacement to a depth of 2 feet. This is intended as a budgetary tool only. Remedial measures will be based on actual conditions in the field and may exceed the recommended budgetary allowance.

Lateral Load Resistance

Resistance to lateral loads is developed by the passive resistance of the soil against the mat foundation and by the friction force or sliding resistance between the concrete mat and the bearing material.

For analysis of foundation base sliding resistance, we recommend a coefficient of friction of 0.35 between mass concrete and anticipated bearing materials. This is an ultimate value and an adequate factor of safety should be used in design. The force which resists sliding is calculated by multiplying the normal force on the base by the coefficient of friction. Full development of the frictional force could require deflection of the base of roughly 0.l to 0.3 inches.

An allowable lateral earth pressure of 130 psf per foot of depth can be used for passive earth pressure resistance. It should be noted that full development of passive pressure will require deflection toward the soil mass on the order of ½-inch.

Seismic Design

UFC 3-301-1 provides seismic accelerations for design for each base/installation throughout the armed forces system. Section 2-1.6.1 of UFC 3-301-01 states that USGS-published values can be used in lieu of base-wide values for design if the specific location of the proposed building or structure is known. The following table presents seismic design parameters for the site based on the soil test borings and the criteria in IBC 2015 and UFC 3-301-01.

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Parameter 2015 IBC Table F.3

UFC 3-301-01

Ss (%g) 0.835 0.84 S1 (%g) 0.305 0.30

Site Class D D

FA 1.166 1.164

FV 1.789 1.800

SMS (%g) 0.974 0.978 SM1 (%g) 0.546 0.540 SDS (%g) 0.649 0.652 SD1 (%g) 0.364 0.360

Risk Category I, II, III, or IV I, II, III, or IV Seismic Design Category D D

Based on the information obtained from the 1996 soil test borings, it is our opinion that the potential for liquefaction of the soils at the site due to earthquake activity is relatively low.

We appreciate the opportunity to serve as your geotechnical consultant for this project, and are prepared to provide any additional services you may require. If you have any questions concerning this report or any of our services, please call us.

Sincerely, GEO-HYDRO ENGINEERS, INC.

A. Marty Peninger Luis E. Babler Senior Geotechnical Engineer Chief Engineer mpeninger@geohydro.com luis@geohydro.com

AMP/LEB/19758.2 - New Wind Turbine - Air National Guard Site - USVI - Geotechnical Evaluation Proposal

APPENDIX

Air National Guard Station

Approximate Scale: 1"=10,000'

New Wind Turbine - Air National Guard Station

St. Croix, US Virgin Islands

Geo-Hydro Project Number 160861.20

Figure 1: Site Location Plan

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PROVIDED GEOTECHNICAL DATA