The URL can be used to link to this page

Home My WebLink AboutNome Banner Ridge Geotechnical Final Report - Aug 2008August 23, 2008 STG, Inc. 11820 South Gambell St. Anchorage, AK 99515 Attention:Jim St. George Subject:Geotechnical Exploration and Tower Foundation Recommendations Banner Ridge Site Nome, Alaska DMA Job No. 4179.005 This letter summarizes our recent geotechnical ﬁeld exploration and foundation recommendations for the proposed wind towers proposed for Banner Ridge in Nome, Alaska. Two site investigation efforts were conducted to support our foundation recommendations. The initial site assessment was conducted on June 30, 2008 by Richard Mitchells of DMA, Lee Wilson of STG and Brian Jackson of Western Community Energy, LLC, (Western) the project developer. Western provided preliminary tower locations in GPS format for the June 30 site work but nearly all provided locations were ﬁeld changed by Brian Jackson during this ﬁeld effort. Based on the ﬁeld revised tower locations conducted concurrently by Western, twenty-two shallow test pits were advanced near the proposed tower sites. The shallow test pits were advanced with a small track hoe to determine the depth to bedrock and general near surface geology and thermal states. The track hoe was unable to advance into rock. Approximate locations for the test pits advanced for the June 30 effort are presented on Plate 1, site location plan. Depth to bedrock and visual bedrock classiﬁcations are summarized below. Sample Number Sample Depth (ft)Rock Type BR-15.5banded phyllitic schist BR-34.5black gneiss BR-56.0phyllitic schist BR-66.5banded phyllitic schist w/thin quartz stringers BR-116.5ﬁne grained gneiss BR-166.0black phyllitic schist BR-196.0banded phyllitic schist BR-216,5mica schist Based on the June 30 ﬁeld work, several proposed tower sites raised concerns for deep or poor bedrock conditions. In particular, tower sites near test pits BR TP-4 and 7 found frozen soil conditions above deeply weathered bedrock. Test pits BR TP-14, 15, and 16 encountered frozen soil conditions with visible ice and the test pits BR-14 and 15 the track hoe could not penetrate to bedrock. Test pit BR TP-17 encountered frozen soil that appeared to be a sheared zone material, ﬁrm bedrock was not encountered at this location due to the equipment not able to advance through frozen soil. Western revised the ﬁeld determined wind tower locations based on their analysis in July 2008. The July 2008 tower sites were provided in GPS format to us and STG. STG’s surveyor ﬁeld located the July revision tower sites; these locations are provided on Plate 1 also. On August 18 -20, 2008, Melanie Hess of DMA returned to Banner Ridge to verify subsurface conditions at several of the survey located tower locations and at select locations were (1) limited test pit data were available and (2) at tower locations of known or suspected poor bedrock conditions. During our August ﬁeld work, we were assisted by Jason Hill of STG Inc. A geotechnical technician from NovaGold observed the drilling and obtained samples under STG’s direction. STG- Banner Ridge Wind TowersDuane Miller Associates, LLC August 23, 2008 Page 2 During our August site work, twenty-two test borings were drilled at the site. The test borings were drilled using an Ingersoll-Rand air track drill rig owned and operated by STG Inc. Subsurface conditions were logged and representative soil and rock chip samples were collected during the investigation. Samples were obtained by collecting rock and soil chips in a wire mesh basket that were returned by the air drill. The samples were sealed in plastic bags. GPS coordinates were recorded with a hand-held instrument and as-built measurements were determined by measuring with a cloth tape from staked wind tower locations. Subsurface conditions were inferred from the type of material returned by the air drill, drilling action, and the driller’s interpretation of subsurface conditions. Representative rock samples may be biased by the air track sampling method since only small chip samples of rock are obtained. This drilling method may result in misrepresenting cobbles, boulders and bedrock. In addition, the small diameter drill bit advances rapidly through fractured rock and may not accurate reﬂect the over burden contact. Eight borings, TB-3 through TB-8, were drilled on the northern portion of banner ridge in the areas of Wind Tower locations WTG 3-2 through 6-3. Most of the borings were drilled on top of the ridge and on the southeast facing slope of the ridge. A very thin organic mat, 1 to 4 inches thick, was underlain by angular cobbles and gravel. Hard bedrock was encountered in all borings between 2 and 8 feet below the surface. Below the bedrock contact, the rock was consistent to the depth drilled. Three borings, TB-11 through TB-13, were drilled in the area of wind tower site WTG 2-3. A thin organic mat, 3 to 6 inches thick, was underlain by brown silty gravel and silty sand. Bedrock was encountered at 11 to 12 feet below the surface. The upper 2 feet of bedrock is likely relatively soft underlain by harder bedrock. The rock appeared to not be consistent, with lighter and darker color intervals, changing at approximate 1 to 2-foot intervals. STG- Banner Ridge Wind TowersDuane Miller Associates, LLC August 23, 2008 Page 3 TB-14 and TB-15 were drilled in the area near wind tower sites WTG 1-1 through 2-2, on the southern portion of Banner Ridge. Bedrock was encountered at approximately 5 and 6.5 feet, respectively. Bedrock appeared consistent to boring termination depths. Six borings were drilled near the wind tower site WTG 3-1, on the southwest facing slope of Banner Ridge. Bedrock was not encountered in this area to the depths explored, approximately 33 to 35 feet below grade. A thin organic mat was underlain by cobbles, silty gravel and silty sand to the depths explored. The material was likely frozen below about 6 to 10 feet deep. Water was returned by the air drill for about 1 to 2 feet at 13 feet in boring TB-20 and TB-22 and at 23 feet in TB-22. Summary ﬁndings from the August 2008 ﬁeld effort are presented below. Of note is the absence of bedrock near tower location WTG 3-1 and the deeper bedrock near tower location WTG 2-3. STG- Banner Ridge Wind TowersDuane Miller Associates, LLC August 23, 2008 Page 4 Air Track Boring Latitude (WGS 84) Longitude (WGS 84)General Location Depth to Bedrock (ft) Total Boring Depth (ft) TB-3 64°34'20.0" 165°25'31.4" 10 ft NW of WTG 6-3 3 22 TB-4 64°34'18.0" 165°25'39.3" 10 ft S of WTG 6-1 4 13 TB-5 64°34'17.9" 165°25'39.5" 10 ft S of WTG 6-1 6 22 TB-6 64°34'15.9" 165°25'38.1" 20 ft SE of WTG 5-3 5 23 TB-7 64°34'09.0" 165°25'38.8" 9 ft N of WTG 4-2 5 22 TB-8 64°34'06.4" 165°25'38.1" 8 ft W of WTG 4-1 8 18 TB-9 64°34'04.2" 165°25'43.0" 11 ft E of WTG 3-3 6 22 TB-10 64°34'02.1" 165°25'47.9"w/in 10 f of WTG 3-2 2 11 TB-11 64°33'54.5" 165°26'01.5" 7 ft E of WTG 2-3 12 23 TB-12 64°33'54.3" 165°26'01.0" 40 ft SE of WTG 2-3 10 23 TB-13 64°33'54.7" 165°26'02.0" 30 ft NW of WTG 2-3 11 23 TB-14 64°33'48.2" 165°26'02.9"110 ft NE of WTG 2-1 5 22 TB-15 64°33'45.5" 165°25'57.5"175 ft NW of WTG 1-1 6.5 11 TB-16 64°34'06.9" 165°25'49.1" 11 ft SE of WTG 3-1 Not Encountered 34 TB-17 64°34'06.9" 165°25'48.5" 30 ft E of WTG 3-1 Not Encountered 35 TB-18 64°34'06.5" 165°25'49.2" 40 ft S of WTG 3-1 Not Encountered 35 TB-19 64°34'07.0" 165°25'50.0" 40 ft E of WTG 3-1 Not Encountered 13 TB-20 64°34'07.5" 165°25'48.9" 60 N of WTG 3-1 Not Encountered 35 TB-21 64°34'06.6" 165°25'46.2"130 ft E of WTG 3-1 Not Encountered 36 TB-22 64°34'07.3" 165°25'52.6"140 ft W of WTG 3-1 Not Encountered 34 Discussion and Recommendations Based on data provided by Western, we understand the towers will be a nominal 100-ft tall 3 leg lattice structure. Design load (per leg) for wind and ice conditions were provided by Western: Uplift:168 kips Download:158 kips Shear:14.6 kips Tower foundation design has been coordinated with the structural engineer, BBFM, Inc. The recommended tower foundation is a steel pipe riser seated to a cast-in-place concrete/grout pad on competent bedrock. The riser will be anchored to the bedrock with grouted anchor bars. Based on the structural engineer’s assessment, the deﬂection at the tower leg base necessary to mobilize the soil passive resistance may exceed the tower foundation pipe riser/base plate connection allowable stress. Accordingly, the foundation design has assumed the tower foundation pipe riser/base plate will control the lateral resistance with minimal passive resistance along the pipe/soil interface. For design purposes, the maximum allowable pipe riser length (tower leg base to concrete pad) for an 18-inch diameter pipe riser is 8-feet. Deeper embedment will require re-analysis by the foundation design team. In addition, an anchored concrete base/steel pipe riser will induce additional uplift force on the anchor(s) and additional download force on the bedrock surface. For the uplift condition, the force couple developed on the pipe riser from the lateral shear force at the tower leg base will be transferred through the anchor rod(s) over their horizontal separation. Thus, a maximum unfactored per anchor uplift load in the range of 70-kips may be developed with an 8-foot long, 18-inch diameter pipe riser with four (4) anchors per leg tensioned through a nominal 3-ft square concrete pad. The material in the test borings within the revised tower locations appeared to be consistent, except at tower locations WTG 2-3 and WTG 3-1. Tower location STG- Banner Ridge Wind TowersDuane Miller Associates, LLC August 23, 2008 Page 5 2-3 encountered bedrock at 10 to 12 feet below grade. This site will require regarding to achieve the recommended maximum rise length (8 feet) or a modiﬁed foundation system. Tower location WTG 3-1 did not encounter bedrock to at least 30 feet below grade, thus the recommended anchored foundation system is not suitable at this location. All other investigated tower locations encountered a metamorphic bedrock below a frost-fractured zone. Based on inferred geology at the revised tower sites, it is reasonable to assume a jointed, metamorphic bedrock suitable for the concrete pad bearing surface should be present no greater than 8 feet below grade at the proposed tower locations. However, actual bearing surface elevation may vary in depth based on site-speciﬁc conditions that may be encountered at time of foundation construction. A variable thickness of frost-fractured rock is present above the in- situ bedrock. The Banner Ridge area has several faults, and fault breccia or fault gouge may be present along fault contacts. If such fractured or poor quality bedrock zones are encountered at the proposed tower sites, we must be notiﬁed immediately to verify or modify the foundation recommendations. We have assumed the tower a foundations will be constructed prior to freeze up 2008. If the tower foundation construction should not occur prior to freeze up, we should review our recommendations prior to initiating construction work. Concurrent with tower foundations, several small transformer/control structures will be constructed near the tower sites. Foundation recommendations for these small structures are discussed after the tower foundation recommendations. Tower Foundation Recommendations The investigated site appears suitable for tower foundation support, provided the tower foundations are seated into hard (non-frost fractured) bedrock. If heavily fractured rock is present under the tower foundations, the following recommendations will require veriﬁcation or modiﬁcation. For the tower, we recommend founding each tower leg on a steel pipe riser over a cast-in-place concrete/grout leveling pad. The cast-in-place concrete/ STG- Banner Ridge Wind TowersDuane Miller Associates, LLC August 23, 2008 Page 6 grout pad will need to be anchored to competent bedrock with a series of grouted rock anchors (threaded bar anchors). The tower foundation design is comprised of three integrated elements, each discussed separately: Rock anchors (bar anchors) Cast-in-place concrete leveling pad Steel pipe riser Tower and ancillary structure foundations will require excavating frost- fractured rock to hard, competent bedrock. We have assumed the contractor will mass excavate the entire tower foundation footprint area rather than excavate individual tower foundation bases. Based on our air track test boring data, we estimate frost fractured rock may extend to 6 to 8 feet below grade, but ﬁnal excavation depths can be expected to vary depending on in-situ conditions encountered in a larger excavation. We do not recommend use of explosives for excavation since improper charge sets may lead to unnecessary over-excavation. We should be advised if the contractor is considering use of explosives at or near the tower foundation area. Rock Anchors (Grouted Threadbar Anchors) The cast-in-place concrete leveling pad and steel pipe riser should be structurally connected to the bedrock with a series of grouted rock anchors under each tower leg pad. Site preparation prior to anchor installation should include removal of frost-fractured rock to a hard bedrock surface. Based on test borings, frost-fractured rock should be expected to extend 6 to 8 feet below existing grade, but actual ﬁeld conditions can be expected to vary under each tower leg. The bedrock surface should be cleared of all debris and deleterious matter with compressed air or hand cleaning. Standing water, snow, ground or seasonal ice should not be present in or on the surface of the bedrock prior to anchor installation or concrete leveling pad construction. After site preparation, we have assumed the bedrock surface under the tower foundations will be relatively uniform but may not be level. While some surface irregularities under the concrete leveling pad can be tolerated, excessive STG- Banner Ridge Wind TowersDuane Miller Associates, LLC August 23, 2008 Page 7 slope, pockets of weathered material or other anomalies should not be present under the concrete leveling pads. If such irregularities are present, select removal with excavation equipment may be necessary. If excessive soft or loose materials or other anomalies are encountered, we should be notiﬁed prior to installing rock anchors or pouring the concrete leveling pad since adjustments to tower foundation position may be necessary. For design purposes, we have assumed a rough surface grade will be present under the concrete leveling pad with a vertical variation of less than 6- inches under each pad. Further, the bedrock surface at each pad need not be at the same elevation, but all prepared rock surfaces under each pad are assumed to be within a three to four foot horizon. Rock anchors should be 150-ksi 1.25-inch nominal diameter DYWIDAG Threadbars (or equivalent). Four anchors per each leg are required. We recommend anchors be a continuous single length, but if necessary, splicing with a manufacturer-supplied coupler is possible. A continuous single length, grouted bar can be installed in a nominal 3 to 4-inch diameter air track borehole. Boreholes should be advanced with a conventional air track or air down-hole hammer drilling equipment. Boreholes should be plumb and true, with minimal bit cuttings and debris along the borehole sidewall and at the bottom of the borehole, prior to anchor installation and grouting. Borehole clearing can be conducted with air lances or a venturi system if ground water is present. If groundwater is present, repeated ﬂushing with a venturi and air lance should be conducted until clear water is returned. Anchor rods should be installed to maintain a minimum 24-inch horizontal separation between anchor rod centerlines to reduce group effect. Closer spacing may be feasible, if necessary, but modiﬁcation to our recommendations will be necessary to accommodate group effects for closer spaced anchors. Some overdrilling of the anchor rod boreholes can be conducted to assure adequate anchor rod embedment. Anchor bars should be inserted into a properly prepared borehole with embedment depths conﬁrmed prior to grouting. Also, anchor boreholes will need to be properly oriented to align with the tower base assembly. Clean standing STG- Banner Ridge Wind TowersDuane Miller Associates, LLC August 23, 2008 Page 8 water in the boreholes is not considered an impediment to installing and grouting anchors. A properly prepared and placed grout will displace borehole water. However, snow, ice or other deleterious materials cannot be present on either the threadbars or the borehole sidewalls. Anchor bars will require manufacturer centralizers at nominal 5 to 6 foot intervals to maintain the anchor in the borehole annulus center during grouting. The basal centralizer should be installed within 2-feet of the bottom of the borehole. The anchors should be supplied with corrosion protection suitable for the expected environment. Full length hot dip galvanizing appears to be the preferred corrosion protection method. Other options may be suitable, pending assessment by an experienced corrosion protection designer. If spliced anchor sections are being considered, a 5-inch diameter borehole will be necessary to accommodate the coupler and grout encasement. If a coupler is used, it should extend at least two (2) feet into the grout. The coupler must be fully encased in grout. Once embedment lengths are veriﬁed, the rock anchor will require grouting. The anchor rods will require a nominal 3-foot free grout section (termed Lf) immediately below the concrete pad. The grouted section below this Lf section is the ﬁxed bond length, termed Lb. The Lf section is necessary to develop adequate pullout resistance of the rock cone under the entire foundation pad. The Lb section is necessary to transfer the uplift loads to the bedrock. In order to facilitate a single grout pour to the bedrock surface, we recommend placing a nominal 1.5 to 2-inch diameter PVC pipe section over the free length (Lf) portion of the anchor rod to the appropriate elevation and sealing the ends of the PVC with heat shrink so that grout cannot enter between the PVC and the anchor rod threads. In this manner, the entire embedded anchor rod section can be grouted to the bedrock surface in one continuous pour. We have assumed the cast-in-place concrete pad will be formed and poured while the anchor rod grout cures. After the concrete pad and anchor rod grout cures, the anchors will be tensioned and locked off to design loads as discussed below. STG- Banner Ridge Wind TowersDuane Miller Associates, LLC August 23, 2008 Page 9 Immediately prior to tensioning and lock off, the small anchor rod section extending through the concrete pad can be grouted. In this manner, the entire anchor rod will be encased in grout with no voids or pockets for water/ice to form. The Fondu grout/sand mixture should be approximately 3 parts Fondu cement to 1 part washed ﬁne sand (by mass). Washed ﬁne sand should pass U.S. Number 40 and be retained on U.S. Number 100 sieve size and not contain any frozen or deleterious matter. A Number 70 washed silica sand is recommended. Potable water is recommended for grout mixture at a water to Fondu plus sand ratio of 0.40 to 0.45 (by mass). Superplasticizers and accelerants appropriate for tremie placement with the contractor’s grout pump system and compatible with Fondu grout can be used. Grout requires tremie placement from the borehole bottom upward to the bedrock surface. Grout should be able to attain a minimum compressive strength of 10,000-pounds per square inch (psi) at 28-days following sampling and testing procedures discussed below. If a 3 to 4-inch diameter borehole is used with the 1.25-inch diameter anchor and a -inch diameter PVC (or similar) tremie pipe, the tremie pipe should be removed concurrent with grout placement. After tremie placement, the grout should be vibratory densiﬁed to remove air voids. We recommend rock anchors be installed to at least 12-feet below the prepared rock surface. This will provide a ﬁxed grout section (Lb) of at least 8- feet and a free length (Lf) of approximately 3-feet to the base of the concrete pad. The anchor will also require sufﬁcient length above the rock surface to accommodate a cast-in-place concrete pad, the steel plate and 2 locking nuts with 2 to 3-inches of free end above the nuts. Assuming full-contact grout along the entire ﬁxed grout (Lb) section, each anchor is expected to develop the adequate resistance to the design uplift with four (4) anchors per leg with a factor of safety of at least 2. As discussed previously, the design shear force imposed at the tower leg base will increase the uplift load on the anchor(s). While the actual load increase will vary with wind direction and anchor orientation, we have estimated the maximum transient load STG- Banner Ridge Wind TowersDuane Miller Associates, LLC August 23, 2008 Page 10 any single anchor would experience with this additional uplift load would be approximately 70-kips. The Hoek and Brown criteria were used to estimate the shear resistance developed along the assumed rock cone interface among the four anchors under each tower leg. This shear resistance combined with the static weight of the rock mass within the cone is expected to develop the design uplift resistance at each tower leg with a factor of safety of at least 1.5. Both the concrete base and anchor rod grout must be allowed to cure adequately prior to tensioning. Curing rates for both the concrete and grout can vary with the amount of water, superplasticizer and accelerant used for the concrete and grout mix. In general, we would recommend that anchor rod tensioning not be conducted until the anchor rod grout and concrete have attained their speciﬁed compressive strengths (10,000 and 4,000-psi, respectively) or as allowed by the design engineers and material testing specialist. However, the cast-in-place concrete pad can be installed within 12 hours of rock anchor grout placement. Representative samples of the grout should be collected at the time of tremie placement following procedures recommended in ASTM C-1107. Retained samples should be submitted for compression testing at a certiﬁed materials testing laboratory. At a minimum, we recommend two (2) grout samples be collected at each anchor, and a 7 and 28-day compressive strength test should be performed per ASTM C-109. The structural engineer or the material testing ﬁrm may recommend additional testing or testing frequency. Cast-in-Place Concrete Pad A cast-in-place concrete pad is recommended between the rock surface and the steel riser section. We estimate pad no greater than 2-inches thick would be suitable, but structural analysis is needed to verify the concrete pad thickness and steel reinforcement size and placement. Rock anchors should penetrate the concrete pad but should not be in direct contact with the concrete during pad pouring and concrete curing. A concrete form should be placed between the anchor rod to maintain a clear space between the concrete and the anchor rod. STG- Banner Ridge Wind TowersDuane Miller Associates, LLC August 23, 2008 Page 11 The PVC sleeve discussed earlier should be adequate for the concrete form. This space will be ﬁlled with grout prior to anchor rod tensioning as discussed previously. For geotechnical design purposes, we have assumed each cast-in-place concrete pad will have a nominal 9 square foot (sf) bearing surface (3-ft square dimension) and a compressive strength of at least 4,000-psi. The concrete pad should be designed to withstand seasonal freeze/thaw cycles as well as seasonal water saturation. Minimum offset between the anchor and the concrete pad edge will be determined by the structural engineer, but is assumed to be at least 6- inches for geotechnical purposes. We expect the concrete will be cast directly on the rock surface to reduce voids between the concrete and the rock surface. Snow, ice, standing water, debris or ﬁll material should not be present between concrete and the surface of the bedrock prior to placing concrete. Download will be resisted by bearing on the rock surface. We estimate a properly prepared, clean metamorphic bedrock surface will develop bearing capacity resistance to the 18,500-pounds per square foot (psf) design load (based on the 9 square foot concrete pad option) with a factor of safety of at least 3. Under the extreme short-term download case (the expected tower leg base shear induced load in addition to the design download), a factor of safety of at least 1.5 for bearing capacity on a properly prepared bedrock surface can be expected. Steel Pipe Riser Based on discussions with BBFM and STG, we have assumed a 18-inch diameter steel pipe riser welded to a base plate with stiffeners will be used above the concrete pad. We have assumed the base plate assembly will be set directly over the rock anchors. To reduce stress concentrations between the base plate and the concrete surface and reduce potential voids, the base plate should be seated on a thin bed of pure Fondu grout placed over the cured concrete immediately prior to placing the base plate/pipe rise assembly. Alternatively, the base plate assembly can be placed over wet concrete to reduce voids under the base plate. A shear key between the base place and the concrete does not appear necessary based on discussions with BBFM. STG- Banner Ridge Wind TowersDuane Miller Associates, LLC August 23, 2008 Page 12 After the cast-in-place concrete pad and rock section grout has cured adequately, the anchor rods must be tensioned. We have assumed tensioning will be conducted against a manufacturer supplied anchor plate directly on the steel base plate atop the concrete pad. We recommend tensioning each grouted anchor to 100-kips (~ 150-percent of design load) then backing down to a lockoff load of 40-kips. Deﬂection and load measurements should be collected during anchor tensioning as part of the construction installation records. At 100-kips tensioning load, the anchors will be subjected to approximately 60-percent their maximum capacity (187.5-kips). DYWIDAG does not recommend tensioning anchor rods in excess of 80-percent the maximum capacity. Also, at these tension loads (up to 100-kips), extreme caution is recommended. If breakage or slippage during tensioning should occur, extensive damage, ﬂying debris and other dangerous conditions may result. Grouted rock anchors at each pad should be tensioned sequentially and in a stepwise manner to balance tensioning loads. We recommend at least a four stage tensioning process. All four grouted anchors (per leg) should be tensioned to an initial seating load of approximately 20 to 25-kips. After an initial seating load, each anchor should be sequentially tensioned to approximately 70-kips then 100-kip load. At each step load, the primary anchor nut should be seated prior to moving the hydraulic cylinder (ram). The 100-kip tension load should be maintained for 15-minutes then step reduced to zero load, then reseated to the lockoff load. Tensioning should not be attempted until both the concrete pad and grout along the bedrock and anchor rod interface have attained their respective speciﬁed compressive strengths. We recommend a hollow plunger cylinder be used for tensioning with a digital pressure gauge (±1-psi resolution), calibrated to the cylinder. At each tensioning stage, pressure should be maintained to allow elastic strain and grout seating to develop. If ﬁeld strain measurements are not collected, we recommend maintaining cylinder pressure until digital pressures are maintained to within ±10-psi of target pressures. Once the lockoff load is attained, the primary anchor nuts require seating. Seating should consist of a tensioning to the 40-kip lockoff load and torque tightening the primary anchor nut until gauge pressure has reduced to near zero. STG- Banner Ridge Wind TowersDuane Miller Associates, LLC August 23, 2008 Page 13 After lockoff, a second locking anchor nut is recommended for each anchor bolt since the anchor assembly will be backﬁlled and not easily inspected. The second locking nut should be installed in accordance with the manufacturer’s recommendations. If desired, a fully grouted end cap can be placed over the entire tensioned and locked off anchor nut assembly. The tower foundations must be backﬁlled with locally available rock or excavation backﬁll. Compaction of coarse rock backﬁll may be impractical. Final grades should slope away from the tower foundations. If the cast-in-place concrete pad is seated and the rock anchors grouted into competent bedrock and tensioned as discussed above, we anticipate settlement (both total and differential) to be less than 0.25-inch during the life of the structure. The pipe risers are considered to behave as short, rigid piles. Lateral loads will be resisted at the pipe riser/base plate connection as discussed previously. Transformer Structure Foundation Recommendations A small transformer structure will be used for each bank of three wind towers. This is a lightly loaded, non heated structure with a ﬁberglass or aluminum building. This structure will be pre-fabricated. These structures can be founded on grouted 4-inch diameter steel open or closed-end pipes into nominal 6-inch diameter air track holes drilled at least four (4) feet into competent bedrock. A ﬁne sand/Fondu grout similar to that recommended for the tower anchors is recommended. A minimum 12-inch center-to-center horizontal separation between multiple pipes is recommended. Lateral resistance will be developed at the bedrock surface with additional resistance developed from rock material along the pipe risers. For design purposes, we recommend the point of ﬁxity be established at the bedrock surface for sustained loads. Lateral resistance developed along the pipe/backﬁll interface can be considered as additional capacity to further resist transient loads. All buried steel foundation components should have corrosion protection suitable for the environment. Upon completion of the ancillary structures work, STG- Banner Ridge Wind TowersDuane Miller Associates, LLC August 23, 2008 Page 14 the site should be backﬁlled and compacted as recommended for the tower foundations with seeding to promote vegetative cover. Construction Activity Control Based on observed surface conditions, we do not anticipate any signiﬁcant adverse thermal impact from construction related site disturbance. However, the regional geology at the proposed tower site is expected to have faulting and other structural geology features. We recommend a trained and experienced DMA engineer or geologist observe the tower foundation bedrock surface prior to anchor drilling and placement. The geologic conditions at each tower anchor borehole should also be observed and logged to conﬁrm that site conditions are in accordance with our recommendations. As part of the construction planning process, provisions should be included for adjusting the tower foundation anchor rod locations to avoid less desirable geologic conditions, if encountered during construction. It has been a pleasure to work with you on this project. Please feel free to contact us if you have any questions. Very truly yours, Duane Miller Associates LLC draft for review, no signature Aug 25, 2008 Richard Mitchells, P.E. STG- Banner Ridge Wind TowersDuane Miller Associates, LLC August 23, 2008 Page 15