Loading...
HomeMy WebLinkAboutDRB160022_115512A (12-31-15) Subsoil Study signed (2)_1455919380.pdf Hepworth-Pawlak Geotechnical, Inc. 5020 County Road 154 Geortech Glenwood Springs,Colorado 8160! Phone:970-945-7988 HEPWORTH—PAWLAK GEOTECHNICAL Far;970-945-8459 cm.ul_hpgeo©hpgeotech.com SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED ADDITION TO FIRE STATION NO. 1 LOT 15,BIGHORN SUBDIVISION 4116 COLUMBINE DRIVE VAIL, COLORADO JOB NO. 115 512A DECEMBER 31,2015 PREPARED FOR: TOWN OF VAIL PUBLIC WORKS ATTN: JOHN KING 1309 ELKHORN DRIVE VAIL, COLORADO 81657 'kin vailgov.com) Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthome 970-468-1989 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY - 1 - PROPOSED CONSTRUCTION - 1 - SITE CONDITIONS - 2 - GEOLOGICAL CONDITIONS -2 - FIELD EXPLORATION - 3 - SUBSURFACE CONDITIONS -4 - ENGINEERING ANALYSIS - 5 - DESIGN RECOMMENDATIONS - 5 - FOUNDATIONS -5 - FLOOR SLABS - 6 - UNDERDRAIN SYSTEM - 7 - SURFACE DRAINAGE - 7 - LIMITATIONS - 8 - REFERENCES - 9 - FIGURE 1 - LOCATION OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 -LEGEND AND NOTES FIGURE 4-GRADATION TEST RESULTS TABLE 1-SUMMARY OF LABORATORY TEST RESULTS Job No, 115 5I2A Gatech PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed addition to the existing Fire Station#1 located at 4116 Columbine Drive,Lot 15, Bighorn Subdivision, Vail, Colorado. The project site is shown on Figure 1. The purpose of the study was to develop recommendations for the foundation design. The study was conducted in general accordance with our agreement for geotechnical engineering services to Town of Vail dated October 12, 2015. A field exploration program consisting of exploratory borings was conducted to obtain information on the subsurface conditions. Samples of the subsoils obtained during the field exploration were tested in the laboratory to determine their classification and other 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 building foundation. This report summarizes the data obtained during this study and presents our conclusions, design recommendations and other geotechnical engineering considerations based on the proposed construction and the subsurface conditions encountered. PROPOSED CONSTRUCTION The proposed addition will be a one story structure attached to the back, western side of the existing fire station, shown on Figure 1. Ground floor will be slab-on-grade and match the elevation of the building floor. Grading for the structure is assumed to be relatively minor with the finish floor elevation at or slightly above adjacent grade. We assume relatively light foundation loadings, typical of the proposed type of construction. If building loadings, location or grading plans change significantly from those described above, we should be notified to re-evaluate the recommendations contained in this report. Job No. 115512A Gatech -2 - SITE CONDITIONS The site is bordered by developed residential lots to the northwest and southeast, Gore Creek to the west, and Columbine Drive to the northeast. The proposed addition is located along the west side of the existing Fire Station#1 in a paved area with a slight slope down to the west. The area appears to have been previously filled to create a relatively flat parking area and a steep slope mainly down to the west to flat terrain adjacent to Gore Creek. Vegetation outside of the pavement area generally consists of grass with brush and pine trees near Gore Creek. GEOLOGICAL CONDITIONS Review of the Town of Vail official geologic hazards maps indicates that potential snow avalanche and rockfall hazards are present on and in the vicinity of Lot 15 (Town of Vail, 2000). Snow Avalanche: The proposed addition on Lot 15 is located in the lower part of the "Possible Avalanche Influence Zone" from the start area located on the valley side to the southwest of the subject site. The snow avalanche hazards zones were defined before construction of the existing residences. The existing residences located to the southwest of Gore Creek should act as snow avalanche retarding structures and will shorten the snow avalanche runout distances and reduce the risk to the proposed addition on Lot 15. Considering the above, in our opinion, the proposed addition on Lot 15 is located beyond avalanche impact area and the risk of snow avalanche reaching the proposed addition is low. If this low risk is not acceptable to the building owners, we expect snow avalanche risk mitigation should be feasible without increasing the potential risks to nearby properties. A consultant experienced in snow avalanche evaluation should be consulted for the mitigation design. Rockfall: Town of Vail rockfall hazards map indicates that Lot 15 is located in the lower part of a potential "High Severity Rockfall" impact area which extends down to Columbine Drive on the northeast side of the lot. Rock blocks associated with past Job No. 115 512A v_-.. . _...CteCh - 3 - rockfalls were not observed on the site but evidence of past rockfall, if present, may have been removed by prior construction grading. Based on aerial photography (Google Earth) and our site visit, two large rockfall blocks were visible approximately 175 and 200 feet south of the proposed addition, and a large rockfall block was observed approximately 125 feet southwest of the project site in Gore Creek. Large rockfall blocks were not observed across Gore Creek in the vicinity of the project site. Considering the limited number of rockfall blocks in the area of Lot 15, the length of horizontal runout from the steep slope to the lot, the intervening basin occupied by Gore Creek and being located at the lower extent of the rockfall hazard zone,in our opinion, we believe the rockfall severity to be low to moderate,but not nonexistent. Although we judge the rockfall hazard to the project site as low to moderate, rockfall by its nature is unpredictable and difficult to quantify. If a future rockfall were to hit the proposed construction, it is likely that it would result in damage to the structure and possibly harm the building occupants. If this rockfall risk potential is not acceptable to the building owners then additional site specific rockfall study will be needed to evaluate the potential rockfall risk and feasibility of possible mitigation methods to reduce the risk. Based on our review of the mapped geologic conditions, review of aerial photography and the observed site conditions, in our opinion,construction of the proposed addition will not increase rockfall risks to adjacent properties or structures including public buildings, roads, street right-of-ways or utility easements or facilities. FIELD EXPLORATION The field exploration for the project was conducted on November 12, 2015. Four exploratory borings were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions. Three borings were requested by the architect and we added a fourth boring located in the building addition area due to the existing fill being encountered. The borings were advanced with 4 inch diameter continuous flight augers powered by a truck-mounted CME-45B drill rig. The borings were logged by a representative of Hepworth-Pawlak Geotechnical, Inc. Job No. 115 512A Gaitstech -4 - Samples of the subsoils were taken with a 1% inch LD. spoon sampler. The sampler was driven into the subsoils at various depths 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. The penetration resistance values are an indication of the relative density or consistency of the subsoils. Depths at which the samples were taken and the penetration resistance values are shown on the Logs of Exploratory Borings, Figure 2. The samples were returned to our laboratory for review by the project engineer and testing. SUBSURFACE CONDITIONS Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. At Borings 1 and 2, located within the proposed building addition footprint, below approximately 7 inches of asphalt, the subsoils consisted of up to 71/2 feet of loose to medium dense, silty sandy gravel fill material with cobbles overlying dense slightly silty, sandy gravel with cobbles and boulders. Drilling in the natural coarse granular soils with auger equipment was difficult due to the cobbles and boulders and drilling refusal was encountered in Borings 1 and 2 at a depth of 141/2. At Borings 3 and 4, located to the north of the existing pavement area,below approximately 1 foot of organic topsoil, the subsoils consisted of 3 to 42 feet of silty, sandy gravel fill material overlying slightly silty, sandy gravel with cobbles and boulders to the maximum depth explored, 11 feet. The fill was occasionally clayey. The purpose of drilling Borings 3 and 4 was mainly to assess the depth and condition of the existing fill. Laboratory testing performed on samples obtained from the borings included natural moisture content, gradation analyses and Atterberg limits. Results of gradation analyses performed on small diameter drive samples (minus 11/2 inch fraction) of the natural coarse granular subsoils are shown on Figure 4. The laboratory testing is summarized in Table 1. No free water was encountered in the borings at the time of drilling and the subsoils were slightly moist to moist. Job No. 115 512A Ggslech -5 - ENGINEERING ANALYSIS The existing fill placement is undocumented and suitability of the fill to support building loads could vary across the addition area. The existing fill should be removed from beneath footing areas down to the natural dense coarse granular soils. The fill soils encountered are mainly granular and should be suitable for use as backfill behind walls and as structural fill below footings, slabs, and pavement areas. The bearing conditions of the subgrade beneath the footing and floor slab areas should be further evaluated at the time of construction and prior to placement of the new structural fill. The existing fill and subsurface profile encountered in Borings 3 and 4 are similar to Borings 1 and 2. If development is proposed in the north part of the lot, we should be contacted for further evaluation. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory borings and the nature of the proposed construction, we recommend the building be founded with spread footings extended down to the natural granular soils or placed on properly compacted structural fill bearing on the natural granular soils. There could be some differential settlement of new foundations with respect to the existing structure which should be considered in the design. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the undisturbed natural granular soils or properly placed structural fill can be designed for an allowable bearing pressure of 3,000 psf. Based on experience, we expect settlement of footings designed and constructed as discussed in this section will be about 1 inch or less. 2) The footings should have a minimum width of 16 inches for continuous walls and 2 feet for isolated pads. Job No. 115 512A —. . Gabel - 6 - 3) Exterior footings and footings beneath unheated areas should be provided with adequate soil cover above their bearing elevation for frost protection. Placement of foundations at least 48 inches below exterior grade is typically used in this area. 4) Continuous foundation walls should be reinforced top and bottom to span local anomalies such as by assuming an unsupported length of at least 10 feet. Foundation walls acting as retaining structures (if any) should also be designed to resist a lateral earth pressure corresponding to an equivalent fluid unit weight of at least 50 pcf. 5) The existing fill and any loose disturbed soils should be removed and the bearing level extended down to competent natural granular soils. Structural fill placed to re-establish design footing bearing levels should be compacted to at Ieast 98% of the maximum standard Proctor density at a moisture content near optimum. The structural fill can consist of the on- site granular soils devoid of vegetation, topsoil and plus 6 inch rocks. The fill should extend laterally beyond the edges of the footings a distance equal to at least 1/2 the depth of the fill below the footings. 6) A representative of the geotechnical engineer should observe all footing excavations prior to footing construction or fill placement to evaluate bearing conditions. Any structural fill should be monitored and tested for compaction by the geotechnical engineer. FLOOR SLABS The existing fill material appears to have a low risk of settlement under lightly to moderately loaded floor slabs. Slab-on-grade construction may be used in the addition provided the risk of some floor slab movement and possible distress is accepted by the owner. If the risk of potential floor slab movement and distress is unacceptable, then the existing fill material below the floor slab area should be removed to a minimum depth of 3 feet below slabs or down to the natural granular soils and replaced with a properly placed and compacted structural fill. Job No, 115 512A 1 - 7 - To reduce the effects of some differential movement, floor slabs should be separated from all bearing walls and columns with expansion joints which allow unrestrained vertical movement. Floor slab control joints should be used to reduce damage due to shrinkage cracking. The requirements for joint spacing and slab reinforcement should be established by the designer based on experience and the intended slab use. A minimum 4 inch layer of road base should be placed beneath interior slabs for support. This material should consist of minus 2 inch aggregate with at least 50% retained on the No. 4 sieve and less than 12%passing the No. 200 sieve. All fill materials for support of floor slabs should be compacted to at least 95% of maximum standard Proctor density at a moisture content near optimum. Required fill can consist of the on-site granular soils devoid of vegetation, topsoil and oversized rocks. UNDERDRAIN SYSTEM It is our understanding the finished floor elevation at the lowest level is at or above the surrounding grade. Therefore, a foundation drain system is not required. It has been our experience in mountainous areas that local perched groundwater can develop during times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can also create a perched condition. We recommend below-grade construction, such as retaining walls, crawlspace and basement areas,be protected from wetting and hydrostatic pressure buildup by an underdrain and wall drain system. If the finished floor elevation of the proposed structure has a floor level below the surrounding grade, we should be contacted to provide recommendations for an underdrain system. All earth retaining structures should be properly drained. SURFACE DRAINAGE The following drainage precautions should be observed during construction and maintained at all times after the addition has been completed: 1) Inundation of the foundation excavations and underslab areas should be avoided during construction. Job No, 115512A - 8 - 2) Exterior backfill should be adjusted to near optimum moisture and compacted to at least 95% of the maximum standard Proctor density in pavement and slab areas and to at least 90% of the maximum standard Proctor density in landscape areas. 3) The ground surface surrounding the exterior of the building should be sloped to drain away from the foundation in all directions. We recommend a minimum slope of 6 inches in the first 10 feet in unpaved areas and a minimum slope of 21/2 inches in the first 10 feet in paved areas. Free-draining wall backfill should be capped with about 2 feet of the on- site finer graded soils to reduce surface water infiltration. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. 5) Landscaping which requires regular heavy irrigation should be located at least 5 feet from foundation walls. LIMITATIONS This study has been conducted in accordance with generally accepted geotechnical engineering principles and practices in this area at this time. We make no warranty either express or implied. The conclusions and recommendations submitted in this report are based upon the data obtained from the exploratory borings drilled at the locations indicated on Figure 1, the proposed type of construction and our experience in the area. Our services do not include determining the presence,prevention or possibility of mold or other biological contaminants (MOBC) developing in the future. If the client is concerned about MOBC, then a professional in this special field of practice should be consulted. Our findings include interpolation and extrapolation of the subsurface conditions identified at the exploratory borings and variations in the subsurface conditions may not become evident until excavation is performed. If conditions encountered during construction appear different from those described in this report, we should be notified so that re-evaluation of the recommendations may be made. Job No. 115 512A . Ger6e.h -9 - This report has been prepared for the exclusive use by our client for design purposes. We are not responsible for technical interpretations by others of our information. As the project evolves, we should provide continued consultation and field services during construction to review and monitor the implementation of our recommendations, and to verify that the recommendations have been appropriately interpreted. Significant design changes may require additional analysis or modifications to the recommendations presented herein. We recommend on-site observation of excavations and foundation bearing strata and testing of structural fill by a representative of the geotechnical engineer. Respectfully Submitted, HEPWORTH -PAWLAK GEOTECHNICAL, INC. Steven L. Pawlak P.E.esz 1 5 2 2 2 . 4 Reviewed t ' ;�/ Ake X 4 /„Li„,y) 4 , David A. Young, P.' . 41110 SLP/ksw cc: Vail Architects—Bill Pierce(bill@vailarchitects.com) REFERENCES Town of Vail, 2000a, Official Avalanche Hazard Map, Town of Vail: Prepared by the Town of Vail, Vail, Colorado (Adopted by the Town Council on October 17, 2000). Town of Vail, 2000b, Official Debris Flow Hazard Map, Town of Vail: Prepared by the Town of Vail, Vail, Colorado (Adopted by the Town Council on October 17, 2000). Town of Vail, 2000c, Official Rockfall Hazard Map, Town of Vail: Prepared by the Town of Vail, Vail, Colorado (Adopted by the Town Council on October 17, 2000). Job Na. 115512A G beck APPROXIMATE SCALE 1" =30' GOREcREEk /008766 RFCOOd 0A1 ` I E ` ` 8 r I qo r 0 ----- ____ _ —7.-A. 1 it —8110_ � � / \ ., ' EDGE OF EXISTING BORING 1 PARKING LOT \\ l BORING 3 1 • \ A p�ToEp \\ N • BORING 2 \ 1 I EkisriN Ors oN PYRE 1 LUMak.06, BORING 4 11 0'js I I \ I \ EXISTING PARKING LOT 1 N. I N. \ 1 (\ COLUMBINE DRIVE \ 115 512A ®Ch LOCATION OF EXPLORATORY BORINGS Figure 1 HEPwQItrH-FAwuu[GEOTECHNICAL BORING 1 BORING 2 BORING 3 BORING 4 ELEV.= 8112.5' ELEV.= 8112.5' ELEV.= 8111.5' ELEV.= 8112.5 8115 8115 — — APPROXIMATE GARAGE FLOOR LEVEL — ___ — 8110 28/12 7/12 18/12 WC 6 8 8110 — :1:.: WC 8.1 -200 i4 _ -200 19 — 64/12 7112 ,. WC=1.9 12/6,50/5 13/12 — WC=7rii ,6 ,� +4=56 ;; -- _ — -200=14 lq: -200-10 '• — a, 8105 >z' 8105 — e q°� QL' pecr: C O _ L7C� co LU , •' 55/12 72/12 'a 14/6,50:5 4 r 73/12 !'x: �4 55 L8'� 8100 ;,-; -200=12 q 8100 ar.• • 92/12 25/6,60/5 — 8095 8095 — Note: Explanation of symbols is shown on Figure 3, 1H 115 512A LOGS OF EXPLORATORY BORINGS Figure 2 Hepworth--Powlak Geotechnlrol LEGEND: IASPHALT PAVEMENT; about 7 inches thick. 2 TOPSOIL; organic silty sand, moist, dark brown. ® FILL; silty sandy gravel,occasionally clayey,cobbles, loose to medium dense, moist to very moist, brown. hq GRAVEL AND COBBLES (GM-GP); slightly silty, sandy, boulders, dense, slightly moist, brown, rounded to subrounded rock. 11 Drive sample; standard penetration test (SPT), 1 3/8 inch I.D. split spoon sample,ASTM D-1586. 7/12 Drive sample blow count; indicates that 7 blows of a 140 pound hammer falling 30 inches were required to drive the SPT sampler 12 inches. T NOTES: 1. Exploratory borings were drilled on November 12, 2015 with 4-inch diameter continuous flight power auger. 2. Locations of exploratory borings were measured approximately by pacing from features shown on the site plan provided. 3. Elevations of exploratory borings were obtained by interpolation between contours shown on the site plan provided. 4. The exploratory boring locations and elevations should be considered accurate only to the degree implied by the method used. 5. The lines between materials shown on the exploratory boring logs represent the approximate boundaries between material types and transitions may be gradual. 6. No free water was encountered in the borings at the time of drilling. Fluctuation in water level may occur with time. 7. Laboratory Testing Results: WC = Water Content (%) +4 = Percent retained on the No. 4 sieve -200 = Percent passing No.200 sieve NP = Non-Plastic 115 512A G iiistech LEGEND AND NOTES Figure 3 HEPWOR H-PAWLAK GEOTECHNICAL I HYDROMETER ANALYSIS I SIEVE ANALYSIS I TIME READINGS U.S STANDARD SERIES I CLEAR SQUARE OPENINGS 1 HR 45 MIN.15 MIN .60MIN19MIN 4 MIN 1 MIN #200 #100 #50 #30 #16 #8 #4 318' 314- 1 112' 3' 5'6' 8' 0 1,... la......,... .11. 'ma II•IMI• MI 11 MII Ill• A II1=•I =. 1 NOMMEIN IIIII = —a-..is--- = . _--_ 10..I,— ate.------ 90 as a-- MEM a i a--- IaMIIl11 INIMIII 20 ia ^-. —a.`--a-II ..y ra i1 .— --A alae--- W a..s. 0 .—aa--- Z _— Z 30 il�..- �a.r-aai-i 715 _ Q �a■r--i-- U1 IMIw-w-�- RIM- a� -.r. Cl) 40 aaar�— _AM���- Q CC INS—�.. _rte ar--i-— H. (L al ali i--IMPI 11 Z - --i --- Z 50 ii_a- 1..---.=RIO r.W.----,MI w V —�C --aaaa--- �,Ai��iaai-i CC 60. • -aa��a•.aa--- 1- CC w W —a—` a'—as—aai—a-- W i=. ...s aaaaaa--- =-- 7D ice^'-- ai�a.�iaa�aar.r -- 2d .� as as i--iia•-- i a-- --- ..p/� — r 80 aaa—a. ��iaaii�i aaaa i-i ia--.aaii..-- r—as aaa-- a-- �as i--- ia--iii J El!MR 11 OU 111MR11 ill ia--a- I] is—a a--a- - I- - .ar-- Iia-- spa-..as--- 100 -'ma ..-ii- 0 .501 .002 .CO5 009 .519 .037 .074 .150 .203 .600 1 t8 236 4 75 5 5 1FS 10 C. 37 5 78.2 152 203 127 DIAMETER OF PARTICLES IN MILLIMETERS CLAY 7050_T 'M I GRAVEL COBBLES FINE I MEDIUM I COARSE FIRE I COARSE - GRAVEL 55 % SAND 33 % SILT AND CLAY 12 % LIQUID LIMIT % PLASTICITY INDEX % SAMPLE OF: Slightly Silty Sandy Gravel FROM: Boring 1 at 10 Feet HYDROMETER ANALYSIS I SIEVE ANALYSIS q TIME READINGS U S STANDARD SERIES I CLEAR SQUARE OPENINGS 45 IN 15 IN 60M1N19MIN 4 MIN 1 MN #L:00 #100 #50 #30 #16 #8 #4 318' 314' 1 12 3 5'6' 8' 100 tea.—Baa-- aa a-- -■as--a- II 111,11 -10 =1 1M 10 ._. --aas--a 90 .... a--.rr-a- 2D aaa-- -------- �� la 80 O is—a- as-aha- .am......11J 1R Z aaila V aml I WI IN i a-- LLzJJ Z 30 . >—...-,I air--....., 7D .76..... . --'-as--- 01 40aa.a �'GiC�-C �— 60 11M M=i1 a Q —�-� 4—p- ---- I- i=- F l a.i as--- H A.ilaa as--a— 50 Z LZu 50 .1- —= =_--- w --ii1- R--- —N Lias MI . MMI i tea.ii--- II 60 aa�= �i-.��..-- 40 CC UJ as-- ,..iaaiaa--- W n rte-�a.iaaa--�alla ice- ai --- aD a - _ 1•13 . 11 MIII MOON a-- Ilf 20 fl .�-aa ii i_-a --III MI ice- ---i--- III a--- FE R MI III IMI i MN ill 100 .--a■ �UMW IMINal_a I.. 001 002 005 009 019 037 !.74 150 -300 .600 1 18 2 36 4 75 9 512 519.0 37 5 76.2 121152 203 DIAMETER OF PARTICLES IN MILLIMETERS CLAY o sl r ' FINE I SANO MEDIUM I COARSE ` FIRE GRAVELI COARSE l GRAVEL 56 % SAND 34 % SILT AND CLAYS 10 % LIQUID LIMIT % PLASTICITY INDEX % SAMPLE OF: Slightly Silty Sandy Gravel FROM: Boring 2 at 4 y,Feet H 115 512A GRADATION TEST RESULTS Figure 4 Hepworth-Pawlak Geotechnicol a a� m. Tw V u] in 0 ca ea at Z O� Z. z, 6. O r 0 N '"" W C/] (ID (I) :r .., (/] C/] Z' "�O 'mobbA4' Z' in rig v� va vs W D� Z V7 I- aw0 C paa CO C.0 H D 0 ca Z J 0 — D - J !!1 0 N LLI tu * U gD z z Z In 0 m LU re0 W 1— co DF O~ >-. w � W O a o4 7 � Ta _ YWCE aJQ zzcw Qm c�la rnGW - - o - a N a I— J a.0- a u, _ O D CC Ce o ran t en en 11 0 o I U) 0 a in CC 0 . r math 0 HDz a; z D r K F CC H w ; 10 d Q• ---- 00 I—O V '`\- C� M ,--i 00 10 0 o w O �L co a D r" Il B, 0 W z a t9 z U Cl, 0 z z I 'k