Loading...
HomeMy WebLinkAboutDRB17-0078_Subsoil report_1489502940.pdf .. MAR5020 County Road 154 Glenwood Springs, CO 81601 Geotechnical Engineering I Engineering Geology Phone:(970)945-7988 Materials Testing I Environmental Fax: (970)945-8454 Email: hpkglenwood@kumarusa.com Office Locations: Parker, Glenwood Springs,and Silverthome,Colorado PRELIMINARY SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCES LOTS 1 AND 2,BLOCK 6,BIGHORN 3rd ADDITION 4367 and 4387 COLUMBINE DRIVE VAIL, COLORADO PROJECT NO. 16-7-508 NOVEMBER 9,2016 PREPARED FOR: CRESTONE BUILDING COMPANY ATTN: SCOTT HOFFMAN 501 WEST HALLAM ASPEN, COLORADO 81611 Scott@crestonebuildine.com TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY - 1 - PROPOSED CONSTRUCTION - 1 - SITE CONDITIONS _ 2- FIELD EXPLORATION - 2 - SUBSURFACE CONDITIONS - 2 - FOUNDATION BEARING CONDITIONS - 3 - DESIGN RECOMMENDATIONS - 3 - FOUNDATIONS - 3 - FOUNDATION AND RETAINING WALLS -4- FLOOR SLABS - 6 - UNDERDRAIN SYSTEM - 6 - SITE GRADING -7 - SURFACE DRAINAGE - 7 - LIMITATIONS - g - FIGURE 1 - LOCATION OF EXPLORATORY PITS FIGURE 2 - LOGS OF EXPLORATORY PITS FIGURE 3 - GRADATION TEST RESULTS TABLE 1- SUMMARY OF LABORATORY TEST RESULTS H-P KUMAR Project No. 16-7-508 PURPOSE AND SCOPE OF STUDY This report presents the results of a preliminary subsoil study for proposed residences to be located on Lots 1 and 2, Block 6, Bighorn 3`d Addition,4367 and 4387 Columbine Drive, 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 accordance with our agreement for geotechnical engineering services to Crestone Building Company dated October 11, 2016. We understand potential geologic hazards that may impact the lots are being evaluated by others. A field exploration program consisting of exploratory pits 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 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 At the time of our report preparation plans for the residences were conceptual and we understand the findings of our study will be considered in the purchase of the Iots. One residence is planned on each of the two lots located in the general area of our exploratory pits shown on Figure 1. The buildings will be 1 to 2 story wood frame structures over walkout basement Ievels. Ground floors may be slab-on-grade or structurally supported over crawlspace. Grading for the structures could be fairly extensive due to the steepness of the lots. We assume relatively light foundation loadings, typical of the proposed type of construction. H-P`KUMAR Project No. 16-7-508 - 2 - When building location, grading and loading information have been developed, we should be notified to re-evaluate the recommendations presented in this report and provide additional analyses as needed. SITE-CONDITIONS The lots are vacant and the ground surface in the building areas appears mostly natural. The terrain is south facing hillside above Columbine Drive. There are several shallow broad swales trending north to south on the lots. In general, the ground surface slopes moderately steep to steep down to the south at grades from about to 25 to 40%. Elevation difference across the assumed building areas is about 12 to 15 feet. The slope grades become steeper along the southern side of the lots adjacent Columbine Drive where cuts were made for construction of the road. Vegetation consists of grass and brush with smaller aspen and Iarger scattered evergreen and pine trees. There are boulders on the ground surface of the lots. FIELD EXPLORATION The field exploration for the project was conducted on October 14, 2016. Four exploratory pits were excavated at the locations shown on Figure 1 to evaluate the subsurface conditions. The pits were dug with a medium sized trackhoe. The pits were logged by a representative of H- P/Kumar. Access onto and across the lots was difficult and locations to excavate the pits were limited to the less steep hillside areas. Samples of the subsoils were taken by disturbed sampling methods. Depths at which the samples were taken are shown on the Logs of Exploratory Pits, 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. The subsoils encountered, below about 1 foot of organic rocky topsoil, consisted of relatively dense, H-P KUMAR Project No. 16-7-508 -3 - slightly silty sandy gravel and cobbles with boulders. Excavation in the dense coarse granular soils with the medium sized trackhoe was difficult due to the cobbles and boulders and excavation refusal was encountered in the deposit at depths from 3 to 41 feet. Laboratory testing performed on samples obtained from the pits included natural moisture content and gradation analyses. The soils were too rocky to obtain undisturbed samples for swell-consolidation testing. Results of gradation analyses performed on disturbed bulk samples (minus 3 inch fraction) of the coarse granular subsoils are shown on Figure 3. The laboratory testing is summarized in Table 1. No free water was encountered in the pits at the time of excavation and the subsoils were slightly moist. FOUNDATION BEARING CONDITIONS The natural coarse granular soils exposed in the pits possess moderate bearing capacity and relatively low settlement potential. We expect the coarse granular soils extend to at Ieast assumed excavation depth at the site, however, it is possible that different soils or bedrock could be encountered in deeper excavations and may require modifications to the foundation design. The excavation subgrade condition should be further evaluated at the time of construction. Additional subsurface drilling could be done prior to construction to better determine the subsoil conditions with depth. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory pits and the nature of the proposed construction, we recommend the buildings be founded with spread footings bearing on the natural coarse granular soils. H-P t KUMAR Project No. 16-7-508 -4- The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the undisturbed natural coarse granular soils should be designed for an allowable bearing pressure of 2,500 psf. The toe pressure of eccentrically loaded (retaining wall) footings can be increased by one-third. 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 18 inches for continuous walls and 2 feet for isolated pads. 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 should also be designed to resist lateral earth pressures as discussed in the "Foundation and Retaining Walls" section of this report. 5) The topsoil and any loose or disturbed soils should be removed and the footing bearing level extended down to the relatively dense natural coarse granular soils and the exposed subgrade compacted as feasible. Boulders and large cobbles encountered near footing elevation should be carefully removed to prevent disturbance of the bearing soils. Voids below footing areas resulting from large cobble or boulder removal should be backfilled with concrete or compacted road base. 6) A representative of the geotechnical engineer should observe all footing excavations prior to concrete placement to evaluate bearing conditions. FOUNDATION AND RETAINING WALLS Foundation walls and retaining structures up to 15 feet in height which are laterally supported and can be expected to undergo only a slight amount of deflection should be designed for a H-P KUMAR Project No. 16-7-508 - 5 - lateral earth pressure computed on the basis of an equivalent fluid unit weight of at least 50 pcf for backfill consisting of the on-site granular soils. Cantilevered retaining structures up to 15 feet in height which are separate from the buildings and can be expected to deflect sufficiently to mobilize the full active earth pressure condition should be designed for a lateral earth pressure computed on the basis of an equivalent fluid unit weight of at least 45 pcf for backfill consisting of the on-site granular soils. The backfill should not contain topsoil or oversized (plus 6 inch) rocks. For foundation and retaining walls taller than 15 feet, we should review our lateral earth pressure recommendations. All foundation and retaining structures should be designed for appropriate hydrostatic and surcharge pressures such as adjacent footings, traffic,construction materials and equipment. The pressures recommended above assume drained conditions behind the walls and a horizontal backfill surface. The buildup of water behind a wall or an upward sloping backfill surface will increase the lateral pressure imposed on a foundation wall or retaining structure. An underdrain should be provided to prevent hydrostatic pressure buildup behind walls. Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum standard Proctor density at a moisture content near optimum. Backfill placed in pavement and walkway areas should be compacted to at least 95% of the maximum standard Proctor density. Care should be taken not to overcompact the backfill or use Iarge equipment near the wall, since this could cause excessive lateral pressure on the wall. Some settlement of deep foundation wall backfill should be expected,even if the material is placed correctly, and could result in distress to facilities constructed on the backfill. Use of a select granular imported material such as road base and increasing compaction to at least 98% standard Proctor density could be done to reduce the backfill settlement potential. The lateral resistance of foundation or retaining wall footings will be a combination of the sliding resistance of the footing on the foundation materials and 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 0.50. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 375 pcf. The H-P KUMAR Project No. 16-7-508 - 6 - coefficient of friction and passive pressure values recommended above assume ultimate soil strength. Suitable factors of safety should be included in the design to limit the strain which will occur at the ultimate strength, particularly in the case of passive resistance. Fill placed against the sides of the footings to resist lateral loads should be a granular material, such as the on-site soils, compacted to at least 95% of the maximum standard Proctor density at a moisture content near optimum. FLOOR SLABS The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab-on-grade construction. 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 free- draining gravel should be placed beneath basement level slabs to facilitate drainage. This material should consist of minus 2 inch aggregate with at least 50% retained on the No. 4 sieve and Iess than 2% 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 topsoil and oversized (plus 6 inch) rocks. UNDERDRAIN SYSTEM Although free water was not encountered during our exploration, 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 system. H-P KUMAR Project No. 16-7-508 - 7 - The drains should consist of drainpipe placed in the bottom of the wall backfill surrounded above the invert level with free-draining granular material. The drain should be placed at each level of excavation and at least 1 foot below lowest adjacent finish grade and sloped at a minimum I% to a suitable gravity outlet. Free-draining granular material used in the underdrain system should contain less than 2% passing the No. 200 sieve, less than 50% passing the No. 4 sieve and have a maximum size of 2 inches. The drain gravel backfill should be at least 11 feet deep and be covered by filter fabric such as Mirafi I40N or 160N. SITE GRADING The risk of construction-induced slope instability at the site appears low provided the buildings are located above the steeper slope areas as planned and cut and fill depths are limited. We assume the cut depths for the basement level will not exceed one level, about 10 to 12 feet. Fills should be limited to about 8 to 10 feet deep, especially at the downhill side of the site where the slope steepens. Embankment fills should be compacted to at least 95% of the maximum standard Proctor density near optimum moisture content. Prior to fill placement, the subgrade should be carefully prepared by removing all vegetation and topsoil and compacting to at least 95% of the maximum standard Proctor density. The fill should be benched horizontally into the hillside. Permanent unretained cut and fill slopes should be graded at 2 horizontal to 1 vertical or flatter and protected against erosion by revegetation or other means. The risk of slope instability will be increased if seepage is encountered in cuts and flatter slopes may be necessary. If seepage is encountered in permanent cuts, an investigation should be conducted to determine if the seepage will adversely affect the cut stability. We should review site grading plans for the project prior to construction. SURFACE DRAINAGE The following drainage precautions should be observed during construction and maintained at all times after the residences have been completed: 1) Inundation of the foundation excavations and underslab areas should be avoided during construction. H-P KUMAR Project No. 16-7-508 - 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 buildings should be sloped to drain away from the foundation in all directions. We recommend a minimum slope of 12 inches in the first 10 feet in unpaved areas and a minimum slope of 3 inches in the first 10 feet in paved areas. This may require a swale uphill of the residences to divert surface water runoff. 4) Free-draining wall backfill should be capped with filter fabric and about 2 feet of the on-site finer graded soils to reduce surface water infiltration. 5) Roof downspouts and drains should discharge well beyond the limits of all backfill. 6) 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 pits excavated at the locations indicated on Figure 1 and to the depths shown on Figure 2, 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 pits 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. N-P;KUMAR Project No. 16-7-508 - 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, H-P KU 0 ssT:f.o . 14.,,Q, i° = t CI G.1 David A. Young, P.1. ='3 k 3P-216 $ V`NiReviewed by: �'�• ^li .1e lingionno --t-. ; -P....-4 4. Steven L. Pawlak, P.E. DAY/ksw H-P KUMAR Project No. 16-7-508 I WHITE RIVER NATIONAL FOREST ••••.....6 ...n.ammo*Oa VII iF OM.M .......=.01.61..1.t 11PGm• alr -. . --- 1..0"G G. .--or Kama — • .. 1{{orf .-... .-r II1I -- _— sot ....r% IrY P r G rr� +.1•u+ faro wray'fa I..+so.r. ` -�PIT 1` . 2 ` Lal ..�.. �,�= ROC�rSQia w �f nr 17tcs rw�'r til�ifir in....---.. ostr torr 1�� {506i. L tewo-�\Y//� r • ��r y�1 w r RSr aa ;IX/Vein •n 1,4 do a " YA.fw - '� ' • enh ♦rf- 1, -.4-"; IMM • .rte- —_ f - 0. 11.0111 ---- . ." 1 LOT 13 WK. . %.',..'" +- -_ ___fit: r � 1 �\ � \ _ --*_::- rt.... ` LOT 12 1 Si REMAINDER LOT 11 \ +:. t _ 1 \ . 1 • I I I. 1 1 5 a 3 1 30 0 30 60 'Ri APPROXIMATE SCALE-FEET 11 16-7-508 H-P-KUMAR I LOCATION OF EXPLORATORY PITS Fig. 1 PIT 1 PIT 2 PIT 3 PIT 4 EL. 8512' EL. 8510' EL 8505' EL 5815' —00— LU G ; W :X f t ..... —I+4=73 X 1 d— tri i+4=65 —200=6 -i yyC=1.5 — W— .sem i—200=8 _I— — W o !. 200-6 —o 5 5— _LEGEND .~ TOPSOIL; ORGANIC SILTY SAND AND GRAVEL WITH COBBLES, FIRM, SLIGHTLY MOIST, DARK ,,ti BROWN. sz rioGRAVEL AND COBBLES (GM); WITH BOULDERS, SANDY, SLIGHTLY SILTY, DENSE, SLIGHTLY MOIST, BROWN. DISTURBED BULK SAMPLE. t PRACTICAL EXCAVATION REFUSAL WITH A MEDIUM SIZED TRACKHOE. NOTES 1. THE EXPLORATORY PITS WERE EXCAVATED WITH A TRACKHOE ON OCTOBER 14, 2016. 2. THE LOCATIONS OF THE EXPLORATORY PITS WERE MEASURED APPROXIMATELY BY PACING FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED. 3. THE ELEVATIONS OF THE EXPLORATORY PITS WERE OBTAINED BY INTERPOLATION BETWEEN CONTOURS ON THE SITE PLAN PROVIDED. 4. THE EXPLORATORY PIT LOCATIONS AND ELEVATIONS SHOULD BE CONSIDERED ACCURATE ONLY TO THE DEGREE IMPLIED BY THE METHOD USED. 5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY PIT LOGS REPRESENT THE APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL. 6. GROUNDWATER WAS NOT ENCOUNTERED IN THE PITS AT THE TIME OF EXCAVATION. 7. LABORATORY TEST RESULTS: WC = WATER CONTENT (X) (ASTM D 2216); +4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D 422); —200 = PERCENTAGE PASSING NO. 200 SIEVE (ASTM D 1140). s i P 1 y= • iil == nr if 16-7-508 H-P- KUMAR LOGS OF EXPLORATORY PITS Fig. 2 HYDROMETER ANALYSISSIEVE ANALYSIS _ TIME READINGS 115. STANDARD SERIES CLEAR SQUARE crevin03 24 NRS 7 HRS 100 45 YIN IS 111$1 &OYIN 199141 /414 WG 1919 I O 1100 150 PAO# 35 116 1110_98_ !4 S/6- 3/4' I 1/3- 3' s"e' 6",, 90 I 1 I I I I ID Il I I It I 1 1 a° I f I 20 I f 7a I I 30 I II 1 d0 1 l I 0.1 I a0 I I 1 so ) _ I I I �r I 4 so EEf r II , 40 I I6U I 1 I 1 !a 1 1- 7D I 20 1 1 I r-."" e0 II 10 I 1 ...-I...-- 90 1 I I I I 1 I I 0 11 I 1 1 I II I h IIIIII I I II I1 1111 I I II 1111 to i L 1111:: t 100 .001 .002 .005 .009 .019 .037 .073 .150 300 I .6013 Ina 1 2.36 A.75 95 IS 38.1 76.2 127 200 .425 2.0I DIAMETER OF PARTICLES IN MILLIMETERS 132 CLAY TO SILT SAND GRAVEL COBBLES FINE I MEDIUM (COARSE FINE I COARSE GRAVEL 65 X SAND 27 X SILT AND CLAY 8 X LIQUID LIMIT PLASTICITY INDEX SAMPLE OF: Slightly Silty Sandy Gravel FROM: Pit I O 3-4' HYDROMETER ANALYSIS SIEVE ANALYSIS TIYC READINGS U.S. STANDARD SERIESCLEAR SQUARE OPENINGS 24 HI RS 7 HRS 100 a5 MIN 15 9114 GOYIM 19944 MAIN IwO 03 9 #100 1S0 140 130 Pit 410 S/ 45 (4 6" 314' I 1.12' 7' s'6' a'a 1 I 1 II i I I 1 ( I 90 ; I I i t I 1 { i I to I I I I i I 1 I I 1 I I I BO I I f : ; 20 I I 1 I ; 1 I I I I• 70 I I 1 + ' t I I t 30 • 1 I I I I I 1 I 11 g 60 `t 1 1 I I S I I res R I I 1 I I 1 i I SD I 1 I 1 I I i 1)1 i I i G i, (1 I 1 .I 11 i I I I } SO i `0 i I 1 • f ""1 1 iI I I t-}— so II I 1 I e I I I I i I I I i I I1 30 �- I l } 1 "' I I 1 I 70 1 I 20 t I 1 i ! I { I I i 4 ea 1 i i I i I I I { I I I 13 I I I I I i I I I ? 90 I I ' I I I I I I I 1 a I I 1 1• 4 11 I '1 1 1 1 1 1 1 I I I r 1 1 1 11 I I I I I 1 I PI t1 I it 11111 I r 1 I 100 .001 .002 .005 .009 .019 037 .075 .150 .300 + .600 1 II :236 4,75 9.5 19 36.1 762 1271 200 I DIAMETER OF PARTICLES IN MILLIMETERS 152 I CLAY TO SILT SAND GRAVEL ' FINE I MEDIUM (COARSE FINE I COARSE COBBLES yn F GRAVEL 73 Y. SAND 21 X SILT AND CLAY 6 X r 1 LIQUID LIMIT PLASTICITY NDEX 3 SAMPLE OF: Slightly Silty Sandy Gravel FROM: Pit 3 0 2-3' These toil results appy only 10 true samples which were tested. The Si foaling report shall not he reproduced. exe.pi In fut. without the wrltlen g approval of Kumar & Associalea. Inc. °' 5 ere analysis felling Is performed In 1; Accordance *115 ASTM 0122, ASTM C136 R And/or ASTM 011.10. 1 16-7-508 H-P- KUMAR GRADATION TEST RESULTS Fig. co 15 11 to ca ea Co • w 0 0 ch r a ,O ..7 z O. Cl Cl ClC .+ J CJ C/] V] C4 d COo mo v) iii ir a. m 7, >, >, aa C E5 W . , D w?X Z CO paW a co n0N J v D CD 0 Ce Ce LU co 0 u 2 5Z NCD H re m )- cz °I- p a =i - s. H W 16 ~Oc ))) J p ra Zzow H JaaZ(I) if- 0 I C — < O can r- N I= - _ 2 a J CO C a e in r, v:› N 0 I .0Z g < G 41 t-u) o -, a o 0 z 2 0 z x _ 1 C91(Q C M N M O W J a a a "' r1 U3