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HomeMy WebLinkAboutB16-0209_HP Geotech - Soils Report - 6-13-16_1512688152.pdf Hepworth-Paw•lak Geotechnical, IncGL26-4St . 5020 County Road 154 �� Gime: Springs,Colorado 81601 Phone:970-945-7983 HEPWORTH-PAWLAK GEOTECHNICAL Fax:970-945.8454 email:hpgeo' hpgeorech.com June 13, 2016 Division Six Construction Attn: Randy Fischer 2636 Davos Trail Vail, Colorado 81657 randy@divisionsixconst.com Job No.116 205A Subject: Subsoil Study for Foundation Design, Proposed Retaining Wall, Lot 2, Block C, Vail Ridge, 2636 Davos Trial, Vail, Colorado Gentlemen: As requested, Hepworth-Pawlak Geotechnical, Inc. performed a subsoil study for design of the retaining wall foundations at the subject site. The study was conducted in general accordance with our agreement for geotechnical engineering services to you dated May 23, 2016. The data obtained and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this report. Proposed Construction: The existing residence is being renovated and a retaining wall is proposed for outside parking at the northwest corner of the property as shown on Figure 1. The driveway and parking area will have hard surfaced pavement. Cut and fill depths are expected to range between about 5 to 10 feet and the wall will extend out beyond the existing driveway edge. Foundation loadings for the residence construction are assumed to be relatively light and typical of the proposed type of construction. If building conditions or foundation loadings are significantly different from those described above, we should be notified to re-evaluate the recommendations presented in this report. Site Conditions: The lot is occupied by tan existing 3-story residence with a walkout lower level located approximately as shown on Figure 1. The ground surface is moderately steep sloping down to the south with roughly 20 feet of elevation difference across the building footprint. Vegetation around the building mainly consists of grass. Subsurface Conditions: The subsurface conditions at the site were evaluated by drilling one exploratory boring at the approximate location shown on Figure 1. The log of the boring is presented on Figure 2. The subsoils encountered, below about 101/2 feet of silty sandy clay and organics mixed fill,consist of inter-layered medium dense/stiff sand and clay and medium dense silty sand and gravel to the boring depth of 49 feet. Results of a gradation analysis performed on a sample of silty sand and gravel (minus 11/2 inch fraction) obtained from the boring are presented on Figure 3. We also observed pits dug down to the existing building foundation at the locations shown on Figure 1 on May 24, 2016. Results of swell-consolidation testing performed on a sample of clay taken from Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthome 970-468-1989 _ ., - Pit 1, presented on Figure 4, indicate moderate to high compressibility under conditions of loading and wetting. It appears the relatively high compressibility may have been partly due to sample disturbance. The laboratory test results are summarized in Table 1. No free water was encountered in the boring or observed in the pits at the time of excavation and the soils were generally moist. Foundation Recommendations: Considering the subsoil conditions encountered in the exploratory boring and the nature of the proposed construction, the retaining wall can be founded on a spread footing placed on the undisturbed natural sand and clay soil designed for an allowable soil bearing pressure of 2,000 psf. The soils tend to compress after wetting and there could be post-construction foundation settlement depending on the loading and depth of wetting. Footings should be a minimum width of 16 inches for continuous walls and 2 feet for columns. The existing fill, topsoil and loose disturbed soils encountered at the retaining wall area should be removed down to the undisturbed natural soils. Exterior footings should be provided with adequate cover above their bearing elevations for frost protection. Placement of footings at least 42 inches below the exterior grade is typically used in this area. We should observe the completed excavation prior to footing or wall construction. Foundation Walls and Retaining Structures: Building foundation walls which are laterally restrained and can be expected to undergo only a slight amount of deflection should be designed for a 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 soils. Cantilevered or site retaining walls which are separate from the residence and can 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 40 pcf for backfill consisting of the on-site soils. 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 near optimum moisture content. 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 large 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 granular structural fill, such as road base, and compaction to at least 98% of standard Proctor density can be used to limit the 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 Job Nor.116 205A Gtech - 3 - 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 0.40. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 350 pcf. The 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 compacted to at least 95% of the maximum standard Proctor density at a moisture content near optimum. Underdrain System: Although free water was not encountered during our exploration, it has been our experience in the area and where there are clay soils that local perched groundwater can also develop during times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can 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. 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 1% 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. Surface Drainage: The following drainage precautions should be observed during construction and maintained at all times after the residence has been completed: 1) Inundation of the foundation excavations and underslab areas should be avoided during construction. 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. Free-draining wall backfill should be covered with filter fabric and capped with about 2 feet of the on-site, finer graded soils to reduce surface water infiltration. 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 12 inches in the first 10 feet in unpaved areas and a minimum slope of 3 inches in the first 10 feet in pavement and walkway areas. 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 the building. 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 Job No.116 205A Gtech -4 - this report are based upon the data obtained from the exploratory boring and pits located as shown on Figure 1 and to the boring depth 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 boring and 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 at once so re-evaluation of the recommendations may be made. 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. If you have any questions or need further assistance, please let us know. Respectfully Submitted, HEPWORTH- PAWLAK GE• .NICAL, INC. • , . 16222 .*, Steven L. Pawlak, P.E , ' Reviewed by: ' y ��J; Daniel E. Hardin, P.E. SLP/ksw Attachments: Figure 1 -Location of Exploratory Boring Figure 2- Log of Exploratory Boring Figure 3 - Legend and Notes Figure 4-Gradation Test Results Figure 5-Swell-Consolidation Test Results Table 1 - Summary of Laboratory Test Results lob No,116 205A Gtech APPROXIMATE SCALE 1 20' irN. 8000 s - _ 1 0 � — \ F______ , --, \ , .\ \ -----i \ I PIT 1 \ \ (8 1/2 DEEP) ■ PIT 2 ` (5 1/2' DEEP)\ I X • 000 \I 1 I LOT 1 EXISTING RESIDENCE _ 1 L.L. = 8011.9' 1 SOO \ LOT 3 I \ GARAGE PIT 3 \ `ADDITION 51/2'DEE8 F.F. = 8029.0' 070 .--.. ( in PROPOSED RETAINING \� I WALL PIT 4 8 pR/VF BORING 1 1/2'DEE` 602 o LOT 2 x'41' BLOCK C --. --. —. — — — — —. -- / — , EXISTING \ BLOCK 1 WALL 1 DAVOS TRAIL t 116 205A gr h LOCATION OF EXPLORATORY BORING Figure 1 Hepworth—Pawlak Geotechnical BORING 1 ELEV.= 8020' 0 0 1/12 — .■ 2/12 189 — Owl DD 106 10 -200...71 WC-135— — • DD 12 10 -200=42 UC 1,500 15/12 WC 7 9 _ DD 109 +4 41 20fon — 25/12 200 24 20 a� a) IL — — 9112 — r WC1 201 DD 103 — 30 -200 78 30 ❑ 10/12 11 WC=19.1 40 DD-=104-200-84 40 UC-2,300 54/12AIM 50 50 NOTE: Explanation of symbols is shown on Figure 3. 116 205AGcrote Hch LOG OF EXPLORATORY BORING Figure 2 Hepworth—Pawlak Geotechnical LEGEND: FILL; mixed silty sandy clay and organics, wood debris, loose/soft,very moist, dark brown. .::j SAND AND CLAY(SC-CL); silty, scattered gravel to gravey, medium dense/stiff, moist, brown, low plasticity SAND AND GRAVEL (SM.GM); sty, medium dense, moist, brown. 61 11 Relatively undisturbed drive sample; 2-inch I.D. Caifornia liner sample, 5/12 Drive sample blow count; indicates that 5 blows of a 140 pound hammer falling 30 inches were required to drive the California sampler 12 inches. NOTES: 1. The exploratory boring was drilled on June 3, 2016 with a 4-inch diameter continuous flight power auger. 2. The exploratory boring location was measured approximately by pacing from features shown on the site plan provided. 3. The exploratory bor ng elevation was obtained by interpolation between contours on the site plan provided. 4. The exploratory boring location and elevation should be considered accurate only to the degree implied by the method used. 5. The lines between materials shown on the exploratory boring log represent the approximate boundaries between material types and transitions may be gradual. 6. No free water was encountered in the boring at the time of drilling . Fluctuation in water level may occur with time. 7. Laboratory Testing Results: WC = Water Content (%) DD = Dry Density(pct) +4 = Percent retained on the No. 4 sieve -200 = Percent passing No. 200 sieve UC = Unconfined Compressive Strength (psf) 116 205AGe~ rC@ch LEGEND AND NOTES Figure 3 Hepworth—Par/10k Geotechnical IHYDROMETER ANALYSIS I SIEVE ANALYSIS 24 HR. 7 HR TIME READINGS U S STANDARD SERIESSIEVE CLEAR SQUARE OPENINGS 0 45 MIN 15 MIN 60MIN19MIN.4 MIN 1 MIN #200 #100 #50 #30 #16 #8 #4 318 314 1 112 3 5 6 8 iaarr_i r.—a....__ aaaa—r—i 100 +. l— ..-1Ii-aair a—MI=MEI•-i ....._moi ia_aaai—moi-i Ell r.._ NMI a_i—.aa—i—.._ I .....-moi —.._ Ell-i —IIII — i ri--lir.._ ami-i 90 10 ili ia a—i-i ..... 11=11O ia_aaaaar—r i-i MM Il ��-� parr r-.-_ i•--II r...--i IIII ME S.itii-ari aaaaaar+�—i __ar.....--i IIIMM i�r—ii-=ill r..a.i ..-a_i�-.....--i 80 20 ` 4i....._i—aaaarr iaaaaaaa� !!a i i.....--i ir�-i i!a1 1ii--r. i—i i------i � �-- arm— -` .-.a i aaaaaaari-i IMMI— i_IM I.ate i-air ar......apIli ..---ii r_-!Ma MI i —,a_-11—i-MA! 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C06E3t.E5 NE I MED'JM I COARSE F•I+E 1 COARSE GRAVEL 41 % SAND 35 % SILT AND CLAY 24 % LIQUID LIMIT PLASTICITY INDEX % SAMPLE OF: Silty Sand and Gravel FROM:Baring 1 at 15 Feet Gg116 205A GRADATION TEST RESULTS Figure 4 Hepworth—Pawlak Geotechnical Moisture Content = 13.6 percent Dry Density = 102 pcf Sample of: Silty Sandy Clay From: Pit 3 at 4 Feet 0 1 Compression upon 2 -wetting 92 92 3 • 4 5 7 • 8 9 10 • 11 _ 0 1 1.0 10 100 APPLIED PRESSURE-ksf 116 205A � _~per_ C.7B`���Ch SWELL-CONSOLIDATION TEST RESULTS Figure 5 Hepworth—Pawlak Geotechnical 2 = 2 « § > § o CL C 0 . 2 D G G_ § 2 CZ 0 k CC CI) 2 -N U U 00 z ci � � >1 rj .0 ID k k CS § 2 § ® � U 2 00 U k rrr, k 7 m > D rn cn � q 00 \ W / q q R q 0§ 2 § q •-4 Du LiLn _ . _ \2 Y -§ k�« 2 UcC - Z §- U Lu § § / >- < / Z 1-1-1 / 9 eC % a, § § k c U Z — N 00 7 — en CL m § § I « CC 2 3 0 / q o > § a en 2 LU 2 . . . .. 4 _ § k LI 1- § -1 _4 - en k 2 -4 of 2 0c 4 D Lu 2 D In D _ _ « vo 400 E - N - � -4 z 2 u I 2 - in co m « o _ N en § al g _ . k - — .