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HomeMy WebLinkAboutSubsoils Report for Foundation DesignG eo"%' �—f oftech HEPWORTH-PAWLAK GEOTECHNICAL Hepworth-Pawlak Geotechnical, Inc. 5020 Count}' Road 154 Glenwood Sprine;, Colorado 51601 Phone: 970-9 5-798 Fax:970-9�5-5=5- email: hpRcu0hPrcorech.corn SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 40, CERISE RANCH LARKSPUR DRI17E GARFIELD COUNTY, COLORADO JOB NO. 106 0353 JUNE 8, 2006 PREPARED FOR: HOLM 3ECK CONSTRUCTION ATTN: LINDSEY HOLM 3ECK 14 DAKOTA COURT CARBONDALE, COLORADO 81623 4 SP�w�wA Parker 303-841-7119 • Colorado Springs 719-633-5562 e Silverthome 970-468-1989 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY.................................................................. _ .. - I - PROPOSEDCONSTRUCTION .................................. ................................................... - I - SITE CONDITIONS ......•••••.............•.. 2- SUBSIDENCE POTENTIAL ••.••••.......... - - FIELDEXPLORATION................................................................... ........................................... .......... - SUBSURFACE CONDITIONS .•.•••. . - DESIGN RECOMMENDATIONS ................... - FOUNDATIONS.......................................................................................................... 4- FOUNDATION AND RETAINING WALLS ................. - FLOORSLABS .............................................. .......................................................... 6 - UNDERDRAINSYSTEM..........................................................................................- 7- SITEGRADING......................................................................................................... 7- SURFACEDRAINAGE............................................................................................. - 8- LIMITATIONS.............................................................................................................. 8- FIGURE I - LOCATION OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 - LEGEND AND NOTES FIGURE 4 - SWELL -CONSOLIDATION TEST RESULTS TABLE 1- SUMMARY OF LABORATORY TEST RESULTS PURPOSE .-N'D SCOPE OF STUDI' This report presents the results of a subsoil study for a proposed residence to be located on Lot 40, Cerise Ranch, Larkspur Drive, Garfield County, 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 geotechnicaI engineering ser<�ices to Holmbeck Construction dated April 14, 2006. 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. compressibility or swell 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 residence will be a single story, wood frame structure over a walkout basement level located between the exploratory borings shown on Figure 1. Basement floor will be slab -on -grade. Grading for the structure is assumed to be relatively minor with cut depths between about 4 to 10 feet. 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. 106 0353 GEICPtech SITE CONDITIONS The site was vacant at the time of our field exploration. The site is located on a south facing hillside with a strong slope between about 12% and 17% grade. There is about 10 to 15 feet of elevation difference across the proposed building area. Minor dry drainages cross the lot. There is an old graded road through the center and an open irrigation ditch across the lower corner of the building envelope. Vegetation consists of pinon pine and juniper trees, sagebrush, grass and weeds. Basalt cobbles and boulders are exposed on the ground surface of the lot. SUBSIDENCE POTENTIAL Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Cerise Ranch development. These rocks are a sequence of gypsiferous shale, fine-grained sandstone/siltstone and limestone with some massive beds of gypsum. There is a possibility that massive gypsum deposits associated with the Eagle Valley Evaporite underlie portions of the lot. Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can produce areas of localized subsidence. Sinkholes were not observed in the immediate area of the subject lot. No evidence of cavities was encountered in the subsurface materials; however, the exploratory borings were relatively shallow, for foundation design only. Based on our present knowledge of the subsurface conditions at the site, it cannot be said for certain that sinkholes will not develop. The risk of future ground subsidence on Lot 40 throughout the service He of the proposed residence, in our opinion, is low; however, the owner should be made aware of the potential for sinkhole development. If further investigation of possible cavities in the bedrock below the site is desired, we should be contacted. FIELD EXPLORATION The field exploration for the project was conducted on May 1, 2006. Two exploratory borings were drilled at the locations shown on Figure 1 to evaluate the subsurface Job No. 106 0353 G,�'GoacC ) conditions. The borings were advanced with 4-inch diameter continuous flight augers powered by a rubber tire, ATV -mounted CME-55 drill rig. The borings were logged by a representative of Hepworth-Pawlak Geotechrucal, Inc. Samples of the subsoils were taken with 1'/6 inch and 2 inch I.D. spoon samplers. The samplers were 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-1 586. 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. The subsoils consist of about 1 foot of topsoil overlying medium dense/stiff to very stiff silty sand and clay with scattered gravel down to the maximum explored depth of 30 feet. The subsoils are debris fan deposits formed from erosion of the hillside to the north of the building area and could be on the order of 50 feet deep in the lower part of the building site. Laboratory testing performed on samples obtained from the borings included natural moisture content and gradation analyses. Results of swell -consolidation testing performed on relatively undisturbed drive samples; presented on Figure 4, generalIy indicate low to moderate cornpressibility under conditions of loading and wetting. The sample from Boring 1 at 4 feet showed a low collapse potential (settlement under a constant load) after wetting. The laboratory testing is summarized in Table 1. Job No. 106 0353 G cPtech No free water was encountered in the borings at the time of drilling or when checked 7 days later and the subsoils were slightly moist at Boring 1 and moist at Boring 2. The hia-her moisture content at Boring 2 could be due to leakage from the nearby open ditch. DESIGN RECOMMENDATIONS FOUNDATIONS The subsoils are hydro compressive and there is a risk of post -construction settlement if the soils below the building become wetted. The building can be founded with spread footings bearing on the natural debris fan soils with a risk of differential settlement and building distress if the bearing soils become wetted. The foundation should consist of heavily reinforced continuous foundation walls that will span and limit the effects of differential settlement. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the undisturbed natural soils should be designed for an allowable bearing pressure of 1,200 psf. Based on experience, we expect initial settlement of footings designed and constructed as discussed in this section will be about 1 inch or less. There could be 1 to 2 inches of additional differential settlement if the bearing soils become wetted, depending on the depth of wetting. 2) The footings should have a minimum width of 20 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 36 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 14 feet. Foundation walls acting as retaining structures should also be Job No. 106 0353 G<�Ttech - 5 - desib -ied 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 natural soils. The exposed soils in footing area should then be moistened and compacted. 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 which are laterally supported 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 » pcf for backfill consisting of the on -site soils. Cantilevered retaining structures which are separate from the residence 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 soils. Backfill should not contain vegetation.. topsoil or oversized rock. 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 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 Job No. 106 0353 GeCPtech 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. 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 0.40. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit ureight 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. FLOOR SLABS soil. are suitable to support lightly loaded slab - The natural on -site soils; exclusive of top on -grade construction. The soils are hydrocompressive and there could be some slab settlement and distress if the sub grade soils become wetted. 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'—) inch aggregate with at least 50% retained on the No. 4 sieve and less 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 soils devoid of vegetation; topsoil and oversized rock. Job No. 106 0353 C_e�CPtech -7- UNDERDRAIN SYSTEM Although free water was not encountered during our exploration, it has been our experience in the area that local perched groundwater can develop during times of heati�}� 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. The drains should consist of drainpipe placed in the bottom of the wall back -fill surrounded above the invert level with free -draining granular material. The drain should be placed at each level of excavation and at least I 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 back -fill should be at least I% feet deep. An impervious membrane; such as 30 mil PVC, should be placed beneath the drain gravel in a trough shape and attached to the foundation wall with mastic to prevent wetting of the bearing soils. SITE GRADING The risk of construction -induced slope instability at the site appears low provided cut and fill depths are limited. We assume the cut depths for the basement level will not exceed one level, about 8 to 10 feet. Fills should be limited to about 8 to10 feet deep. 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 90% of the maximum standard Proctor density. 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 pennanent cuts, an investigation should be conducted to determine if the seepage will job No. 106 0353 Ge PteCh adversely affect the cut stability. This office 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 residence has been completed: 1) Inundation of the foundation excavations and underslab areas should be avoided during construction. 2) Exterior back -fill 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 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. Free -draining wall back -fill should be capped with about 2 feet of the on - site, finer grained soils to reduce surface water infiltration. 4) Roof downspouts and drains should discharge well beyond the limits of all backill . 5) Sprinkler heads and landscaping which requires regular heavy irrigation, such as sod, should be located at least 5 feet from foundation Malls. Consideration should be given to use of xeriscape to help limit the risk of wetting by landscape irrigation. L511TATIONS 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 GecPtech Job No. 106 0353 -9- 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 possibilit-v 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. 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 CH1tiz1C_AL, INTC. VIC 38469 Trevor L. Knell, P. 9' Reviewed by:SS "° °. •��G NA_ Steven L. Pawlak, P.E. TLK/ksw Job No. 106 0353 H&eCh 6575 ry r APPROXIMATE SCALE I 6550 V 1" = 60' I \ 6525 - \ I � 6500 647s —,_ i LOT 40 s4 I ` so LOT 41IC3:. i sA2� I iI BORI 1 APPROXIMATE BUILDING BUILDING I AREA f �- y J OLROP oG�� BORING 2 p O I ��� LOT 39 I C3 1 _ ---- -.--- -- ksp_ 6380 �-- LZ 1060353 th LOCATION OF EXPLORATORY BORINGS FIGURE 1 6400 6395 6390 �-- L7 6385 Lu Lu w z O H= w w 6380 6375 6370 6365 j 1060353 BORING 1 ELEV. =6400' 37/12 we =g DD= 5i 19/12 50/5 VC=5.7 )D=121 ]00=68 3/5 f5 =5.9 1 24 0=62 BORING 2 ELEV. =6390' 17/12 7/12 8/12 WC=18.9 ]p=107 200— 7S JCS 2900 7/12 JC=12.5 D-122 1/6 6395 6390 6385 - I �- w w u_ z O 6380 j L_Uj w 6375 6370 6365 Note: Explanation of symbols is shown on Figure 3. P� LOGS OF EXPLORATORY BORINGS S FiGURE 2 LEGEND: Fq TOPSOIL; sandy clay, root zone. CLAY (CQ; sandy, silty, stiff, slightly moist, light brown. SAND AND CLAY (SC-CL); silty, scattered gravel, medium dense/stiff to very stiff, slihtl at Baring 2, fight brown to brown, slightly calcareous. gy moist at Boring 1, moist Relatively undisturbed drive sample; 2-inch I.D. California liner sample. Drive sample; standard penetration test (SPT), 1 3/8 inch I.D D. split spoon sample; ASTM D-1586. 37/12 Drive sample blow count; indicates that 37 blows of 140 pound hammer falling 30 inches were required to drive the California or SPT sampler 12 inches. NOTES: 11. Exploratory borings were drilled on May 1, 2006 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 Figure 1. 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 or when checked 7 days later. Fluctuation in water level may occur with time. 7. Laboratory Testing Results: WC = water content ( fo } DD = [dry Density (pcf ) -200 = percent passing No. 200 sieve UC = Unconfined Compressive Strength ( psi: ) 106 0353 � � _ _ __ -- HEPwoRrH-PAWLAKGE0TZCHN1CAL LEGEND AND NOTES I FIGURE 3 0 ` I Moisture Content = 5.2 percent I I 1 Dry Density = tY 93 pcf 1 Sample of: Silty Sand and Clay 1 f' r From: Boring 1 at 4 Feet 2 0 3 { { I { w cif 1' rr- 1 0- I GompresSion 0 4 upon wetTing 5r i 1 6 I 0.1 r i 1.0 10 i ! APPLIED PRESSURE (ksf) 100 i f + ' Moisture Content = 13.5 percent I i Dry Density = 122 pcf r Sample of: Silty Sand and Clay with Gravel I From: Boring 2 at 14 Feet I1 1 1 f coCr U) f I Lu . f g 2 f No movement 0 I l i f upon wer U I t ! P ing i � f I 1 0.1 j � 0 10 APPLIED PRESSURE (ksf) 100 106 0353P-�i ..--.... r` SWELL CON :HNICAL - SOLIDATION TEST RESULTS FIGURE 4 n / \ C; 2 = # ) 2 \ \ 9 \ 4>1 >1 cn > > —cz i$= 2nw $// i R Z0 Z ° < S k LLJ <z / Z �- LU m LLJ L § r-=)k $ ! - � g =i LLJ 2 < -j Q Lo zicUJ CO < 2CO 2 q < i iƒ2� U- 0 � o R $ < 0 7 3: a a W § 2 m / / - �§ / 4 o q < ^ _ < Z Er a y 2 a C </§ © L 220 § � \ / O 2 2. 0Z A m