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HomeMy WebLinkAboutB12-0385 SUBSOIL STUDY Hepworth-Pawlak GeoLechnical, Inc. 0020 Co Road 154 � IIrW00d S 71Ingi, l�il UrlU G( c'f)tech l �IQII' Phone:970-945-7988 HEPWORTH-PAWLAK GEOTECHNICAL Fax:970-945-8454 email:hjigc,�hpgeotech.com SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE 2801 SNOWBERRY DRIVE LOT 9, VAIL INTERMOUNTAIN VAIL, COLORADO JOB NO. 112 097A JUNE 11, 2012 PREPARED FOR: MOUNTAIN CI HOLDINGS, LTD. C/O DW DANTAS CONSTRUCTION, LLC ATTN: DAVE DANTAS P.O. BOX 2322 AVON, COLORADO 81620 RECEIVED By David Rhoades at 2:57 pm, Aug 23, 2012 Parker 303-841-7119 Colorado Springs 719-633-5562 • Silverthorne 970-468-1989 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY . .. . . . .. ... . ... .. . .. ... .. . .... . ..... . .. .. .. .. ... . ... .. .... ... .. .. ..... .. - 1 - PROPOSED CONSTRUCTION.... .. .. .. . .. .. .. . .. . ... ... . ... . .. ..... . ... .. . .. .. .. ..... . . . ... .. .. . . .. ... . . .... . - 1 - SITECONDITIONS. . .. .. ..... .... .. ..... .. .. .. . .. .. .. . .. . . .. . .. . .. .. . ........ . .... .. .. . . . .. . ... . . . . . . . .. . .. .. .. ..... . - 2 - FIELDEXPLORATION .... ... . .. ... .. .. . .... .. .. ... . .. . . . . . .. ..... ..... .. .... ... .. .. . .. .. .... . . . . . . . ... .. .. ... .. .. - 2 - SUBSURFACE CONDITIONS ... .. ... .. .. .. . .. ... ... . . .. . .. .... ... . .. . .... . ..... .. .. .. . . .. ... .. .. . . .. ... . ..... . - 3 - FOUNDATION BEARING CONDITIONS . . ... . .. . .. .. . .. ..... .. .... . .. .. . . . .. . ... . . . . . . . . .. . .. . . ... .. .. - 3 - DESIGN RECOMMENDATIONS . .. . .. .. ... .. . .. . .. . . .. . .. ... ..... ...... .. . . . .. ... .. .. .. . . . . . .. . . . . ... . ..... . - 4 - FOUNDATIONS . ... . .. ... . . .... .. ... .. ... .. .. .. ... .. . .. . . . .. . .. ..... ..... . . .. .. . .. .. . .. .. . ... . . .. . . . . ... .. .. .. . . ... - 4 - FOUNDATION AND RETAINING WALLS . . .. . .... .... .. . .... . .. ... .. .. .. . . . . . .. .... . . .. .. ...... . . - 5 - FLOORSLABS . . . ... . .. .. . . . .... .. .. . .. . .. .. .. . .. .. . . . . ... . . . . . .. .. ...... ........ . .. .. . .. .. .. .. . . . . . .. . . . . ... . ..... . - 6 - UNDERDRAINSYSTEM... . .... .. . .. .. ... .. .. . . .. . . ... . .. ..... . . ... . ... .. . .. .. ... .. . ... . . . . . . . . ... .. .. . . . . ... - 7 - SITEGRADING . ... . .. .. . . . ... . .. . . . .. . .. . . .. . .. .. . ... . ... . ... .. ... .... . .. .... . .. .. . .. .. .. . . . . . .. . . .. .. .. .. ...... . . - 7 - SURFACEDRAINAGE .. ..... .... . .. .. .. . .. ... .. . .. . . . .. . ... . . .. ..... ....... . . . .. ... .. .. .. . . . . . . . . . .. .. .. ..... . - 8 - LIMITATIONS ... . ... . . . .. .. .. . .. ..... ... .. .. . .. .. ... .. . .. . ... . . .. . .. ... ..... .. .... . .. .. . .. .. .. .. . . ... . ..... . .. .. .. . .... - 9 - FIGURE 1 - LOCATION OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 - LEGEND AND NOTES FIGURE 4 - SWELL-CONSOLIDATION TEST RESULTS FIGURE 5 - GRADATION TEST RESULTS TABLE 1 - SUMMARY OF LABORATORY TEST RESULTS PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located at 2801 Snowberry Drive, Lot 9, Vail Intermountain, 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 Mountain Cl Holdings, LTD c/o DW Dantas Construction, LLC dated April 24, 2012. 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 assumed construction and the subsurface conditions encountered. PROPOSED CONSTRUCTION Development of the lot was conceptual at the time of our field exploration. The residence will be built in the southwest corner of the lot and extend a short distance down the steep hillside. The proposed residence is assumed to be a 3-level structure with the main floor level near road grade and the lower floor level possibly up to 25 feet or more below that. We assume that the floor levels will step down the hillside to keep cut depth at about 15 feet. We assume relatively light to moderate foundation loadings, typical of the assumed type of construction. When building loadings, location and grading plans have been determined, we should be notified to re-evaluate the recommendations contained in this report. Job No. 112 097A Ptech - 2 - SITE CONDITIONS The proposed residence is located on a natural, northeast facing hillside of about 30 to 40% grade at the front, south building setback line and about 60 to 70% grade down the hillside in the lower building area. Off of Snowberry Drive is a steep sided knoll about 6 feet high before reaching the building area. Elevation difference is on the order of 20 feet across the assumed building footprint. Vegetation consists mostly of grass, weeds and brush with scattered aspen and conifer trees. A drainage swale crosses the lot from southeast to northwest and occupies roughly the northeastern half of the lot that could be the site of old landslide activity associated with the steep hillside and wet ground conditions. We assume this is outside of the current building area. Evaluation of the hillside stability on the lot is beyond the scope of our study. FIELD EXPLORATION The field exploration for the project was conducted on May 1 , 2012. Two exploratory borings were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions. The borings were advanced with 4 inch diameter continuous flight augers powered by a track-mounted CME 45 drill rig. The track-mounted drill rig was needed due to the steep, irregular terrain and could only access the south end of the proposed building area. The borings were logged by a representative of Hepworth-Pawlak Geotechnical, Inc. Samples of the subsoils were taken with 1 % and 2 inch I.D. spoon samplers. The samplers were driven into the subsurface materials 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 and hardness of the bedrock. 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. Job No. 112097A G80)t@ch - 3 - 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 at Boring 1 and 4% feet of mainly granular fill at Boring 2 overlying medium dense, silty to clayey sand and gravel with cobbles down to the drilled depth of 28 feet at Boring 1 . At Boring 2, the granular soils transitioned to stiff, sandy clay/clayey sand with gravel down to about 18% feet where apparent weathered shale was encountered. Drilling with auger in the deeper coarse granular soils and the bedrock was difficult due to the cobbles and boulders or cemented rock and drilling refusal was encountered in the deposits at both borings. Laboratory testing performed on samples obtained from the borings included natural moisture content and density, gradation analyses, liquid and plastic limits and unconfined compressive strength. Results of swell-consolidation testing performed on relatively undisturbed drive samples, presented on Figure 4, indicate low to moderate compressibility under conditions of loading and wetting. The fine fraction of the soils typically has low plasticity. The laboratory testing is summarized in Table 1 . Free water was encountered in the Boring 1 at a depth of about 22 feet and Boring 2 was dry to the drilled depth of 23 feet. The upper soils were typically moist. FOUNDATION BEARING CONDITIONS Medium dense granular soils were encountered in Boring 1 and in the upper part of Boring 2 that should be suitable for support of lightly loaded spread footings with low settlement risk. The clayey soils encountered with depth in Boring 2 will have higher settlement potential and lower bearing capacity than the coarse granular soils. The proposed extensive excavation depths will increase the risk of foundation movements and precautions will need to be taken to maintain stability of cut slopes. Footings will need to be set back from downhill cut slope faces to not adversely impact slope stability. Temporary excavation shoring will be needed or footing bearing levels will need to be Job No. 112 097A C Ptech - 4 - dropped to set back behind an imaginary line of 1 horizontal to 1 vertical extended up from the bottom of the adjacent footing. Groundwater seepage may be encountered and the excavation should be dewatered from outside of footing areas as needed. We should review the foundation plans when the excavation depths and methods have been determined. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory borings and the nature of the proposed constriction, we recommend the building be founded with spread footings bearing on the natural granular soils. 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 should be designed for an allowable bearing pressure of 2,000 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. 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 12 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. Sob No. 112 097A GGC�JttGCh - 5 - 5) The existing fill, topsoil and any loose or disturbed soils should be removed and the footing bearing level extended down to the relatively dense natural granular soils. The exposed soils in footing areas should then be moistened as needed and compacted. If water seepage is encountered, the footing areas should be dewatered before concrete placement. 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 high 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 55 pcf for backfill consisting of the on-site granular soils. Foundation retaining walls greater than 15 feet tall should be designed for a uniform lateral earth pressure of 26H in psf where H is the wall height in feet (a 20 foot high wall would have a uniform lateral earth pressure of 520 psf). 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 granular 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. Job No. 112 097A G�PtGCh - 6 - 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 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 0.45. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 375 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 a granular material 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 Job No. 112 097A GecPtech - 7 - 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 granular soils devoid of vegetation, topsoil and oversized rock. UNDERDRAIN SYSTEM Although free water appears to have been encountered below assumed excavation depths, it has been our experience in mountainous areas that the groundwater level can rise and local perched groundwater can 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 2 feet deep. SITE GRADING The north and eastern part of the lot may be an old landslide complex related to the very steep sloping hillside and wet conditions from the drainage. We assume this is outside of the current proposed building area that should be evaluated as development planning progresses. The proposed development methods should not adversely impact slope Job No. 112 097A ~Ptech - 8 - stability anywhere on the lot. Relatively extensive excavation depth is assumed for the residence development and there is a risk of construction-induced slope instability at the site. We assume the cuts will be sloped to a stable grade or shoring will be provided to maintain stability. Fills should be limited to about 8 feet deep and not be placed on existing steep slopes at the downhill side of the residence. 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. The fill should be benched into the portions of the hillside exceeding 20% grade. 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. This office should review site grading plans for the project prior to construction. Further evaluation of the possible slope instability area should be performed and consider the grading plan of the proposed development. 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. Job No. 112 097A C-�cPtech - 9 - 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 backfill should be capped with at least 2 feet of the on- site soils to reduce surface water infiltration. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. Surface water runoff should not be concentrated and directed onto the steep down slope without adequate protection against erosion and slope instability. 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 assumed 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. 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 Job No. 112 097A G<cgt@ch - lo - 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. J�N�xt.aP Steven L. Pawlak, P. *p 1 5222 s� i a Reviewed b Y J,,r. s`yoira��w�•"o �Tf Qf COV��Po Daniel E. Hardin, P.E. SLP/ljg Job No. 112 097A 8iPtech APPROXIMATE SCALE 8050 _ LOT 1 " = 30' 8060 — \ 1 8070 _ \ \ \ \ \ \ \ \ \ \ \ Cbo LOT — 807 PROPOSED ` RESIDENCE \ \ — / AREA — \ 8060 — ( ' @[% ING SETBACK LINE 8p90 • BORING 1 \ / — '- � \ BORING 2 � 8108 ° SNOWBERRY DRIVE 112 097A LOCATION OF EXPLORATORY BORINGS Figure 1 He worth—Pawlak Gaotechniccl BORING 1 BORING 2 ELEV.= 8107' ELEV. = 8101 ' 8110 8110 8105 8105 13/12 8100 8100 o' 12/12 9/12 WC=6.7 . 0. 8095 +4=27 m -200=25 8095 � 5 0 O LL 0 52/12 18/12 'ro o _m 0 *0 > ED 8090 8090 w WC=14.6 27/12 14/12 DD-107 : % WC=18.6 •°t WC=8.5 DD=113 1 -200=75 8085 +4=37 LL=34 8085 -200=13 P1=16 0 UC=4,200 16/12 10/6,40/6 WC-12.3 DD=125 8080 8080 8075 8075 Note: Explanation of symbols is shown on Figure 3. H -T 1 �7$�,I@Ch LOGS OF EXPLORATORY BORINGS Figure 2 He worth—Pawlak Geotechniccl g LEGEND: ® FILL; silty clayey sand and gravel with cobbles, loose, moist, mixed brown. ® TOPSOIL; organic sandy silt and clay, gravelly, firm, moist, black. SAND AND GRAVEL (SM-GM); silty, cobbles, medium dense, moist, brown. CLAY (CL-SC) ; sandy to very sandy, gravelly, stiff/medium dense, moist, mixed brown. SAND AND GRAVEL (SC-GC) ; clayey, cobbles, possible boulders, medium dense, moist, mixed brown. Probable Shale Bedrock; hard to very hard, cemented with depth, moist to slightly moist, grey. Minturn Formation. Relatively undisturbed drive sample; 2-inch I.D. California liner sample. ■ Drive sample; standard penetration test (SPT), 13/8 inch I.D. split spoon sample, ASTM D-1586. 52x/12 Drive sample blow count; indicates that 52 blows of a 140 pound hammer falling 30 inches were required to drive the California or SPT sampler 12 inches. �'1 Free water level in boring and number of days following drilling measurement was taken. Depth at which boring had caved when checked on May 2, 2012. TPractical drilling refusal with augers. NOTES: 1 . Exploratory borings were drilled on May 1 , 2012 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. Water level readings shown on the logs were made at the time and under the conditions indicated. Fluctuations in water level may occur with time. No free water was encountered in Boring 2. 7. Laboratory Testing Results: WC = Water Content (%) -200 = Percent passing No. 200 sieve DD = Dry Density (pcf) LL = Liquid Limit (%) +4 = Percent retained on the No. 4 sieve PI = Plasticity Index (°/O) UC = Unconfined Compressive Strength s H 112 097A tec 1 LEGEND AND NOTES Figure 3 HEPWORTHPAWLAK GEOTECHNICAL Moisture Content = 14.6 percent Dry Density = 107 pcf Sample of: Sandy Clay with Gravel From: Boring 2 at 13 Feet 0 1 Compression upon oR 2 wetting C 0 Z a 3 E 0 U 4 5 6 0. 1 1 .0 10 100 APPLIED PRESSURE - ksf Moisture Content = 12.3 percent Dry Density = 125 pcf Sample of: Weathered Shale From: Boring 2 at 18 Feet OR 0 0 0 ro 1 °L No movement 0 upon 2 wetting 0.1 7 . 0 70 100 APPLIED PRESSURE - ksf 112 097A Hp �(,)"�.-,:,t,CAL SWELL-CONSOLIDATION TEST RESULTS Figure 4 HEPWORTHPAWLAK GEOTECHNI �R•r.r:Ta n 3 r a�:n r_a•�� . 1 II 11 1 1 - ------- --------- 11 - ------------------ 0 1 . • • Ml - ------------------ - ------------------ --------------------- - - ------------------- - ------------------- - ------------------ ------------------ - ------------------- - ------------------ - ------------------ - ----------------- ------------------ - ------------------ - ------------------- - ------------------- - ------------------ ------------------ - ----------------- - -------- --------- ---------_---------- - ------------------- - ------- ---------- - ----------------- - ------------------ 11 11 11 11 • , • • 1 , , 11 . 11 • 1 I 1 • • - - - c GRADATION HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE 1 Job No. 112 097A SUMMARY OF LABORATORY TEST RESULTS SAMPLE LOCATION NATURAL NATURAL GRADATION ATTERBERG LIMITS MOISTURE DRY PERCENT UNCONFINED BORING DEPTH CONTENT DENSITY GRAVEL SAND PASSING LIQUID PLASTIC COMPRESSIVE SOIL OR (%1 (%) S NO 200 EVE LIMIT INDEX STRENGTH BEDROCK TYPE n % PC % 10/0) (PSF) 1 9 & 14 6. 7. 7 27 48 25 Silty Gravelly Sand 19 & 24 8.5.5 37 50 13 Silty Gravelly Sand 2 13 (top) 14. 6 107 Sandy Clay with Gravel 13(bottom) 18 .6 113 75 34 16 47200 Sandy Clay 18 12.3 125 Weathered Shale