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HomeMy WebLinkAboutSubsoil StudyRECEIVEO V kiErW©RTH-PAWLAh GEOTECHNICAL October 31, 2005 20055 Hepworth-Pawlak Geotechnical, Inc. 5020 County Road 154 Glenwood Springs, Colorado 81601 Phone: 970-945-7988 Fax: 970-945-8454 email: hpgeo®hpgeotech.com Ron York 18091 Riverchase Court Alva, Florida 33920 Job No. 105 758 Subject: Subsoil Study for Foundation Design and Percolation Testing, Proposed Residence, Lot 21, Teller Springs, Garfield County, Colorado Dear Mr. York: As requested, Hepworth-Pawlak Geotechnical, Inc. performed a subsoil study and percolation testing for foundation and septic disposal designs at the subject site. The study was conducted in accordance with our agreement for geotechnical engineering services to you dated August 23, 2005. The data obtained and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this report. We previously performed a subsoil study for a residence located several hundred feet south of this site and presented our findings in a report dated October 26, 1998, Job No. 198 613. Proposed Construction: Design of the residence has not been determined. For the purposed of this report, we assume that the proposed residence will be a two story wood frame structure located in the area of Pits 1-4 as shown on Figure 1. Ground floor will be slab -on -grade. Cut depths are expected to be about 3 feet. Foundation loadings for this type of construction are assumed to be relatively light and typical of the proposed type of construction. The septic disposal system is proposed to be located south of the residence. 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 site was vacant at the time of our site visit. Topographically, the site consists of two relatively flat alluvial terraces which slope slightly down to the east. There is a steep slope between the terraces which is about 15 feet high. The septic field site is on the upper terrace and the residence site is on the lower terrace. Two irrigation ditches cross the lot, one along the western side of the lot and the other on the lower terrace. The Roaring Fork River is located east of the lot. Vegetation consists of cottonwood trees, brush, grass and weeds. Parker 303-841-7119 • Colorado Springs 719-633-5562 a Silverthorne 970-468.1989 -2 - Subsidence Potential: Teller Springs is underlain by Pennsylvania Age Eagle Valley Evaporite bedrock. The evaporite contains gypsum deposits. Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can produce areas of localized subsidence. During previous work in the area, a few sinkholes were observed in the terraces close to the Roaring Fork River at Teller Springs. 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 pits were relatively shallow, for foundation design only. Based on our present knowledge of the site, it cannot be said for certain that sinkholes will not develop. In our opinion, the risk of future ground subsidence throughout the service life of the residence at Lot 21 is low and similar to other lots in the area but the owner should be 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. Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating four exploratory pits in the building area and one profile pit in the septic disposal area at the approximate locations shown on Figure 1. The logs of the pits are presented on Figure 2. The subsoils encountered, below about 1 to 11/z feet of topsoil, consist of 1 1/2to 3 feet of loose silty sand and sandy silt overlying relatively dense slightly silty sandy gravel with cobbles and boulders down to the bottom of the pits, 41/2 to 51/2 feet. Results of swell -consolidation testing performed on relatively undisturbed samples of the sandy silt and silty sand, presented on Figures 3 and 4, indicate moderate to high compressibility under loading and wetting. Results of gradation analyses performed on samples of the slightly silty sandy gravel (minus 3 to 5 inch fraction) obtained from the site are presented on Figure 5. The laboratory test results are summarized in Table 1. Free water was observed in Pits 1-4 at 4 to 41/2 feet deep at the time of excavation. The soils above the water level were moist to very moist. Foundation Recommendations: Considering the subsoil conditions encountered in the exploratory pits and the nature of the proposed construction, we recommend spread footings placed on the undisturbed natural gravel soil or granular structural fill designed for an allowable soil bearing pressure of 2,500 psf for support of the proposed residence. Footings should be a minimum width of 16 inches for continuous walls and 2 feet for columns. Loose and disturbed soils and existing loose sand and silt encountered at the foundation bearing level within the excavation should be removed and the footing bearing level extended down to the undisturbed natural gravel soils. Structural fill used to re- establish design bearing level should extend beyond the edges of the footings a distance equal to at least the depth of fill below the footing and be compacted to at least 100% of Job No. 105 758 Gtech -3 - standard Proctor density at near optimum moisture content. Exterior footings should be provided with adequate cover above their bearing elevations for frost protection. Placement of footings at least 36 inches below the exterior grade is typically used in this area. 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 be designed to resist a lateral earth pressure based on an equivalent fluid unit weight of at least 45 pcf for the on-site soil as backfill. Floor Slabs: The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab -on -grade construction. The loose sand and silt soils may be suitable for support of floor slabs but should be further evaluated at the time of 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 laver of free -draining gravel should be placed beneath basement level slabs to facilitate drainage. This material should consist of minus 2 inch aggregate with less than 50% passing 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 gravel soils or imported granular soils devoid of vegetation, topsoil and oversized rock. Underdrain System: Free water was encountered'at relatively shallow depth and it has been our experience that the water level will rise during spring runoff and seasonal irrigation. Since it is not practical to permanently dewater the site or even temporarily during a flood event, we recommend below -grade construction, such as crawlspace and basement areas, be avoided. We assume the ground floor will be slab -on -grade, elevated slightly above the surrounding ground. An underdrain system is not required for this slab - ori -grade condition. 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 Job No.105 758 P$eCh 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 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. Percolation Testing: Percolation tests were conducted on October 15, 2005 to evaluate the feasibility of an infiltration septic disposal system at the site. One profile pit and three percolation holes were dug at the locations shown on Figure 1. The test holes (nominal 12 inch diameter by 12 inch deep) were hand dug at the bottom of shallow backhoe pits and were soaked with water one day prior to testing. The soils exposed in the percolation holes are similar to those exposed in the Profile Pit shown on Figure 2 and consist of 1/2 foot of topsoil overlying slightly silty sandy gravel with cobbles and boulders down to the pit depth of 8 feet. The percolation test results are presented in Table 2 and ranged between 2 to 20 minutes per inch. Based on the subsurface conditions encountered and the percolation test results, we recommend that a civil engineer design the infiltration septic disposal system. 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, 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 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 Job No. 105 758 G�c�teCh -5 - 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 if we may be of further assistance, please let us know. Respectfully Submitted, HEPWORTH - PAWLAK GEOTECHNICAL, INC. Daniel E. Hardin, P. � - 24443 t Reviewed b ��,Q�,pe"®���; Steven L. Pawlak, P.E. DEH/ksw attachments Figure 1— Location of Exploratory Pits and Percolation Test Holes Figure 2 — Logs of Exploratory Pits Figures 3 to 4 — Swell -Consolidation Test Results Figure 5 — Gradation Test Results Table 1— Summary of Laboratory Test Results Table 2,— Percolation Test Results cc: Gamba Associates — Attn: Mike Gamba Job No. 105 758 qtr Ch a� v LL Q N n PIT 1 PIT 2 PIT 3 PIT 4 _ 0 =_ WC=31 .0 D=84 -hWG=8354.7 0=34 DD -200=81 _ - +4=64 -200=8 10 LEGEND: TOPSOIL; silt, sandy, clayey, organic, firm, moist, dark brown. WC=37.4 DD=84 -200=73 PROFILE PI -11 0 SAND AND SILT (SM -ML); interlayered sandy silt and silty sand, loose, moist to very moist, brown. 0 �e +4=70 -200=8 j 5 R° `p•a o. GRAVEL (GP -GM); sandy, slightly silty, with cobbles and boulders, dense, wet, brown, rounded rock. ••o 2" Diameter hand driven liner sample. Disturbed bulk sample. _J Free water level in pit at time of excavating. 10 NOTES: 1. Exploratory pits were excavated on October 14, 2005 with a Cat 416B backhoe. 2. Locations of exploratory pits were measured approximately by pacing from features shown on the site plan provided. 3. Elevations of exploratory pits were not measured and the logs of exploratory pits are drawn to depth. 4. The exploratory pit locations 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 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. 7. Laboratory Testing Results: WC = Water Content (%) DD = Dry Density (pcf) +4 = Percent retained on the No. 4 sieve -200 = Percent passing No. 200 sieve 105 758 G~(1j'� LOGS OF EXPLORATORY PITS Figure 2 0 1 U 4 5 6 7 0.1 0 1 2 c 3 0 LD n 4 E 0 U 6 6 105 758 Moisture Content = 34.7 Dry Density = 85 Sample of: Sandy Silt From: Pit 1 at 2.5 Feet No movement upon wetting 1.0 10 APPLIED PRESSURE - ksf Moisture Content = 31.0 Dry Density = 84 Sample of: Silty Sand From: Pit 2 at 2 Feet Compression upon percent pcf percent pcf M 1.0 10 100 APPLIED PRESSURE - ksf SWELL -CONSOLIDATION TEST RESULTS I Figure 3 H6PWOFZTH-PAWLAK GEOTECHNICAL 0 1 2 c 0 .C/) cn 3 m a E 0 C.� 4 5 6 7 8 9 0.1 1.0 Moisture Content = 37.4 percent Dry Density = 84 pcf Sample of: Sandy Silt From: Pit 4 at 2 Feet APPLIED PRESSURE - ksf Compression upon wetting Ins 100 105 758 1 1 SWELL -CONSOLIDATION TEST RESULTS Figure 4 H FpwoRTH-PAWLAK GEOTECHNICAL ƒ / k � _ w � m � U) � :D Lr) LU � U) LU � � O � O 3 :5 LL O � Q � � � LO ± � a k \� C Ln � § E o = ® q •7 � � / � _ / 2 2 coo q\ \ )_ &\§ / o=� \/E ux �\ 2 \\co ;} \0 ce � . CD Lu u) 7 00 5&»@ CL a. z 7 q q 2 = k \ / 7 & S 7 0 e _ [ §00 7 7 z c &§LU y :D S 25§ m m y z=u \ \ kn § 0 m g a ± ( m $ HEPWORTH-PAWLAK TABLE PERCOLATION TEST RESULTS HOLE NO. HOLE DEPTH (INCHES) LENGTH OF INTERVAL (MIN) WATER DEPTH AT ST IN ERV ART OF L INCHES WATER DEPTH AT END INTERVAL (INCHES DROP IN WATER LEVEL (INCHES) AVERAGE PERCOLATION TE (MIF N /INCH) P-1 27 10 Water added 6 4 3/4 1 1/a 20 4 3/44 3/4 6 5 1 5 41/2 1/2 41/2 4 1/2 4 31/2 1/2 31/2 3 1/2 P-2 35 10 Water added 6 5 1 20 5 41/4 3/4 61/4 51/4 1 5 1/4 43/4 1/2 4 3/4 4 1/4 1/2 4 1/4 3 3/4 1/2 33/4 3 1/4 1/2 P-3 30 10 water added 10 Water added 10 Water added 10 Water added 1 Water added 1 Water added 1 Water added 5 0 5+ 2 5 0 5+ 5 0 5+ 5 0 5+ 5 41/2 1/2 41/2 4 1/2 4 31/2 1/2 Note: Percolation test holes were hand dug in the bottom of backhoe pits and soaked on October 14, 2005. Percolation tests were conducted on October 15, 2005. The average percolation rates were based on the last three readings of each test.