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Home My WebLink AboutSoils Report 06.16.2016Gtech HEPWORTH - PAWLAK GEOTECHNICAL June 16, 2016 Mike Freeman 720 Minter Avenue Glenwood Springs, Colorado 81601 (Mfreeman 81601 @amail.com) 1-1cpwortli-I iw1at; Geotechnical, Inc. 5020 County Road 154 Glenwood Springs, Color:Jo 81601 Phone. 970-945-7988 Fax. 970-945-8454 L111 ill iipgco llpgmieih.cum Job No. 116 207A Subject: Subsoil Study for Foundation Design and Percolation Test, Proposed Residence, Lot 18, Mountain Springs Ranch, Mountain Springs Road, Garfield County, Colorado Dear Mr. Freeman: As requested, Hepworth-Pawlak Geotechnical, Inc. performed a subsoil study and percolation test for foundation design and septic disposal feasibility at the subject site. The study was conducted in accordance with our agreement for professional services to you dated May 23, 2016. The data obtiiined and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this report. Evaluation of potential geologic hazard impacts on the site is beyond the scope of this study. Proposed Construction: The proposed residence will be one story with a loft above a walkout basement and located roughly as shown on Figure 1. Ground floors could be structural above crawlspace or slab -on -grade. Cut depths are expected to range between about 4 to 10 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 and downhill of the residence. If building conditions or foundation Ioadings are significantly different from those described above, we should be notified to re-evaluate the recommendations presented in this report. Site Conditions: The vacant lot is located in the southwest part of the subdivision on a moderately to strongly sloping, south-southeast facing hillside. The building site is located in a small open area of grass and weeds surrounded by dense brush and stands of aspen and scrub oak trees. Scattered basalt boulders are visible on the ground surface and adjacent hillsides. Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthorne 970-468-1989 Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating two exploratory pits in the building area and a profile pit with three percolation test holes in the proposed septic disposal area at the approximate designated locations shown on Figure 1A. The logs of the pits are presented on Figure 2. The subsoils encountered, below about 2 to 21/2 feet of topsoil, consist of medium stiff to very stiff sandy silty clay with scattered basalt cobbles and boulders. Results of swell - consolidation testing performed on relatively undisturbed samples of the sandy silty clay, presented on Figure 3 and 4, indicate low compressibility under existing moisture conditions and light Ioading and variable low expansion potential when wetted. Results of a USDA gradation analysis performed on a sample of silty clay loam obtained from Profile Pit are presented on Figure 5. The laboratory test results are summarized in Table 1. No free water level was observed in the pits at the time of excavation and the soils were moist to slightly moist with depth. Minor seepage was observed in Pit 1 at the interface of the topsoil and the clay soils. Foundation Recommendations: The stiff to very stiff clay soils are typically expansive when wetted which could result in differential movement of lightly loaded footings. Considering the subsoil conditions encountered in the exploratory pits and the nature of the proposed construction, spread footings placed on the undisturbed natural soil designed for an allowable soil bearing pressure of 2,500 psf and minimum dead load pressure of 1 800 psf can be used with a risk of movement. The soils tend to heave when wetted and there could be post -construction foundation movements on the order of 1 to 2 inches. Placing a minimum 3 feet of structural fill such as road base below the footing would 1. to mitigate the movement otent'• •and c be used a means to eliminate the minimum dead load. The expansion potential of the clay soils should be further evaluated at the time of construction. Footings should be a minimum width of 16 inches for continuous ti walls and 2 feet for columns. The topsoil and Ioose disturbed soils encountered at the foundation bearing level within the excavation should be removed to expose the undisturbed natural soils. Voids created by boulder removal should be backfilled with structural fill. 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 12 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 60 pcf for the on-site soil as backfill. A representative of the geotechnical engineer should observe all footing excavations prior to concrete placement to evaluate bearing conditions. The structural fill should be compacted to at least 98% of standard Proctor density and extend to at least 11/2 feet beyond the footing edge. Floor Slabs: The natural clay soils, below the topsoil, can be used to support lightly loaded slab -on -grade construction with a high risk of heave it the bearing soils are wetted. Job ND. I 16 207A GL415tech -3 - Use of a crawlspace is recommended to minimize potential floor movement. If slab -on - grade is used, a minimum 3 feet of road base is recommended to reduce the heave potential. To reduce the effects of some differential movement, floor slabs where used 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 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 should consist of imported granular soils devoid of vegetation, topsoil and oversized rock. Underdrain System: Although free water level was not encountered during our exploration, seepage was observed at the topsoil -clay soil interface. It has been our experience in mountainous areas and where there are clay soils that local perched groundwater can develop during times of heavy precipitation or seasonal runoff. Significant seasonal runoff at the topsoil interface has been noted throughout the Mountain Springs Ranch development. 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 11/2 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 Ieast 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 Job No, 116 207A Gtech -4 - covered with filter fabric and capped with about 2 feet of the on-site, fine grained 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. A swale will be needed uphill to direct surface runoff around the residence. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. 5) Landscaping which requires irrigation should be located at least 5 feet from the building and beyond any foundation wall backfill. Percolation Testing: One profile pit and three percolation test holes were excavated on June 7, 2016 at the locations shown on Figure IA. The subsoils exposed in the Profile Pit consisted of about 2 feet of topsoil overlying silty clay loam with massive structure. The results of a USDA gradation analysis performed on a sample of silty clay loam obtained from Profile Pit 1 are presented on Figure 5. The sample tested has an USDA Soil Texture Classification of Silty Clay Loam. No free water or evidence of a seasonal perched water table was observed in the pits and the soils were slightly moist to moist. Percolation test holes were hand dug and soaked with water on June 7, 2016. Percolation testing was conducted on June 8, 2016 by a representative of Hepworth - Pawlak Geotechnical, Inc. The percolation rates in Holes 2 and 3 were on the order of 960 minutes per inch. Percolation Hole I showed no water movement. The rates are slower than typical of a conventional infiltration disposal system. The percolation test results are summarized on Table 2. A professional engineer should design the 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 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 at once so re-evaluation of the recommendations may be made. Job No. 116 207A Meth 5 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 if we may be of further assistance, please let us know. Respectfully Submitted, HEPWORTH - PAWLAK GEOTECHNICAL, INC. Tom C Brunner - Staff Engineer Reviewed by: Steven L. Pawlak, P.E. TCB/ksw attachments Figure 1 — Location of Building Area Figure IA — Location of Exploratory Pits and Percolation Test Holes Figure 2 — Logs of Exploratory Pits Figures 3 and 4 - Swell -Consolidation Test Results Figure 5 - USDA Gradation Test Results Table 1 — Summary of Laboratory Test Results Table 2 — Percolation Test Results Job No. 116 207A G ech LOT 18 35.5ac. MOUNTAIN SPRINGS RANCH 1 1 APPROXIMATE SCALE 1' :-: 150' 116 207A HEPWORTH•PAWLAK GEOTECHNICAL LOCATION OF BUILDING AREA MOUNTAIN SPRINGS ROAD Figure 1 APPROXIMATE SCALE 1`- 20° 116 207A I-1 - i - 106 PROPOSED RESIDENCE - - - PIT 2 PROPOSED DRIVEWAY - Hepworth—Pawlak Geotechnical El PIT 1 - - 104 • - - • PROFILE 0 ®- PIT -- , P2 -s - Q P1 - A P3 LOCATION OF EXPLORATORY PITS AND PERCOLATION TEST HOLES 0A Figure 1A 0 5 10 LEGEND: ,1 _ J PIT 1 ELEV.= 106' WC= 18.2 013=- 104 PIT 2 ELEV.= 103 WC 209 DD. 99 WC 12 7 OD113 PROFILE PIT ELEV. 99 J _J GRAVEL 1 SAND 13 SILT 50 CLAY 36 TOPSOIL; organic silty clay, soft to medium stiff, moist to very moist, dark brown to black CLAY (CL); slightly sandy, silty, medium stiff to stiff, most, reddish brown CLAY (CL); sandy, silty, with occasional basalt cobbles and boulders, stiff to very stiff, sightly most, brown to reddish brown. 2" Diameter hand driven liner sample. Disturbed bulk sample. Seepage NOTES: 0 5 10 1. Exploratory pits were excavated on June 7, 2016 with a 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 obtained by interpolation between contours shown on the site plan provided. 4. The exploratory pit locations and elevations should be considered accurate on y to the degree implied by the method used. 5. The lines between materials shown on the exploratory pit logs represent material types and transitions may be gradual. 6. Water seepage shown on the logs was observed at the time excavation. may occur with time. No seepage was observed in Pit 2 and Profile Pit. 7. Laboratory Testing Results: WC = Water Content (%) DD = Dry Density (pcf) Gravel = Percent retained on No. 10 Sieve Sand = Percent passing No. 10 sieve and retained on No. 325 sieve Silt = Percent passing No. 325 sieve to particle size .002mm Clay = Percent smaller than particle size .002mm the approximate boundaries between Development and fluctuations in water leve' 116 207A H H EPWORTH•PAWLAK GEOTECHNICAL LOGS OF EXPLORATORY PITS Figure 2 Compression 0 1 2 3 4 0 Moisture Content 18.2 percent Dry Density = 104 pct Sample of: Sandy Silty Clay From: Pit 1 at 5 y Feet No movement upon wetting 0.1 1.0 10 APPLIED PRESSURE - ksf 100 Moisture Content = 20.9 percent Dry Density = 99 pcf Sample of: Slightly Sandy Silty Clay From: Pit 2 at 3 Feet 2 0 m 0 a 3 E 0 U 4 Expansion upon wetting 0.1 116 207A 1.0 10 APPLIED PRESSURE - ksf HEPWORTH.PAWLAK GEOTECHNICAL SWELL -CONSOLIDATION TEST RESULTS 100 Figure 3 Compression - Expansion % 0 1 2 3 Moisture Content - 12.7 percent Dry Density = 113 pct Sample of: Silty Clay From: Pit 2 at 7 Feet Expansion upon wetting 01 10 APPLIED PRESSURE - ksf 10 100 116 207A HEPWORTH•PAWLAK GEOTECHNICAL SWELL -CONSOLIDATION TEST RESULTS Figure 4 RCENT RETAIN • HYDROMETER ANALYSIS SIEVE ANALYSIS 24 R 7 }{p TIME READINGS 1 M 0 45 MIN 15 MIN 60MIN19MIN 4 MIN 10 20 30 40 50 60 70 BO 90 100 W#1 ✓— i i i IN #325 #140 #60 #35 #18 #10 #4 U S STANDARD SERIES --ter rrw! —#111,4•1 N#IMIN# .ir —tea! s_ali i! r►M 11W— ra'� —ri — rl rrr�lii _Jilt.—!ri! l !ii I is#1WI .� PM Irl itrs W I — M#Ep! w.■i rrr�i NI .rl r...mmii W 11 =1#11#1.•• ! ! ! ^www ! ir.— #m11#1•11.1 — i IrJm --- i-- i�� #.0 ##11,1011 --- �—rrl� 1111#1 w— rr# rr#i ii !i rrw� !i irwwlw.....r rr�ili i !i rrr� i! #11 #1,0 rl.rr i....w.� #11i! rrrrti! rrrrr i! rrr� i! rrr� i! —rl11#2•1•......1..1 rrr� i rrrr !--.—wiV IMI_ri rrrl i! 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O ci r ri — co Lei z 0 r a 0 0 DEPTH (11) I 5 1 e; 3to5 SAMPLE! O. — cam! 0 O L HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE 2 PERCOLATION TEST RESULTS JOB NO. 116 207A HOLE NO. HOLE DEPTH (INCHES) LENGTH OF INTERVAL (MIN) WATER DEPTH AT START OF INTERVAL (INCHES) WATER DEPTH AT END OF INTERVAL (INCHES) DROP IN WATER LEVEL (INCHES) AVERAGE PERCOLATION RATE (MIN./INCH) P 1 46" 30 6118 6118 0 No Perc 6118 6118 0 6118 6118 0 6118 6118 0 6118 6118 0 P 2 40" 30 6 1/2 6 1/2 0 960 6'/2 6'/z 0 61/2 61/2 0 6 1/2 6 1/2 0 6'/2 6 318 1/8 P 3 46" 30 6 718 6 718 0 .__. 960 6 7/8 6 718 0 6 718 6 3 1/8 6' 6' 0 6' 6' 0 Note: Percolation test holes were hand dug in the bottom of backhoe pits and soaked on June 7, 2016. Percolation tests were conducted on June 8, 2016. The average percolation rates were based on the last three readings of each test. G e Jt h HEPWORTH—PAWLAK GEOTECHNICAL June 23, 2016 Mike Freeman 720 Minter Avenue Glenwood Springs, Colorado 81601 mfreeman81601@gmail.com l €I.i,u,Irrlt-I',rt.1 G.ottcluia.,i1. is« 5020'G,untr BEI IL€ € i_ (ilcriwUlk} Srrin;;:, (2 Itir,,:In 81601 rltil,n� 970-945-79;11:: Fix 970.945 845-1 cru.ui €irgrl,t•I,rgrnti_LlI conn Job No. 116 207A Subject: Slab -On -Grade Alternative Recommendations, Proposed Residence, Lot 18, Mountain Springs Ranch, Mountain Springs Road, Garfield County, Colorado Gentlemen: We understand from Ed Walters that slab -on -grade is preferred in the walkout level basement and garage. Since importing aggregate base course is costly, alternative subgrade treatment of the clay soils is desired if feasible. We previously conducted a subsoil study for design of foundations at the site and presented our findings in a report dated June 16, 2016, Job No. 116 207A. The options presented in our previous report to mitigate the expansive clay soil condition were to: 1) use structural floor above crawlspace, and 2) replace the clay soils with 3 feet of aggregate base course. Other options with higher movement risk consist of: 1) moisture condition to at least optimum the onsite clay soils and replace them compacted to 3 feet, and 2) use a slip joint at the bottom of slab bearing partition walls to allow for slab movement without affecting the upper structure provided the owner accepts the risk of movement and distress. Slip joints which allow at least 1 �h inches of vertical movement are recommended. This detail is also important for wallboards, stairways and door frames. Other recommendations presented in our previous report which are applicable, including further evaluation of the expansion potential below slab -on -grade areas at the time of excavation should also be followed. If you have any questions or need further assistance, please call our office. Sincerely, HEPWORTH — PAWLAK GE 1 ICAL, INC. Steven L. Pawlak, P.E. SLP/ksw cc: The Walters Compnfo@walter5company.net) •11't'.ar 11 1 1..841 I _ f 11 C) 0 l Ill, ,r i.l \f1r1111TL 7 I Q.fil l..CA? O 071)..4A1.1..100