The URL can be used to link to this page

Home My WebLink AboutSoils Report 09.11.2020ir CTL I THOMPSON GEOTECHNICAL ENGINEERING INVESTIGATION JOHNSON RESIDENCE COLORADO HIGHWAY 325 GARFIELD COUNTY, COLORADO Prepared For: MICHAEL JOHNSON 124 West 2nd Street Rifle, CO 81650 Project No. GS06486.000-120 September 11, 2020 TABLE OF CONTENTS SCOPE 1 SUMMARY OF CONCLUSIONS 1 SITE CONDITIONS 2 PROPOSED CONSTRUCTION 3 SITE GEOLOGY 3 SUBSURFACE CONDITIONS 3 SITE EARTHWORK 5 Foundation Wall Backfill 5 FOUNDATION 6 STRUCTURALLY -SUPPORTED FLOORS AND CRAWL SPACES 7 FOUNDATION WALLS 7 SUBSURFACE DRAINAGE 8 SURFACE DRAINAGE 9 CONCRETE 10 CONSTRUCTION OBSERVATIONS 10 STRUCTURAL ENGINEERING SERVICES 11 GEOTECHNICAL RISK 11 LIMITATIONS 12 FIGURE 1 — VICINITY MAP FIGURE 2 — AERIAL PHOTOGRAPH FIGURE 3 — SUMMARY LOGS OF EXPLORATORY PITS FIGURE 4 — GRADATION TEST RESULTS FIGURE 5 — FOUNDATION WALL DRAIN CONCEPT TABLE I — SUMMARY OF LABORATORY TESTING MICHAEL JOHNSON JOHNSON RESIDENCE PROJECT NO. GS06486.000-120 SCOPE This report presents the results of our geotechnical engineering investiga- tion for the Johnson Residence proposed along Colorado Highway 325 in Gar- field County, Colorado. We conducted this investigation to evaluate subsurface conditions at the site and provide geotechnical engineering recommendations for the planned residence. Our report was prepared from data developed from our field exploration, laboratory testing, engineering analysis, and our experience with similar conditions. This report includes a description of the subsurface con- ditions observed in exploratory pits and presents geotechnical engineering rec- ommendations for design and construction of foundations, floor systems, below - grade walls, and details influenced by the subsoils. Recommendations contained in this report were developed based on our understanding of the proposed con- struction. A summary of our conclusions is below. SUMMARY OF CONCLUSIONS 1. Our exploratory pits (TP-1 and TP-2) excavated at the site encoun- tered about 6 inches of topsoil over soils consisting predominantly of silty to clayey sand to depths of 10.5 feet and 8 feet. The clayey to silty sand was intermixed with silty sandy clay The soils below a depth of 8 feet in TP-2 were silty to clayey gravel with cobbles. Groundwater was not found in our exploratory pits. Soils at anticipated foundation elevation for the building will consist predominantly of clayey to silty sand. We judge the residence can be constructed on a footing foundation supported by the undis- turbed, natural soils at the site. Design and construction criteria for the footing foundation are provided in the report. A perimeter foundation drain should be constructed around the crawlspace below the residence. Ground surface adjacent to the residence should be graded to provide for rapid removal of surface water away from the building. MICHAEL JOHNSON JOHNSON RESIDENCE PROJECT NO. GS06486.000-120 SITE CONDITIONS The Johnson Residence is proposed on a 60-acre parcel (Garfield County Parcel No. 212718200027), which is located along the east side of Colorado Highway 325 in Garfield County, Colorado. A vicinity map with the location of the site is shown on Figure 1. The site is located near the mouth of the canyon (Rifle Gap) that is below the dam of Rifle Gap Reservoir. Rifle Creek Golf Course is south of the parcel. Rifle Creek trends down to the south across the subject property, adjacent to the road. An aerial photograph of the site is included as Figure 2. The pro- posed building site is located on the valley floor between Rifle Creek and the south -trending Rifle Creek Canon Ditch along the base of the steep canyon slope to the east. Overall ground surface in the proposed building area is gently sloping to the south and southwest at grades Tess than 5 percent. Vegetation in this area consists of native grass and scattered sage. Hydrophilic plants, such as willows, are adjacent to the creek. A photograph of the site is below. Looking south across building site MICHAEL JOHNSON JOHNSON RESIDENCE PROJECT NO. GS06486.000-120 PROPOSED CONSTRUCTION We were provided with plans for the residence by Drummond House Plans (dated December 17, 2018). The plans indicated the Johnson Residence will be a two-story, wood -frame building. We understand the client desires a crawl space below the main level floor. Several covered decks will be on the building exterior. Foundation loads along perimeter walls are likely to be be- tween 1,000 and 3,000 pounds per linear feet with maximum interior column loads of less than 50 kips. We should be informed if construction plans change so we can provide geotechnical/geo-structural engineering input. SITE GEOLOGY As part of our geotechnical engineering investigation, we reviewed the ge- ologic map by the U.S. Geological Survey (USGS), titled, "Geologic Map of the Rifle Quadrangle, Garfield County, Colorado", by Ralph R. Shroba and Robert Scott (dated 1997). The overburden soils at the site are mapped as undivided alluvium and colluvium. The silty to clayey sand and gravel soils found in our ex- ploratory pits excavated at the site are consistent with the descriptions on the map- ping. The soils are underlain at depth by bedrock units of the Wasatch formation. SUBSURFACE CONDITIONS To investigate subsurface conditions at the site, we excavated two explor- atory pits (TP-1 and TP-2) at the approximate locations shown on Figure 2. Ex- ploratory excavation operations were directed by our representative who logged subsoils encountered in the pits and obtained representative samples of the soils. Graphic logs of the soils exposed in the pits are included as Figure 3. MICHAEL JOHNSON JOHNSON RESIDENCE PROJECT NO. GS06486.000-120 Our exploratory pits TP-1 and TP-2 encountered about 6 inches of topsoil over soils consisting predominantly of silty to clayey sand to depths of 10.5 feet and 8 feet, respectively. The clayey to silty sand was intermixed with silty sandy clay. The soils below a depth of 8 feet in TP-2 were silty to clayey gravel with cobbles. Groundwater was not found in our exploratory pits. The pits were back - filled immediately after exploratory excavation operations were completed. A photograph of the soils excavated from TP-2 is below. Soils excavated from TP-2 Samples of the soils obtained from our exploratory pits were returned to our laboratory for pertinent testing. One sample of the clayey to silty gravel se- lected for gradation analysis contained 54 percent gravel, 26 percent sand, and 20 percent silt and clay (passing the No. 200 sieve). A sample of the silty sandy clay selected for engineering index testing exhibited a liquid limit of 23 percent and a plasticity index of 5 percent. Gradation test results are results are shown on Figure 4. Laboratory testing is summarized on Table I. MICHAEL JOHNSON JOHNSON RESIDENCE PROJECT NO GS06486.000-120 SITE EARTHWORK We anticipate the foundation excavation will extend to depths of 3 to 5 feet to attain bottom of crawl space elevation. Our subsurface information indicates the subsoils at this depth will consist predominantly of clayey to silty sand. Sides of excavations deeper than 4 feet should be sloped to meet local, state, and fed- eral safety regulations. The natural soils at the site will likely classify as Type B or C soils based on OSHA criteria. Sides of excavations should be no steeper than 1 to 1 (horizontal to vertical) in Type B soils and 1.5 to 1 in Type C soils. The contractor is responsible for identification of soil types and proper sloping and excavation sloping. Free groundwater was not encountered in our exploratory pits during ex- cavation. We do not expect that excavations to the anticipated depths (3 to 5 feet) will penetrate a free groundwater table. We suggest excavations be sloped to a gravity discharge or to a temporary sump where water from precipitation and runoff can be removed by pumping. Foundation Wall Backfill Proper placement and compaction of foundation backfill is important to re- duce infiltration of surface water and settlement of backfill. This is especially im- portant for backfill areas that will support pavements, such as in the driveway. The excavated soils free of rocks larger than 4 inches in diameter, organics and debris can be reused as backfill adjacent to foundation wall exteriors. Backfill should be placed in loose lifts of approximately 10 inches thick or less, moisture -conditioned to within 2 percent of optimum moisture content and compacted to at least 95 percent of maximum standard Proctor dry density MICHAEL JOHNSON JOHNSON RESIDENCE PROJECT NO. GS06486.000-120 (ASTM D 698). Our representative should test moisture content and density of the backfill during placement. FOUNDATION Our subsurface information indicates the soils at anticipated foundation el- evation for the building will consist predominantly of clayey to silty sand. We judge the Johnson Residence can be constructed on a footing foundation sup- ported by the undisturbed, natural soils at the site. Our representative should be called to observe the completed foundation excavation to confirm that conditions are as anticipated from our investigation and suitable for support of footings. Recommended design and construction criteria for footings are below. 1 The residence can be constructed on a footing foundation sup- ported by the undisturbed, natural clayey to silty sand. 2. Footings can be sized using a maximum allowable bearing pres- sure of 2,500 psf. Continuous wall footings should have a minimum width of at least 16 inches. Foundations for isolated columns should have minimum dimensions of 24 inches by 24 inches. Larger sizes may be re- quired, depending upon foundation loads. 4. Grade beams and foundation walls should be well reinforced, top and bottom, to span undisclosed loose or soft soil pockets. We rec- ommend reinforcement sufficient to span an unsupported distance of at least 12 feet. The soils under exterior footings should be protected from freezing. We recommend the bottom of footings be constructed at a depth of at least 36 inches below finished exterior grades. The Garfield County building department should be consulted regarding required depth. MICHAEL JOHNSON JOHNSON RESIDENCE PROJECT NO. GS06486 000-120 STRUCTURALLY -SUPPORTED FLOORS AND CRAWL SPACES The main level floor in the residence will be structurally -supported with a crawl space below. The required air space in crawl spaces depends on the ma- terials used to construct the floor and the potential expansion of the underlying soils. Building codes normally require a clear space of at least 18 inches be- tween exposed earth and untreated wood floor components. For non -organic systems, we recommend a minimum clear space of 12 inches. Utility connections, including water, gas, air duct, and exhaust stack con- nections to appliances on structural floors should be capable of absorbing some deflection of the floor. Plumbing that passes through the floor should ideally be hung from the underside of the structural floor and not laid on the bottom of the excavation. It is prudent to maintain the minimum clear space below all plumbing lines. Control of humidity in crawl spaces is important for indoor air quality and performance of wood floor systems. We believe the best current practices to control humidity involve the use of a vapor retarder or vapor barrier (10 mil mini- mum) placed on the soils below accessible subfloor areas. The vapor re- tarder/barrier should be sealed at joints and attached to concrete foundation ele- ments. FOUNDATION WALLS Foundation walls which extend below -grade should be designed for lateral earth pressures where backfill is not present to about the same extent on both sides of the wall, such as in crawl spaces. Many factors affect the values of the design lateral earth pressure. These factors include, but are not limited to, the MICHAEL JOHNSON JOHNSON RESIDENCE PROJECT NO. GS06486.000-120 type, compaction, slope, and drainage of the backfill, and the rigidity of the wall against rotation and deflection. For a very rigid wall where negligible or very little deflection will occur, an "at -rest" lateral earth pressure should be used in design. For walls that can de- flect or rotate 0.5 to 1 percent of wall height (depending upon the backfill types), design for a lower "active" lateral earth pressure may be appropriate. Our experi- ence indicates typical below -grade walls in residences deflect or rotate slightly under normal design loads, and that this deflection results in satisfactory wall performance. Thus, the earth pressures on the walls will likely be between the "active" and "at -rest" conditions. For backfill conforming with recommendations in the Foundation Wall Backfill section, that are not saturated, we recommend design of below -grade walls at this site using an equivalent fluid density of at least 40 pcf. This value as- sumes deflection; some minor cracking of walls may occur. If very little wall de- flection is desired, a higher design value for the at -rest condition using an equiva- lent fluid pressure of 50 pcf is recommended. SUBSURFACE DRAINAGE Water from precipitation and surface irrigation frequently flows through rel- atively permeable backfill placed adjacent to a residence and collects on the sur- face of less permeable soils at the bottom of foundation excavations. This pro- cess can cause wet or moist conditions after construction. To reduce the likeli- hood water pressure will develop outside foundation walls and the risk of accu- mulation of water in below -grade areas, we recommend provision of a foundation drain adjacent to the perimeter of the crawlspace. The drain should consist of a 4-inch diameter, slotted pipe encased in free -draining gravel. The drain should MICHAEL JOHNSON JOHNSON RESIDENCE PROJECT NO. GS06486 000-120 lead to a positive gravity outlet or to a sump where water can be removed by pumping. The foundation drain concept is shown on Figure 5. SURFACE DRAINAGE Surface drainage is critical to the performance of foundations, floor slabs, and concrete flatwork. Surface drainage should be designed to provide rapid runoff of surface water away from the residence. Proper surface drainage and ir- rigation practices can help control the amount of surface water that penetrates to foundation levels and contributes to settlement of soils that support the building foundation. Positive drainage away from the foundation and avoidance of irriga- tion near the foundation also help to avoid excessive wetting of backfill soils, which can lead to increased backfill settlement and possibly to higher lateral earth pressures, due to increased weight and reduced strength of the backfill. We recommend the following precautions. The ground surface surrounding the exterior of the residence should be sloped to drain away from the building in all directions. We recommend a minimum constructed slope of at least 12 inches in the first 10 feet (10 percent) in landscaped areas around the resi- dence, where practical. 2. The residence should be provided with roof gutters and down- spouts. The downspouts should discharge well beyond the limits of all backfill. Splash blocks and/or extensions should be provided at all downspouts so water discharges onto the ground beyond the backfill. Landscaping should be carefully designed and maintained to mini- mize irrigation. Plants placed close to foundation walls should be limited to those with low moisture requirements. Sprinklers should not discharge within 5 feet of foundations. Plastic sheeting should not be placed beneath landscaped areas adjacent to foundation walls or grade beams. Geotextile fabric will inhibit weed growth yet still allow natural evaporation to occur. MICHAEL JOHNSON JOHNSON RESIDENCE PROJECT NO. GS06486 000-120 CONCRETE Concrete in contact with soil can be subject to sulfate attack. We meas- ured a water-soluble sulfate concentration of 0.0 percent in a sample of the sub- soils from the site. For this level of sulfate concentration, ACI 332-08 Code Re- quirements for Residential Concrete indicates there are no special requirements for sulfate resistance. In our experience, superficial damage may occur to the exposed surfaces of highly permeable concrete. To control this risk and to resist freeze thaw deteri- oration, the water-to-cementitious materials ratio should not exceed 0.50 for con- crete in contact with soils that are likely to stay moist due to surface drainage or high-water tables. Concrete should have a total air content of 6% +/-1.5%. We recommend all foundation walls and grade beams in contact with the subsoils (in- cluding the inside and outside faces of garage and crawl spaces) be damp - proofed. CONSTRUCTION OBSERVATIONS We recommend that CTL I Thompson, Inc. be retained to provide con- struction observation and materials testing services for the project. This would allow us the opportunity to verify whether soil conditions are consistent with those found during this investigation. If others perform these observations, they must accept responsibility to judge whether the recommendations in this report remain appropriate. It is also beneficial to projects, from economic and practical stand- points, when there is continuity between engineering consultation and the con- struction observation and materials testing phases. MICHAEL JOHNSON JOHNSON RESIDENCE PROJECT NO. GS06486.000-120 STRUCTURAL ENGINEERING SERVICES CTL I Thompson, Inc. is a full -service geotechnical, structural, materials, and environmental engineering firm. Our services include preparation of struc- tural framing and foundation plans. We can also design earth retention systems. Based on our experience, CTL I Thompson, Inc. typically provides value to pro- jects from schedule and economic standpoints, due to our combined expertise and experience with geotechnical, structural, and materials engineering. We would like the opportunity to provide proposals for structural engineering services on your future projects. GEOTECHNICAL RISK The concept of risk is an important aspect of any geotechnical evaluation. The primary reason for this is that the analytical methods used to develop ge- otechnical recommendations do not comprise an exact science. The analytical tools which geotechnical engineers use are generally empirical and must be tem- pered by engineering judgment and experience. Therefore, the solutions or rec- ommendations presented in any geotechnical evaluation should not be consid- ered risk -free and, more importantly, are not a guarantee that the interaction be- tween the soils and that the proposed structure will perform as desired or in- tended. What the engineering recommendations presented in the preceding sec- tions do constitute is our estimate, based on the information generated during this evaluation and our experience working with these conditions, of those measures that are necessary to help the building perform satisfactorily. This report has been prepared for the exclusive use of the client for the purpose of providing geotechnical design and construction criteria for the pro- posed project. The information, conclusions, and recommendations presented MICHAEL JOHNSON JOHNSON RESIDENCE PROJECT NO GS06486 000-120 herein are based upon consideration of many factors including, but not limited to, the type of structures proposed, the geologic setting, and the subsurface condi- tions encountered. The conclusions and recommendations contained in the re- port are not valid for use by others. Standards of practice continuously change in the area of geotechnical engineering. If the proposed residence is not con- structed within three years, we should be contacted to determine if we should up- date this report. LIMITATIONS Our exploratory pits provide a reasonable picture of subsurface conditions below the site. Variations in the subsurface conditions not indicated by the pits will occur. Our representative should be called to observe the completed founda- tion excavation to confirm that conditions are as anticipated from our investiga- tion and suitable for support of footings. This investigation was conducted in a manner consistent with that level of care and skill ordinarily exercised by geotechnical engineers currently practicing under similar conditions in the locality of this project. No warranty, express or im- plied, is made. If we can be of further service in discussing the contents of this report, please call. CTL I THOMPSON, INC John Mechling, P.E. Senior Principal Engineer MICHAEL JOHNSON JOHNSON RESIDENCE PROJECT NO. GS06466.000-120 Reviewed By: iiv'-'.. \DY'y\31-3' 8 •-:) 9 mes D. Kellog 'g}.Pt,E. c.„i._ )...,::.„ ision Manage . !j 0 1500 3000 1-1 SCALE: 1" = 3000' i Mlchael Johnson Johnson Residence NOTE: SATELLITE IMAGE FROM GOOGLE EARTH (DATED 6/17/2016) Project No. GS06486.000-120 Vicinity Map Flg. 1 op 1 0 50 100 iinm SCALE: 1" = 100' LEGEND: TP-1 APPROXIMATE LOCATION OF ■ EXPLORATORY PIT NOTE: SATELLITE IMAGE FROM GOOGLE EARTH (DATED 6/17/2016) Michael Johnson Johnson Residence Project No. GS06486.000-120 Aerial Photograph FIB. 2 w w w z 0 1- 5,780 5,775 5,770 TP-1 TP-2 EL. 5780 EL. 5776 5,780 5,775 - 5,770 5,765 5,765 5,760 5,760 MICHAEL JOHNSON JOHNSON RESIDENCE CTLIT PROJECT NO. GS06486.000-120 w w w z 0 F w -J W LEGEND: 0 TOPSOIL, SAND, CLAYEY, MOIST, BROWN. 7.7 CLAYEY SILTY SAND, INTERMIXED WITH SANDY SILTY CLAY, GRAVEL, SLIGHTLY MOIST, MEDIUM STIFF, BROWN, TAN, RUST. (SC-SM, CL-ML) GRAVEL, CLAYEY TO SILTY, COBBLES, • SLIGHTLY MOIST TO MOIST, MEDIUM ' • DENSE, TAN, BROWN. (GC, GC -GM) BULK SAMPLE FROM EXCAVATED SOILS. NOTES: 1. EXPLORATORY PITS WERE EXCAVATED WITH A TRACKHOE ON AUGUST 26, 2020. 2. GROUND WATER WAS NOT FOUND IN OUR EXPLORATORY PITS AT THE TIME OF EXCAVATION. PITS WERE BACKFILLED IMMEDIATELY AFTER EXPLORATORY EXCAVATION OPERATIONS WERE COMPLETED. 3. LOCATIONS OF EXPLORATORY PITS ARE APPROXIMATE. ELEVATIONS WERE ESTIMATED FROM GOOGLE EARTH. 4. THESE LOGS ARE SUBJECT TO THE EXPLANATIONS, LIMITATIONS AND CONCLUSIONS CONTAINED IN THIS REPORT. Summary Logs of Exploratory Pits FIG. 3 HYDROMETER ANALYSIS SIEVE ANALYSIS 25 HR. 7 HR. TIME READINGS U.S. STANDARD SERIES CLEAR SQUARE OPENINGS 45 MIN. 15 MIN. 60 MIN. 19 MIN. 4 MIN. 1 MIN. '200 "100 '50 '40 '30 '16 "10 '8 '4 3/8" 3/4" 1Y,," 3" 5"6" 8" 100 PERCENT PASSING 0 0 0 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 PERCENT RETAINED .001 0 002 .005 .009 .019 .037 .074 .149 DIAMETER .297 OF 0.42 PARTICLE .590 1 IN MILLIMETERS 19 2 0 2.38 4 76 9 52 19.1 36.1 76.2 127 200 152 CLAY (PLASTIC) TO SILT (NON -PLASTIC) SANDS GRAVEL FINE 1 MEDIUM I COARS FINE I COARSE rCOBBLES Sample of GRAVEL, CLAYEY (GC) From TP - 2 AT 8-9 FEET GRAVEL 54 % SAND SILT & CLAY 20 % LIQUID LIMIT PLASTICITY INDEX 26 % 1 HYDROMETER ANALYSIS I SIEVE ANALYSIS 25 HR, 7 HR. TIME READINGS U.S. STANDARD SERIES CLEAR SQUARE OPENINGS 45 MIN. 15 MIN. 60 MIN. 19 MIN. 4 MIN. 1 MIN. '200 '100 50 '40 30 '16 '10 '8 4 3/8" 3/4" 1%" 3" 5'6 B 100 0 - • - -- [10 !ERCCNT PASSINNC 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 00 0 0 PERCENT RETAINED « -_- .... _ __ .- __._ - .. -, - -1_ ,,. y- i . - ."" , , , . .. , • ., .001 0 002 .005 .009 .019 .037 .074 .149 .297 590 119 2 0 238 4.76 9 52 19.1 0.42 DIAMETER OF PARTICLE IN MILLIMETERS . 36.1 76.2 127 200 152 CLAY (PLASTIC) TO SILT (NON -PLASTIC) SANDS GRAVEL FINE I MEDIUM I COARS FINE P COARSE I COBBLES Sample of From MICHAEL JOHNSON JOHNSON RESIDENCE PROJECT NO. GS06486.000-120 GRAVEL SILT & CLAY PLASTICITY INDEX % SAND % LIQUID LIMIT Gradation Test Results OA OA OA FIG. 4 NOTE: DRAIN SHOULD BE AT LEAST 2 INCHES BELOW BOTTOM OF FOOTING AT THE HIGHEST POINT AND SLOPE DOWNWARD TO A POSITIVE GRAVITY OUTLET OR TO A SUMP WHERE WATER CAN BE REMOVED BY PUMPING. SLOPE PER REPORT BACKFILL MIRADRAIN G200N OR EQUIVALENT ATTACH PLASTIC SHEETING SLOPE TO FOUNDATION WALL PER OSHA COVER ENTIRE WIDTH OF GRAVEL WITH NON -WOVEN GEOTEXTILE FABRIC (MIRAFI 140N OR EQUIVALENT). Mlchael Johnson Johnson Residence ^ STRUCTURAL FLOOR lizatimL 2" MINIMUM 8" MINIMUM i- OR BEYOND 1:1 SLOPE FROM BOTTOM OF FOOTING (WHICHEVER IS GREATER) 4-INCH DIAMETER PERFORATED DRAIN PIPE. THE PIPE SHOULD BE PLACED IN A TRENCH WITH A SLOPE OF AT LEAST 1/8-INCH DROP PER FOOT OF DRAIN. CRAWL SPACE --r FOOTING OR PAD "MUD SLAB" OR VAPOR BARRIER Foundation Wall Drain Concept TABLE I SUMMARY OF LABORATORY TESTING PROJECT NO. GS06486.000-120 EXPLORATORY PIT DEPTH (FEET) MOISTURE CONTENT (%) DRY DENSITY (PCF) ATTERBERG LIMITS LIQUID LIMIT (%) PLASTICITY INDEX (%) SOLUBLE SULFATES (%) PERCENT GRAVEL (%) PERCENT SAND (%) PASSING NO. 200 SIEVE (%) DESCRIPTION TP-1 TP-2 TP-2 6-7 5-6 8-9 0.00 39 SAND, CLAYEY (SC) 23 5 54 CLAY, SANDY SILTY (CL-ML) 54 26 20 GRAVEL, CLAYEY (GC) l Page 1 of 1