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HomeMy WebLinkAboutSubsoils Investigation Reportffi CTLITHOMPSON GEOTECHNICAL ENGINEERING INVESTIGATION BRETTELBERG WEST DECK REMODEL 11101 COUNTY ROAD 117 GARFIELD COUNTY, COLORADO Prepared For: Brettelberg Condominium Association 11101County Road 117 Glenwood Springs, CO 81601 Attention: Adam Roberts BCA President Project No, GS06866.000-1 20 July 25, 2024 CTLlThompson. lnc. .WI, Fort Collins, Q.oloracLgSpdn$, Glenwood Sprinqs, Pueblo, Summit County - Colorado Chevenne, V1&oming and Bozeman, Montana ffi SCOPE CTllThompson, lnc. (CTLIT) has completed a geotechnical engineering investigation for the Brettlelberg West Deck Remodel planned for the Brettelberg Condomiums at 11101County Road 117 in Garfield County, Colorado. We con- ducted this investigation to evaluate subsurface conditions at the site and provide geotechnical engineering recommendations for the proposed construction. The scope of our investigation was set forth in our Proposal No. GS 24-0077 . Our re- port was prepared from data developed from our field exploration, laboratory test- ing, engineering analysis, and our experience with similar conditions. This report includes a description of subsurface conditions found in our exploratory pits and provides geotechnical engineering recommendations for design and construction of foundations, earth retaining walls, slab-on-grade construction, and details influ- enced by the subsoils. SITE CONDITIONS The Brettleberg Condominiums are located at11101 County Road 117 in Garfield County, Colorado. A vicinity map with the location of the site is included as Figure 1. The building is at the base of Sunlight Mountain Resort and consists of three, two to three-story rectangular sections that step down the hill. lt appears that initial construction of the building may have commenced in the late 1960's. Extending from the west side of the building is a series of wooden decks. The decks are at four different levels that are connected by three flights of wooden stairs. The east end of the existing decks appear to be supported by the west foundation wall of the building. The west end of the decks are supported on isolated footing pads. Existing decks and stairs were constructed above original slab-on-grade patios and stairs. An aerial photograph of the site is shown on Fig- ure 2. BRETTLEBERG CONDOMINIUM ASSOCIATION BRETTLEBERG WEST DECK REMODEL cTLIT PROJECT NO. GS06866.000-120 Page 1 of 11 ffi Previous work was done to mitigate water intrusion at the Brettelberg Con- dominiums. A foundation drain was installed to mitigate water intrusion into the G level (walkout basement level). Additionally, an interceptor drain was constructed in the upper parking lot at the south side of the building. CTLIT provided geotech- nical consultation regarding the water intrusion under our project number GS05785-145 (report dated August 27,2013). PROPOSED CONSTRUCTION We reviewed a concept plan by John L. Taufer & Associates, lnc. dated April 1 0,2024. The planned construction consists of removal and replacement of the upper two decks at the west side of the building. The proposed construction is shown on Figure 3. We expect the uppermost deck will be replaced with a concrete slab-on- grade. Structural fill will be required to raise grades for the slab. We expect struc- tural fill thickness of about 6 feet. The lower of the two decks will be replaced with a structurally supported slab. Both slabs will have embedded snowmelt tubing. Earth retaining walls with a maximum height of about 6 to 7 feet will be con- structed to facilitate the grade drop at the west and north sides of the upper slab. We anticipate maximum excavation depths on the order of 4 feet for the retaining wall footings. SUBSURFACE CONDITIONS Subsurface conditions were investigated by observing excavation of two exploratory pits (TP-1 and TP-2) at the site. The pits were excavated with a track- hoe on June 19,2024 at the approximate locations shown on Figures 2 and 3. Exploratory excavation operations were directed by our representative, who BRETTLEBERG CONDOMINIUM ASSOCIATION BRETTLEBERG WEST DECK REMODEL crLlr pRoJEcr No. Gs06866.000-120 Page2of1l ffi logged subsurface conditions and obtained representative samples of the soils. Graphic logs of the subsoils encountered in our pits are shown on Figure 4. Subsurface conditions encountered in our exploratory pits excavated at the site were about 6 inches of aggregate base course or 3 inches of topsoil un- derlain by natural clayey gravelwith cobbles and boulders to the maximum ex- plored depth of 8 feet. Some boulders had maximum dimensions of about 2 feet. Groundwater was not found in our exploratory pits. The pits were backfilled im- mediately after exploratory excavation opcrations wcrc completcd. A photograph of the soils and rock found in our exploratory pits is below. Soils and rock excavated from TP-1 t]RETTLEBERG CONDOMINIUM ASSOCIATION BRETTLEBERG WEST DECK REMODEL cTLIT PROJECT NO. GS06866.000-120 Page3of11 ffi Samples of the soils obtained from our exploratory pits were returned to our laboratory for pertinent testing. Gradation analyses indicated 53 percent gravel, 22 percent sand, and 27 percent silt and clay size material (passing the no. 200 sieve). The silt and clay portion of the sample exhibited a liquid limit of 32 percent and a plasticity index of 15 percent. The sample tested did not include rocks larger than 3 inches, which are present in the in-situ soils. The soil had a tested water-soluble sulfate content of 0.03 percent. Gradation test results are shown on Figure 5. Laboratory testing is summarized on Table l. SITE EARTHWORK Excavations All existing deck foundation elements and slabs should be removed from below the new construction. Excavation depths of about 4 feet will likely be re- quired to construct the new foundations. Our subsurface investigation indicates excavations to this depth at the site can be made with conventional, heavy-duty excavating equipment. Medium-size boulders could be encountered. Sides of ex- cavations must be sloped or retained to meet local, state, and federal safety reg- ulations. The subsoils at the site will likely classify as Type C soils based on OSHA standards governing excavations. From a "trench" safety standpoint, tem- porary slopes deeper than 5 feet that are not retained should be no steeper than 1.5 to 1 (horizontal to vertical) in Type C soils. The contractor's competent per- son is responsible for determining the actual OSHA soil type when excavations are made and for maintaining safe excavations. We do not anticipate excavations for foundations at this site will penetrate a free groundwater table. Excavations should be sloped to gravity discharges or to temporary sumps where water from precipitation and seepage can be re- moved by pumping. BRETTLEBERG CONDOMINIUM ASSOCIATION BRETTLEBERG WEST DECK REMODEL cTLIT PROJECT NO. GS06866.000-120 Page4of11 ffi Structural Fill Structural fill will be required to raise grades for the slabs. Additionally, voids may result from removal of boulders and/or existing foundation elements, which will require structuralfill. CTLIT should be called to observe excavated ar- eas to confirm that conditions are suitable for placement of structural fill. We anticipate that the excavated soils can generally be reused as struc- tural fill, providcd they are free of rocks larger than 4 inches, organics, and de- bris. As an alternative, an aggregate base course or pit run can be imported to the site for use as structural fill. A sample of desired import soil material should be submitted to our office for approval, prior to hauling. Structuralfill soils should be moisture conditioned to within 2 percent of optimum moisture content, placed in loose lifts of 8 inches thick or less, and com- pacted to at least 98 percent of standard Proctor (ASTM D 698) maximum dry density. Moisture content and density of structural fill should be checked by a CTLIT during placement. Observation of the compaction procedure is necessary. Backfill Proper placement and compaction of backfill soils adjacent to foundations and earth retaining walls is important from a lateral earth pressure standpoint. lt is also important to reduce infiltration of surface water, which can cause differen- tial movement of foundations and consolidation settlement of backfill soils. We expect the the soils excavated from the site can generally be used as backfill, provided they are free of rocks larger than 4-inches in diameter, organics, and debris. BRETTLEBERg CONBOMINIUM ASSOCIATION BRETTLEBERG WEST DECK REMODEL cTLIT PROJECT NO. GS06866.000-l 20 Page 5 of 11 ffi Backfill soils should be placed in loose lifts of approximately 10 inches thick or less, moisture-conditioned to within 2 percent of optimum moisture con- tent and compacted to at least 95 percent of standard Proctor (ASTM D 698) maximum dry density. Backfill soils within zone that will support new structures or slabs should be compacted to at least 98 percent of ASTM D 698 maximum dry density. Moisture content and density of the backfill should be checked by CTLIT during placement. Observation of the compaction procedure is necessary. FOUNDATIONS We recommend constructing the planned earth retaining walls on footing foundations. Support columns for the structurally supported slab can be con- structed on footing pads. The footings and footing pads should be constructed on natural gravel and cobble soil or densely compacted, structural fill. The structural fill should be in accordance with recommendations in the Structural Fill section. Recommended design and construction criteria for footing foundations are presented below. These criteria were developed based on our analysis of field and laboratory data, as well as our engineering experience. Footings and footing pads should be supported by the natural gravel and cobble soil or densely compacted, structural fill. Footings and footing pads on the natural gravel and cobbles or densely compacted, structural fill can be designed for a maximum net allowable soil bearing pressure of 3,000 psf. The weight of backfill soils above footings can be neglected for bearing pressure calculation. A friction factor of 0.45 can be used to calculate resistance to slid- ing between concrete foundations and the recommended founda- tion soils. Continuous footings should have a minimum width of 16 inches. Footing pads for support columns should have minimum dimen- sions of 24 inches by 24 inches. Larger sizes may be required, de- pending upon foundation loads. 1 2 3 4. BRETTLEBERG CONDOMINIUM ASSOCIATION BRETTLEBERG WEST DECK REMODEL cTLIT PROJECT NO. GS06866.ooo-120 Page6of11 ffi Footings should be well-reinforced. We recommend reinforcement sufficient to span an unsupported distance of at least 10 feet. The soils under footings and footing pads should be protected from freezing. We recommend the bottom of footings and pads be con- strucled at a depth of at least 42 inches below finished exterior grades. The Garfield County building department should be con- sulted regarding frost protection requirements. SLAB.ON.G RADE CONSTRUCTION The upper deck will be replaced with a slab-on-grade. Slabs with embed- ded snowmelt tubing can be sensitive to movement and damage. All existing deck foundation elements and slabs should be removed from below the new slab. Structuralfill below the slab should be in accordance with recommendations in the Structural Fill section. We recommend that the slab-on-grade be separated from foundation walls and retaining walls with slip joints, which allow free vertical movement of the slab. The slab should be well-reinforced to function as an independent unit. Frequent controljoints should be provided, in accordance with American Concrete lnstitute (ACl) recommendations, to reduce problems associated with shrinkage and curl- ing. EARTH RETAINING WALLS Earth retaining walls adjacent to the new slab-on-grade will need to resist appropriate lateral earth pressures. Many factors affect the values of the design lateral earth pressure. These factors include, but are not limited to, the type, compaction, slope, and drainage of the backfill soils, as wells as the rigidity of the wall against rotation and deflection. 5. 6. BRETTLEBERG CONDOMINIUM ASSOCIATION BRETTLEBERG WEST DECK REMODEL cTLIT PROJECT NO. GS06866.000-120 Page7ofl1 ffi 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 about 0.5 to 1 percent of wall height, design for a lower "active" lat- eral earth pressure may be appropriate. For backfill soils conforming with recommendations in the Backfill section that are not saturated, we recommend an equivalent fluid density of at least 55 pcf for the at-rest condition. Retaining walls that are free to rotate and allow the active earth pressure condition to develop can be designed using an equivalent fluid density of at least 40 pcf for on-site material backfill. These equivalent densi- ties do not include allowances for surcharges, or hydrostatic pressures. SUBSURFACE DRAINAGE Water from precipitation, snowmelt, and irrigation can infiltrate relatively permeable backfill behind the earth retaining walls. This can cause hydrostatic pressure behind walls and reduce shear strength of the backfill soils. We recom- mend provision of drains behind the walls. The drains should consist of 4-inch diameter, slotted, PVC pipe encased in free-draining gravel. A prefabricated drainage composite should be placed ad- jacent to the retaining walls. Care should be taken during backfill operations to prevent damage to drainage composites. The drains should discharge via posi- tive gravity outlets. Gravity outlets should not be susceptible to clogging or freez- ing. SURFACE DRAINAGE Surface drainage is critical to the performance of foundations and con- crete flatwork. Proper surface drainage and irrigation practices can help control the amount of surface water that infiltrates the soils and contributes to movement BRETTLEBERG CONDOMINIUM ASSOCIATION BRETTLEBERG WEST DECK REMODEL cTLIT PROJECT NO. GS06866.000-1 20 Page8of11 ffi of foundations and slabs-on-grade. Positive drainage away from foundations and slabs avoidance of irrigation near the structures also help to avoid excessive wet- ting of backfill soils. The ground surface surrounding the decks and patios should be sloped to rapidly convey surface water away from the structures. We recommend a mini- mum constructed slope of at least 6 inches in the first 5 feet around the decks and slabs. Backfill soils adjacent to the planned structures should be moisture- treated and compacted pursuant to recommendations in the Baokfill section. lrri- gation of landscaping should be limited to the minimum amount sufficient to maintain vegetation; application of more water will increase likelihood of founda- tion and slab movements. Plants placed close to foundation walls should be lim- ited to those with low moisture requirements, CONCRETE Concrete in contact with soil can be subject to sulfate attack. One sample of the soil from the site that was tested contained 0.03 percent water soluble sul- fates (see Table l). Pursuant to ACI 318-19, this sulfate concentration is con- sistent with a sulfate exposure class of S0. For this exposure class, there are no special cement requirements for sulfate resistance in concrete that is in contact with the subsoils. ln our experience, superficial damage may occur to the exposed surfaces of highly permeable concrete, even though sulfate levels are relatively low. To control this risk and to resist freeze-thaw deterioration, the water-to-cementitious materials ratio should not exceed 0.50 for concrete 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 60/o +l- 1.5o/o. BRETTLEBERG CONDOMINIUM ASSOCIATION BRETTLEBERG WEST DECK REMODEL cTLIT PROJECT NO. GS06865.000-120 Page9ofl1 ffi CONSTRUCTION OBSERVATIONS We recommend that CTLIT be retained to provide construction observa- tion and materials testing services for the project. This would allow us the oppor- tunity to verify whether soil conditions are consistent with those found during this investigation. lf others perform these observations, they must accept responsibil- ity to judge whether the recommendations in this report remain appropriate. lt is also beneficialto projects, from economic and practical standpoints, when there is continuity between engineering consultation and the construction observation and materials testing phases. 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. We never have complete knowledge of subsurface conditions. Our analysis must be tempered with engineering judgment and experience. Therefore, the recommendations in any geotechnical evaluation should not be considered risk-free. We cannot pro- vide a guarantee that the interaction between the soils and the proposed struc- tures will lead to performance as desired or intended, Our recommendations rep- resent our judgment of those measures that are necessary to increase the chances that the structures will perform satisfactorily. lt is critical that all recom- mendations in this report are followed. LIMITATIONS This report has been prepared for the exclusive use of the Brettelberg Condominium Association. The information, conclusions, and recommendations herein are based upon consideration of many factors including, but not limited to, BRETTLEBERG CONDOMINIUM ASSOCIATION BRETTLEBERG WEST DECK REMODEL cTLlr PROJECT NO. GS06866.000.120 Page 10 of 11 ffi 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 geotechnical engineering. The recommendations provided in this report are ap- propriate for about three years. lf the proposed project is not constructed within three years, we should be contacted to determine if we should update this report. Our exploratory pits provide a reasonable characterization of subsurface conditions at the site. Variations in the subsurfacc conditions not indicated by thc pits will occur. 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. lf we can be of further service in discussing the contents of this report, please call. CTLITHOMPSON, I Reviewed by: Craig A. Burger, P. Principal Engineer D. Kellogg, Principal Engineer Lo n{ 36754 K t.t.0 zf BRETTLEBERG CONDOMINIUM ASSOCIATION BRETTLEBERG WEST DECK REMODEL cTLIT PROJEcT NO. GS06866.000-120 Page 11 of 11 ffi o 500 1000 NOTE: SCALET l'- 1000' BRETTELBERG CONDOMINIUM ASSOOLATION BRETTEI-BERO WEST DECK REMODEL SATELLITE IMAGE FROM MAXAR (DATED 2024) Vicinity Map omrnrum rg oTUT PROJECT NO. GSO6866.OOO-120 Flg. 1 LIGEND: TP_1 APPROXIMATE LOCATION OFI EXPLORATORY PIT ffi 20 40 NOTE: SCALET 1'= 40' BRETTELBERG CONDOMINIUM ASSOSCIATION BRETIEIJERO WEST DECT( REMODEL SATELLITE IMAGE FROM GOOGLE EARTH (oereo SEPTEMBER 21, 2oz1) r:.r. ll . .r, ; I .,-.:. Aerial Photograph TP_2 TP_1 oTUT PROJECT NO. GSO6866.OOO-120 Flg. 2 LEGEN D: TP_l APPROXIMATE LOCATION OFI EXPLOMTORY PIT NOTE: tr 0 10 20 SCALE: 1' * 20' oN' BASE DRAWING BY JOHN L. TAUFER & ASSOCIATES INC (DATED APR|L 1O, 2024) ":.< - -'atfi' atGt I rP-2 @l , ,i Mt,A4C6ffiIrF -E?- r^* ,.---- -d26' a at TP-1 )/t BRETTELBERG CONDOMINIUM ASSOCIATION BRETTEI.BERG WEST DECK REMODEL Proposed Construction Hg 3 :_-\- * oTUT PROJECT NO. GSO6866.OOO-120 TP-1 TOPSOIL, CLAY, SANDY, ORGANICS, MOIST, BROWN FILL, GRAVEL, SILTY, DENSE, SLIGHTLY MOIST TO MOIST, BROWN. GMVEL, CLAYEY, OCCASIONAL COBBLES AND BOULDERS, DENSE, SLIGHTLY MOIST TO MOIST, BROWN. (GC) INDICATES HAND.DRIVEN CALIFORNIA LINER SAMPLE OBTAINED. INDICATES BULK SAMPLE FROM EXCAVATED SOILS. DEPTH AT WHICH DEEPER PIT EXCAVATION WAS NOT PRACTICAL- EXPLORATORY PITS WERE EXCAVATED WITH A TRACKHOE ON JUNE 19,2024. PITS WERE BACKFILLED IMMEDIATELY AFTER EXPLORATORY EXCAVATION OPERATIONS WERE COMPLETED. 2. GROUNDWATER WAS NOT FOUND IN OUR EXPLOMTORY PITS AT THE TIME OF EXCAVATION. 3, THESE LOGS ARE SUBJECT TO THE EXPLANATIONS, LIMITATIONS AND CONCLUSIONS IN THIS REPORT. TP-2 ffi 0 55 0 10 15 F uJ Lrlt! IF o_ uJo F UJut 1r JF o-uo 10 15 LEGEND:NOTES: 1. w F F l- BRETTELSERG CONDOMINIUM ASSOCIATION BRETTELBERG WEST DECK REMODEL cTUT PROJECT NO. OS0€8€r'.000-120 Summary Logs of Fi[E,rratbry FIG.4 7a ffi ffi SANDS GRAVEL FINE MEDIUM COARS FINE COARSE COBBLEScLAy (PLAST|C) TO StLT (NON-PLASTTC) ANALYSIS SIEVE ANALYSIS 10 20 30 40 50 60 70 80 90 100 ------t- -t--t- 9706o f60 Fz 850(r uJa40 oulz Ft!tFzuloEua '4 127 200 152 90 80 100 30 z0 10 0 .001 0.002 .005 .009 .019 .037 9.52 19.1 36.1 76.2 5"6' 8' U.S. STANDARD SERIE$ '100 '50'40'30 '10 '10'8 CLEAR SQUARE OPENINGS 3/8" 314" 1vi' TIME READINGS 60 MtN. 19 MrN. 4 MrN. 1 MlN. '200 .o74 .149 .297 .590 't.19 2.0 2.38 4.76 o.42 DIAMETER OF PARTICLE IN MILLIMETERS 25 HR. 7 HR. 45 MIN, 15 MIN Somple of GRAVEL, cLAyEY (ec)From TP - 1 AT 5.7 FEET GRAVEL 51o/o SAND 22 SILT & CIAY 27 % LIQUID LIMIT 32 PLASTICITY INDEi i5 o/ % o/o GRAVELSANDS FINE MEDIUM COARS FINE COARSE COBBLEScLAY (PLASTIC) TO StLT (NON-PLASTIC) ANALYSIS 't0 20 30 40 50 60 70 80 s0 100 oulzaFut Fzulo d. UJL 127 200 152 100 90 80 otoz6 po Fz u50d.l!oqo 30 ?o 10 0 9.52 19.1 36.1 76.2.001 0.002 .005 .009 .019 .037 TIME READINGS 60MtN. 19MlN. 4MlN. 1MlN. '200 U.S. STANDARD SERIES '100 '50 '40'30 '16 '10 '8 CLEAR SOUARE OPENINGS 3/8' 314" ln', 3' 5"6' .o74 .'149 .257 .590 1.19 2.0 2.38 4.76 0'42 DIAMETER OF PARTICLE IN MILLIMETERS 25 HR. 7 HR. 45 MtN. 15 MtN. Somple of From BRETTELBERG CONDOMINIUM ASSOCIATION BRETTELBERG WEST DECK REMODEL PROJECT NO. GS06866.000-120 GRAVEL SILT & CLAV PLASTICITY INDEX o/o SAND % LIQUID LIMI' oh o/o oA Gradation Test Results FIG.5 TABLE I SUMMARY OF LABORATORY TESTING GTLIT PROJECT NO. cs06866.m0-120 ffi D=SCRIPTION GRAVEL, CLAYEY (GC) PASSING NO.200 SIEVE (o/o\ 27 PERCENT SAND (%\ 22 PERCENT GRAVEL (%) 51 SOLUBLE SULFATES P/o\ 0.03 ATTERBERG LIMITS PLASTICITY INDEX (o/"\ 15 LIQUID LIMIT e) 32 DRY DENSIry (PCF) MOISTURE CONTENT (o/o) 9.8 DEPTH (FEET) 5-7 EXPLORATORY PIT TP-1 Page 1 of 1 Table of Contents PROPOSED CONSTRUCTION 2 4 4 5 5 6 SITE EARTHWORK FOUNDATIONS... FIGURE 1 -VICINITY MAP FIGURE 2 - AERIAL PHOTOGRAPH FIGURE 3 - PROPOSED CONSTRUCTION FIGURE 4 - SUMMARY LOGS OF EXPLORATORY PITS FIGURE 5 - GRADATION TEST RESULTS TABLE I - SUMMARY OF LABORATORY TESTING BRETTLEBERG CONDOMINIUM ASSOCIATION BRETTLEBERG WEST DECK REMODEL cTLIT PROJECT NO. GS06866.000-120