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Home My WebLink AboutStormwater Design Report STORMWATER MANAGEMENT DESIGN REPORT THE PINES TRACT 2A, AMENDED KIRK SUBDIVISION Prepared for: JDS Architects 719 W Mendenhall St, Bozeman, MT 59715 Prepared by: Project Number: 210528 February 2023 INTRODUCTION The Pines Site Plan Phase II project proposes the development of Tract 2A, Amended Kirk Subdivision within the City of Bozeman. It is proposed that the site will have a single building with 12 units located along the northern property line with a paved parking area and trash enclosure. As shown on the Drainage Area Map found in Appendix A, a combination of site grading, sheet flow, & drywells will be utilized in managing the post-development stormwater runoff. The proposed dry well was designed using a 10-year, 2-hour storm event, and was checked against the half-inch requirement according to the City Design Standards. The calculations and volumes used in the design approach can be found in Appendix B following this report. A Geotechnical report for the site is included in Appendix C. DRAINAGE DESIGN The proposed development of this site will result in hardscape on 69% of the .44 acres. The runoff generated from the parking lot, building, and surrounding sidewalk. The runoff will flow into one of 2 storm inlets and be collected in the proposed drywell. A 10-year 2-hr storm on this site will generate 920 ft3 of runoff. The proposed drywell was designed to retain 945 ft3. GROUNDWATER A geotechnical report was prepared for Tract 1A, the report stated that no groundwater was encountered in the soil investigation. The test pits excavated down to 7.5 ft and the proposed drywell is sized to go no deeper than 7.5 ft bgs. The geotechnical report is attached in Appendix C. APPENDIX A DRAINAGE AREA MAP APPENDIX B DRAINAGE AREA AND RETENTION SYSTEM CALCULATIONS DRAINAGE AREA # 1 DRY WELL #1 1. Calculate Area and Weighted C Factor Contributing Area DA # C Area (ft 2) C * Area Hardscape 1 0.95 13080 12426 Landscape 1 0.20 6086 1217 Total 19166 13643 A = Area (acres) 0.44 C = Weighted C Factor 0.71 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.71 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres) 0.44 Q = runoff (cfs) 0.13 V = REQUIRED VOL (ft3)920 REQUIRED VOLUME (1/2" runoff per DSSP II.A.4) 1. Determine Area of Hardscape within Drainage Area #1 Contributing Area Area (ft 2 ) Hardscape 13080 2. Calculate 1/2" runoff volume over hardscape (aka Runoff Reduction Volume [RRV] as calculated in Montana Post- Construction Storwater BMP Manual - Equation 3-1) RRV = [P*Rv*A]/12 P = Water quality rainfall depth 0.50 inches Rv = Dimensionless runoff coefficient 0.66 0.05 + 0.9*I I = Percent impervious cover (decimal) 0.68 decimal A = Entire drainage area 0.44 acres RRV = Runoff Reduction Volume 0.012 acre-ft RRV = Runoff Reduction Volume 530 cubic feet Because the runoff volume from the 10-yr, 2-hr storm (for flood control) is greater than the runoff volume produced by the half inch rainfall (for water quality) the proposed Dry Well #1 is sized to handle the larger volume (920 cf). 1. Calculate Drywell Volume Existing Soil Condition GP poorly graded gravel Percolation Rate (min/in) 2.6 (see Circular DEQ 8, Percolation Rate (ft/hr) 0.217 Appendix C) Porous Media in Drywell Washed Rock Void Ratio of Media 40.00% Media Offset Dist. From Drywell (ft) 7.5 Infiltration Drywell Media Area (ft2)303.6 Infilitration Volume (ft3)132 perc rate x media area x 2 hours Media Void Volume Media Bed Depth (below MH) 1.50 Media Volume (ft2)2174.43 Media Storage Volume (ft2 870 Manhole Volume Manhole Volume (ft3)75 Proposed Manhole Depth (ft) 6 (round up to nearest 3") Provided Storage Volume (ft3)945 Provided Volume Inc. Perc. (ft3)1,077 Design Storm Infiltration Time (hrs)14 APPENDIX C GEOTECHNICAL REPORT 3528 Fieldstone Dr. Bozeman, MT 59715 (406)-582-4406 March 20, 2013 LMF Group 3506 Fieldstone Dr. W. Bozeman Montana 59715 RE: Geotechnical Report –Proposed Pines Apartments, Bozeman, MT Dear LMF Group: A geotechnical investigation has been completed at the site of the proposed apartments. The purpose of the investigation is to assess subsurface soil and groundwater conditions and provide pertinent geotechnical recommendations for design of the foundation elements and pavements. This report presents the findings and recommendations. Project Background The proposed project is located on Tract 1A of Amended Subdivision Plat F-1-D, Gallatin County Montana as shown on Attachment A, Figure 1. The proposed apartment building will consist of a two story, wood framed structure. The building foundations will consist of a slab on grade with perimeter wall footings bearing a minimum of 3.5 feet below grade. Anticipated wall loads could range from 1500 to 3000 plf. Concentrated floor loads likely will not exceed 750 pounds. The footprint of the proposed building is approximately 125 feet by 40 feet (+/-5,000 square feet). If building loads differ from these assumptions we should be contacted to allow review and adjustment of the recommendations if deemed necessary. Existing Conditions The building site consists of a grassed lawn area. The site is located on essentially flat laying ground. An existing wooden storage shed is located on the southwestern edge of the site. The site slopes gently to the northwest. The Pines Apartments- Geotechnical Investigation Page 2 of 9 H:\31\BD 31 (2013)\The Pines\The Pines Geotechnical Report.doc Field Work The surface and subsurface conditions were studied during a field investigation on March 5, 2013. The field investigation consisted of visual reconnaissance of the site and excavation of three (3) test pits. The test pits were excavated at the approximate locations shown on Figure 1 of Attachment A. Locations of the test pits were determined by pacing from existing features. A Zaxis 120 excavator was utilized to excavate the test pits to depths ranging from 6 to 7.5 feet. Mr. Greg Underhill, PE, Geotechnical Engineer observed the excavations, obtained soil samples, and visually classified the soils according to ASTM D 2488. Summary logs of the test pits were recorded during excavation and are included in Attachment B. Elevations shown on the test pits area approximate. The stratigraphic contacts shown on the individual test pit logs represent the approximate boundaries between soil types. The actual transitions may be more gradual or abrupt. The soil and groundwater conditions depicted are only for the specific dates and locations reported, and therefore, are not necessarily representative of other locations and times. Findings Soil Profiles and Characteristics The soil profiles encountered in the test pits were relatively consistent throughout the site. In general, the test pit profiles consisted of a layer of dark brown topsoil (OL) overlying stiff to medium stiff, medium to light brown lean clay (CL). The clay layer is underlain by poorly graded gravel with sand and cobbles (GP). Topsoil varied in thickness from 10 to approximatly15 inches. The lean clay deposits ranged in thickness from approximately 0.5 to 2.8 feet. Depth to the gravel layer ranged from 1.5 in the northwest corner to approximately 3 feet in the northeast corner. Logs of the test pits are presented in Attachment B. Photographs of the test pits are presented in Attachment C. Following is a description of the soils encountered in the test pits. Topsoil and organic matter: The Pines Apartments- Geotechnical Investigation Page 3 of 9 H:\31\BD 31 (2013)\The Pines\The Pines Geotechnical Report.doc This material consisted of organic lean clay (OL) with abundant roots. Larger diameter tree roots were encountered Test Pit 3 which was located in the northwest corner of the site. The topsoil material is considered unsuitable for support of foundation elements or concrete floor slabs. Lean Clay (CL) Lean clay (CL) was present beneath the topsoil throughout the entire site. In general, the clay layer was slightly moist to moist and medium stiff in consistency. The clay is considered suitable for support of the interior slab–on-grade floor provided is it properly prepared (see interior floor slab). Interior footings should be placed on the native gravles or structural fill placed on the native gravels. The clay is moisture sensitive and could become unworkable during times of wet weather. Poorly graded gravel with sand and cobbles (GP): Poorly graded gravel with sand and cobbles (GP) was present beneath the lean clay layer in all of the test pits. The gravel layer was generally medium dense and slightly moist. The gravel horizon was relatively homogeneous, both vertically and horizontally, with some variances in the maximum cobble size. The gravel deposits exhibit high bearing strength and low compressibility. These materials will provide suitable bearing support for foundation elements. Groundwater Groundwater was not encountered in any of the test pits excavated for this investigation. Groundwater levels and soil moisture may vary according to season, irrigation practices, and change in land use. Foundation Recommendations All footings should be placed on the native gravel deposits or structural fill. The lean clay is suitable for support of interior concrete slabs provided moisture conditions are suitable to allow proper compaction. The clay is moisture sensitive and could become unworkable during times of inclimate weather. If this is the case, the wet clay would have to be removed to dryer clay that The Pines Apartments- Geotechnical Investigation Page 4 of 9 H:\31\BD 31 (2013)\The Pines\The Pines Geotechnical Report.doc would allow proper compaction or down to the native gravel. Structural fill could then be placed to final floor subgrade. Footing subgrades should be evaluated during excavation and prior to placement of concrete to ensure all footings bear on native gravel or structural fill if needed. Extra care should be taken not to disturb the native gravel soils at the bottom of the footing excavations. A flat bar should be placed across the excavator bucket teeth or smooth edge excavator bucket utilized to prevent disturbance of the gravel at footing subgrade from ripping action by the excavator teeth. Footing subgrades should be proof rolled and compacted with a minimum of 4 passes with a vibratory compactor. Soft or unsuitable material should be removed to competent gravel and replaced with structural fill. Soil conditions should be confirmed at the time of construction to ensure conditions are not significantly different than those assumed for this report. Mr. Underhill should be contacted if conditions vary so revisions can be made on these recommendations, if deemed necessary. Allowable Bearing Pressure (footings placed on structural fill) The proposed apartment building may be supported on conventional spread footings bearing on native gravel or structural fill placed on the native gravel compacted as specified (see Structural fill placement considerations). Footings placed in this manner may be designed utilizing a net allowable bearing pressure of 2,500 pounds per square foot. Total settlement for footings designed and placed in this manner is estimated to be less than one inch with minor differential settlements. Wall footings should be a minimum of 16 inches in width and isolated column footings should have minimum dimensions of 24 inches. Footings should be placed a minimum of 3.5 feet below the final exterior finished grade surface for frost considerations. Lateral Load Resistance It is assumed that the foundation backfill material will be either lean clay or gravel. The Pines Apartments- Geotechnical Investigation Page 5 of 9 H:\31\BD 31 (2013)\The Pines\The Pines Geotechnical Report.doc For resistance to lateral load, the following values may be utilized:  At Rest (lean clay or gravel). ..................................................................................................... 55 psf/ft*  Active (lean clay or gravel). ...................................................................................................... 40 psf/ft*  Passive (lean clay or gravel ) .................................................................................................. 250 psf/ft**  Coefficient of friction between concrete and gravel,  = 0.45 *These recommendations are based on the assumption of no hydrostatic pressures and horizontal back slope. ** Assuming no hydrostatic pressure and horizontal backfill and neglecting the first foot of backfill depth and gravel backfill is compacted as specified in following the sections. Seismic design in accordance with IBC procedures should be based on assuming site class “D” conditions (stiff soil profile). Structural fill placement considerations If structural fill is required under footings the structural fill limits should extend a minimum of 1 foot beyond the footing edge for each foot of depth below the footing. For example the total excavation width for a two foot wide footing one foot above the gravel layer would be 4 feet. Care should be taken not to disturb the native gravel subgrade. After removal of the clay, structural fill may then be placed to footing subgrade. Structural fill should be compacted to not less than 95 percent of maximum dry density at plus or minus 2 percentage points of optimum moisture content in accordance to ASTM D 698. For a material that does not exhibit a well- defined moisture density curve, 75 percent of relative density according to ASTM D 4253 or D 4254 is required. If structural fill is needed the native onsite gravel or the following granular materials may be utilized for structural fill: ¾ inch minus washed crushed gravel, standard ¾ -inch minus road mix or sandy gravel meeting the following gradation: e Pines Apartments Recommended Structural Fill Gradation Sieve Size Percent Passing by Dry Weight 4-inch 100 No. 4 30-70 No. 200 0-10 Liquid limit less than 25% and Plasticity Less than 6 The Pines Apartments- Geotechnical Investigation Page 6 of 9 H:\31\BD 31 (2013)\The Pines\The Pines Geotechnical Report.doc Other granular materials may be acceptable but should be evaluated and approved by the geotechnical engineer. Quality control is essential during fill placement. Frequent compaction testing and consistent placement procedures with properly sized compaction equipment are required to produce a consistent and unyielding subgrade for successful support of the structural elements. Allowable temporary excavation slopes All temporary excavations must conform to OSHA Standards for Excavations, 29 CFR Part 1926.652 Appendix B to Subpart P. Gravel soils or soils below the groundwater table would be classified as type C soils. Type C soils require excavation slope angles not exceed 1.5H:1V (Horizontal to Vertical) or be shored. Temporary shoring (sheet piling) may be required if excavation limits impinge on adjacent infrastructure elements. The nature and extent of subsurface variations and groundwater conditions between test pit locations may not become evident until construction. Soil conditions should be evaluated at the time of construction by the contractor’s responsible person to ensure compliance to OSHA requirements. The stability of the construction excavations and associated worker safety is the responsibility of the contractor. Interior concrete floor slabs Subgrade for interior slabs should consist of properly compacted native clay or structural fill. A minimum of 4-inches of 3/4inch minus washed crushed base gravel is recommended between the either the clay or structural fill and concrete floor slab. Prior to placement of the base gravel the clay or structural fill subgrade should be moisture conditioned and compacted to 95 percent of maximum dry density at plus or minus 2 percent of optimum moisture in accordance to ASTM D 698. For structural design of interior concrete slabs-on-grade placed in this manner a modulus of subgrade reaction of 200 pci may be used. All concrete slab subgrades soil should be evaluated during construction by proof-rolling with heavy equipment and compacting as specified. Pavements The Pines Apartments- Geotechnical Investigation Page 7 of 9 H:\31\BD 31 (2013)\The Pines\The Pines Geotechnical Report.doc Paved areas immediately adjacent to the building will consist of parking and parking access from South 20th Avenue. Vehicle usage will be predominately automobiles and light trucks with occasional city sanitation trucks. Dependent upon site grading and removal of topsoil and organics the soils at subgrade elevation are anticipated to consist of lean clay. The following pavement section is recommended. The Pines Recommended Flexible Pavement Thicknesses MATERIAL THICKNESS (inches) Parking Areas and Access Lanes Hot Plant mix bituminous surface (MPWSS Type) 3 Crushed base (MPWSS 3/4" or 1-1/2” minus) 8 The commonly used standard specification for private sector projects in Montana is the Montana Public Works Standard Specifications (MPWSS), Sixth Edition, April 2010. It is recommended that gravel base courses meet the requirements of MPWSS Sections 02335 and 02234. Section 02510 should be referenced regarding specifications for Asphalt Concrete Pavement. Prior to pavement section placement, topsoil and organics should be stripped and the existing subgrade scarified to a depth of at least eight inches and compacted to not less than 95 percent of maximum dry density at optimum moisture content according to ASTM D 698. It is noted that fill, debris or other unsuitable subgrade may be encountered at some locations. These areas should be sub-excavated to a depth of 1 foot or to suitable native gravel and replaced with compacted granular fill. The gravel base material should be compacted to not less than 95 percent of maximum dry density at plus or minus 2 percent of optimum moisture content according to ASTM D 698 or ASSHTO T99. Landscape and Irrigation Considerations Surface grading should provide positive flow away from all structures. Landscaped areas, within 10 feet of foundations, should provide a minimum 5 percent slope away from the building. All roof down spouts should have extensions and/or splash guards that will route storm water away from the structure foundation areas. The Pines Apartments- Geotechnical Investigation Page 8 of 9 H:\31\BD 31 (2013)\The Pines\The Pines Geotechnical Report.doc Earthwork Site preparation should consist of stripping the existing vegetation and loose surficial materials from the areas beneath any footing, floor slab or pavement area. All exposed surfaces should be free from mounds and depressions, which could prevent uniform compaction. If unexpected fills or obstructions are encountered during site clearing, such features should be removed and the excavation thoroughly cleaned prior to backfill placement and/or construction. Fill, footings, or pavement should not be placed on frozen subgrade. Excavation: Based on the materials encountered in the test pits, the site soils may be excavated with conventional earthmoving equipment. Backfill placement under concrete slabs, pavement, and walkways: Prior to granular fill placement under pavement or concrete slab locations, the subgrade should be scarified to a depth of six inches and compacted to 95 percent of maximum dry density at plus or minus 2 percentage points of optimum moisture content according to ASTM D 698 (Standard Proctor). Soft areas should be over-excavated and replaced with structural fill. General oversite fills: Not less than 90 percent of maximum dry density at optimum moisture content according to ASTM D 698 (Standard Proctor). Fill should be placed and compacted in horizontal lifts, using equipment and procedures that will produce recommended moisture contents and densities throughout the lift. Uncompacted fill lifts should not exceed 10 inches in thickness. Quality control is essential during fill placement. Frequent compaction testing and consistent placement procedures with properly sized compaction equipment are required. If density tests indicate compaction is not being achieved, the fill should be scarified, conditioned to near optimum moisture content, re-compacted, and re-tested. Fill should not be placed in a frozen state or upon frozen subgrade. The Pines Apartments- Geotechnical Investigation Page 9 of 9 H:\31\BD 31 (2013)\The Pines\The Pines Geotechnical Report.doc Limitations The conclusions and recommendations presented in this report assume that site conditions are not substantially different than those exposed by the explorations. If during construction, subsurface conditions are observed or appear to differ from those encountered in the explorations, Mr. Underhill should be advised at once so that these conditions can be reviewed and recommendation can be is considered necessary. If there is a substantial lapse of time between submission of this report and the start of work at the site, and conditions have changed due to natural causes or construction operations at or near the site, it is recommended that this report be reviewed to determine the applicability of the conclusions and recommendations considering the changed conditions. The report was prepared for the use of the Owner and Architect in the preliminary planning of the site. It should be made available to prospective contractors for information on factual data only and not as a warranty of subsurface conditions. Any conclusions by a construction contractor or bidder relating to construction means, methods, techniques, sequences, or costs based upon the information provided in this report are not the responsibility of the Owner or Mr. Underhill. The contractors and bidders are encouraged to perform additional investigations necessary to develop subsurface data for construction purposes. If you have any further questions, please do not hesitate to call at 406-539-3707. Sincerely, __ Greg Underhill P.E. Senior Geotechnical Engineer Attachments ATTACHMENT A THE PINES APARTMENTS FIGURES S.19THAVE. S.20THAVE. S.19THAVE. S.20THAVE. TEST PIT LOCATIONS SITE PLAN - GEOTECHNICALFI G-1THE PINES BOZEMAN, MONTANA ATTACHMENT B THE PINES APARTMENTS TEST PIT LOGS 0 1.5 3 4.5 6 7.5 9 10.5 4834.5 4833 4831.5 4830 10" +/- Topsoil, moist, medium stiff, black, organics 0.8 4834.1Lean Clay, CL; moist, medium stiff, medium to light brown (root zone transition) 2.3 4832.6Poorly Graded Gravel with sand and cobbles, GP; slightly moist, medium dense, medium to light brown 6.0 4828.9Test Pit terminated at 6.0 feet No groundwater observed Project No.: 31.0000 LOG OF TEST PIT TP-1 CLIENT LMF Group PROJECT The Pines Apartments TEST PIT LOCATION South Center Edge of Building SITE Bozeman, MT DOWL HKM 920 Technology Blvd., Suite A Bozeman, Montana 59718 Telephone: (406) 586-8834 Fax: (406) 586-1730 STARTED 3/5/13 FINISHED 3/5/13 CONTRACTOR EXCAVATOR Zaxis 120 Ex OPERATOR MODEL LOGGED BY G. Underhill APPROVED BY G. UnderhillDEPTH(FT.)MATERIAL DESCRIPTION GRAPHICLOGELEVATION(FT.)BULK/GRABSAMPLES POCKETPENE-TROMETER,TSF10 20 30 40 PL LL ADDITIONALDATA/REMARKS Surface Elevation: 4834.9 M.C. Sheet 1 of 1 0 1.5 3 4.5 6 7.5 9 10.5 4834.5 4833 4831.5 4830 4828.5 14" +/- Topsoil, moist, medium stiff, black, organics 1.2 4834.3Lean Clay, CL; moist, medium stiff, medium to light brown 3.1 4832.4Poorly Graded Gravel with sand and cobbles, GP; slightly moist, medium dense, medium to light brown 7.0 4828.5Test Pit terminated at 7.0 feet No groundwater observed Project No.: 31.0000 LOG OF TEST PIT TP-2 CLIENT LMF Group PROJECT The Pines Apartments TEST PIT LOCATION Northeast Corner of Building SITE Bozeman, MT DOWL HKM 920 Technology Blvd., Suite A Bozeman, Montana 59718 Telephone: (406) 586-8834 Fax: (406) 586-1730 STARTED 3/5/13 FINISHED 3/5/13 CONTRACTOR EXCAVATORZaxis 120 Excavator OPERATOR MODEL LOGGED BY G. Underhill APPROVED BY G. UnderhillDEPTH(FT.)MATERIAL DESCRIPTION GRAPHICLOGELEVATION(FT.)BULK/GRABSAMPLES POCKETPENE-TROMETER,TSF10 20 30 40 PL LL ADDITIONALDATA/REMARKS Surface Elevation: 4835.5 M.C. Sheet 1 of 1 0 1.5 3 4.5 6 7.5 9 10.5 4833 4831.5 4830 4828.5 4827 12" +/- Topsoil, moist, medium stiff, black, organics 1.0 4833Lean Clay, CL; sligthly moist, medium stiff to stiff, medium to light brown abundant roots to 1.9' 1.9 4832.1Poorly Graded Gravel with sand and cobbles, GP; slightly moist, medium dense, medium to light brown, cobble size predominately 3" to 6"with occasional 8" 7.5 4826.5Test Pit terminated at 7.5 feet No groundwater observed Project No.: 31.0000 LOG OF TEST PIT TP-3 CLIENT LMF Group PROJECT The Pines Apartments TEST PIT LOCATION Northwest Corner of Building SITE Bozeman, MT DOWL HKM 920 Technology Blvd., Suite A Bozeman, Montana 59718 Telephone: (406) 586-8834 Fax: (406) 586-1730 STARTED 3/5/13 FINISHED 3/5/13 CONTRACTOR EXCAVATORZaxis 120 Excavator OPERATOR MODEL LOGGED BY G. Underhill APPROVED BY G. UnderhillDEPTH(FT.)MATERIAL DESCRIPTION GRAPHICLOGELEVATION(FT.)BULK/GRABSAMPLES POCKETPENE-TROMETER,TSF10 20 30 40 PL LL ADDITIONALDATA/REMARKS Surface Elevation: 4834.0 M.C. Sheet 1 of 1 ATTACHMENT C THE PINES APARTMENTS TEST PIT PHOTOGRAPHS The Pines Test Pit Photos Page 1 Test Pit-1 Excavated Gravel Test Pit-1 Excavation . The Pines Test Pit Photos Page 2 Test Pit-2 Excavated Gravel Test Pit-2 Excavation The Pines Test Pit Photos Page 3 Test Pit-3 Excavated Gravel Test Pit-3 Excavation