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HomeMy WebLinkAbout20250932 5 Tomahawk Ln Area Variance Stormwater Narrative 11182025C.T. MALE ASSOCIATES Engineering, Surveying, Architecture, Landscape Architecture & Geology, D.P.C. 50 Century Hill Drive, Latham, NY 12110 518.786.7400 FAX 518.786.7299 www.ctmale.com Stormwater Narrative for 5 TOMAHAWK LANE CITY OF SARATOGA SPRINGS, NY May 20,2025 C.T. Male Associates Engineering, Surveying, Architecture, Landscape Architecture & Geology, D.P.C. (C.T. Male) has evaluated the pre -development and post -development drainage conditions at 5 Tomahawk Lane, located in the City of Saratoga Springs, New York. The project site is situated at the southern end of a residential neighborhood on the northwest side of Saratoga Lake. The property features a grade change of approximately 50 feet, sloping from Tomahawk Lane down toward the lake. A small residence previously stood on the higher portion of the site, near the current location of a parking area and a water supply well. This residence was demolished in 2008 to make way for the construction of the existing home, which sits further down the slope. k-, A new structure is proposed to be constructed on the sloped area between the parking area and the existing house. This new building will serve as a garage and storage facility, with an enclosed stairway providing access to the current residence. Supporting documentation for the project— including segments of a topographic and boundary survey, as well as conceptual design sketches —is provided in Appendix A. The proposed garage and storage building will cover approximately 970 square feet, with an additional 230 square feet dedicated to the enclosed stairway, for a total project footprint of approximately 1,200 square feet. Although the City of Saratoga Springs does not require stormwater management for residential ,developments that disturb less than one acre, the Lake George Park Commission Stormwater Regulations have been referenced for guidance due to the property's proximity to Saratoga Lake. These regulations require infiltration capacity of 1.5 gallons per square foot of new impervious surface for minor projects. Based on a net increase of 1,200 square feet of impervious area, the design calls for an infiltration device with a minimum storage capacity of 1,800 gallons (or 240 cubic feet). The roof drainage from the new structure will be directed into one or more drywells that meet or exceed the required 240 cubic feet of storage. I Civil Engineering 9 Environmental Services 9 Survey Services * Land Services * Architecture • Energy & Building Systems Services 9 Electrical Engineering IN 1 �fx- April 42024 Geotechnical Evaluation � EdProposed Garage and Storage Re Building � 5 Tomahawk Lane Saratoga Springs Saratoga County, New York Prepared for: Mr. Clad Kelman 5032 24th Circle Boca Raton, FL 33431 Prepared by: C.T. MALE ASSOCIATES 50 Century Hill Drive 100743 Latham, New York 12110 (518) 786-7400 FAX (518) 786-7299 IC.T. Male Associates Project No: 23.3612 Unauthorized alteration or addition to this document is a violation of the New York © Copyright 2024 State Education Law. C.T. MALE ASSOCIATES ENGINEERING, SURVEYING, ARCHITECTURE, LANDSCAPE ARCHITECTURE & GEOLOGY, D.P.C. C.T. MALE ASSOCIATES GEOTECHNICAL EVALUATION PROPOSED GARAGE AND STORAGE BUILDING 5 TOMAHAWK LANE SARATOGA SPRINGS, NEW YORK 1.0 INTRODUCTION .................................................................................................... 1 2.0 SITE AND PROJECT DESCRIPTION ................................................................... 1 3.0 SUBSURFACE INVESTIGATION PROGRAM .......... 6 ........................................ 2 4.0 SUBSURFACE CONDITIONS ............................................................................... 2 4.1 Geologic Mapping ........................................................................................ 2 4.2 Conditions Disclosed by Subsurface Investigation .................................3 5.0 GEOTECHNICAL EVALUATION ........................................................................ 4 5.1 Slope Stability ............................................................................................... 4 5.2 Foundation Support ..................................................................................... 4 5.3 Site Class Assessment .................................................................................. 5 6.0 GEOTECHNICAL RECOMMENDATIONS ........................................................5 6.1 Site Preparation ................................. ! .......................................................... 5 6.2 Excavation Considerations ......................................................................... 6 6.3 Foundation and Foundation Walls ............................................................6 6.4 Garage Floor Slab ......................................................................................... 7 7.0 CLOSURE .................................................................................................................. 8 Table 1: Imported Structural Fill Table 2: Design Parameters for Foundation Walls Appendix A: Concept Drawings Appendix B: Test Boring Location Plan Appendix C: Test Boring Log Appendix D: Websoil Survey Report 01 Appendix E: Slope Stability Analysis Screenshots C.T. MALE ASSOCIATES 3.0 SUBSURFACE INVESTIGATION PROGRAM The site's subsurface conditions were investigated at a location within the parking area at the top of the slope, to the west of and adjacent to the proposed structure footprint. Its location coordinates, as recorded using a cellular phone, are noted on the test boring log. The approximate location of the test boring is shown on the Test Boring Location Plan contained in Appendix B. The test boring was advanced by a subcontractor operating a Central Mine Equipment Model 55 track -mounted drill rig. It was advanced and cased against collapse through rotary drilling of 31/4-inch inside diameter hollow stem augers. As the augers were advanced with depth, the overburden was sampled and its penetration resistance determined in general accordance with the procedures of ASTM D-1586, "Standard Method for Penetration Testing and Split -Barrel Sampling of Soils". The sampling and penetration resistance testing were conducted on a continuous basis to a depth of 12 feet, and at nominal 5-foot intervals thereafter until the planned drilling depth of 77 feet was reached. In addition, two undisturbed soil samples were collected in general accordance with the procedures of ASTM D-1587, "Standard Practice for Thin -Walled Tube Sampling of Fine -Grained Soils for Geotechnical Purposes"'. A pocket penetrometer and torvane were used to measure the approximate unconfined compressive strength and shear strength of several of the recovered cohesive soil samples, the results of which are noted on the test boring log. A representative from our firm monitored the test boring, recorded the standard penetration resistances, field classified the recovered samples, and placed representative portions of the samples in glass jars. The samples placed in jars were brought to our geotechnical laboratory, examined, and, where necessary, refinements made to the field classifications. The log of the test boring presenting the soil descriptions in accordance with the Burmister identification system and the records maintained in the field are presented in Appendix C along with a sheet and key that explains the terms and symbols used in its preparation. 4.0 SUBSURFACE CONDITIONS 4.1 Geologic Mapping The surficial soils at the site have been mapped by the United States Department of Agriculture (USDA), Natural Resources Conservation Service, to a depth of 72 inches. The Soil Survey of Saratoga County, New York produced by this agency provides this mapping in some detail. For this project site, the detailed mapping is presented as a Custom Soil Resource Report in Appendix D. The surficial soils at the site are identified in this report as consisting of Windsor loamy sand (Wn) and Limerick -Saco complex (Lm). The Windsor loamy sand has a profile typically consisting of loamy sand overlying sand. The Limerick -Saco has a profile consisting of layers of silt loam, very fine sandy loam and loamy fine sand. -2- C.T. MALE A 1.0 INTRODUCTION This report presents the findings of an investigation and geotechnical evaluation of the subsurface conditions present at the site of a proposed new garage and storage building located at 5 Tomahawk Lane, Saratoga Springs, New York. The site's subsurface conditions have been investigated through the advancement of one conventional test boring and the performance of pocket penetrometer and torvane tests on selected samples recovered from the test boring. From our evaluation of the subsurface conditions disclosed by this work, we have identified the Seismic Site Class applicable to the project and have developed recommendations for the design and construction of foundations, floor slabs, and below grade foundation walls for the new structure. In developing these recommendations., we have analyzed the stability of the slope on which the new structure will be constructed and developed recommendations for limiting the weight imposed on the slope by the new structure to maintain the slope's existing stability. This study has been performed in accordance with the geotechnical portion of the scope of work presented in our proposal dated August 31, 2023 and contract agreement dated November 8, 2023. 2.0 SITE AND PROJECT DESCRIPTION The project site is located at 5 Tomahawk Lane at the southern end of a residential neighborhood on the northwest side of Saratoga Lake in Saratoga Springs, New York. The site grades drop approximately 50 feet across the property from Tomahawk Lane down to Saratoga Lake. A small residence was formerly present on the higher side of the property, where a small parking area and a water supply well are currently located. This residence was removed in 2008 prior to construction of the current residential building located further down the slope within the property. It is our understanding that a new structure is to be constructed off the high parking area on the slope leading down to the current house. The structure is planned to consist of a garage and storage building supported on a single foundation system. The structure will have a garage finished floor elevation of 198 feet and have an exercise room and bathroom above the garage. The garage portion of the structure will be 32 feet by 22 feet in plan and will be connected to the storage building with plan dimensions of 20 feet by 12 feet. Documents provided for this project and site, including portions of a topographic and boundary survey along with conceptual sketches, are included in Appendix A. - 1 - The sites soils have also been identified on the Surficial Geologic Map of New York, Hudson -Mohawk Sheet, prepared by the New York State Geologic Survey. Although this map identifies one material occupying the site, this being fluvial gravel (fg), which is indicated to consist of outwash sand and gravel deposits, the property is nearby the boundary of this deposit with lacustrine sands (Is). The lacustrine sands are indicated to consist of well -sorted and stratified, generally near -shore sand deposits. 4.2 Conditions Disclosed by Subsurface Investigation The test boring was advanced through a surficial layer of asphalt and its subbase, and into fill material and the underlying layers of native silty sand and silty clay. The fill material was found to primarily consist of medium to coarse sand. The standard penetration resistance values (N-values) within this material typically ranged from 4 to 15 blows per foot with a weighted average value of 10 blows per foot., indicating that the soils have an overall loose relative density. The fill extended to a depth of 9 feet below existing site grades. The upper sequence of native material underlying the fill consisted primarily of fine sand with minor to near equal amounts of silt and frequent clayey silt lenses. The N-values within this material ranged from 2 to 17 blows per foot with a weighted average value of 8 blows per foot, indicating that the soils are loose. This upper sequence of native soil extended to a depth of 37 feet to 40 feet below grade. The lower sequence of native material encountered within the test boring consisted of silt with minor to equal amounts of clay, minor amounts of fine sand, and frequent fine sand and silt lenses. The N-values within this material ranged from 2 to 6 blows per foot with a weighted average value of 4 blows per foot, indicating that the soils are of a soft to medium consistency. The stratum's undrained shear strength, as - measured through pocket penetrometer testing, typically ranged from 750 to 1,250 pounds per square foot, with these values being typical of cohesive soils of a soft to medium consistency. The test boring was terminated at the planned termination depth of 77 feet within this lower sequence of native soil. Although groundwater measurements were not taken during or at the completion of drilling, recovered soil samples became wet beginning at a depth of 11.5 feet, which is assumed to be the depth to the groundwater table at the test boring location on the date the test boring was advanced. -3- C.T. MALE ASSOCIATES 5.1 Slope Stability The stability of the slope was analyzed based on the results of the subsurface investigation performed at the site. Geostudio 2023, a software program developed by GEO-SLOPE International, Ltd., was used to analyze the stability of the slope, both for its existing condition, and for several alternatives for siting the new structure at the location proposed for its construction. For its existing condition (without the new structure present), our stability analyses indicated that the slope has a factor of safety against failure between 1.25 and 1.30, with the lower value being determined for a deep-seated failure and the higher value for a surficial failure of the slope. This range in factor of safety is considered to be the absolute lower limit of generally accepted values for slope stability evaluations in geotechnical engineering practice. Accordingly, to maintain or improve the slope's factor of safety associated with construction of the proposed structure,, the net load imposed to construct the finished floor at elevation 198.0 feet must be essentially zero. This may be accomplished either by using Geofoam as the majority of the fill material to establish the proposed finished floor elevation of 198.0 feet or by incorporating a "basement" level below the first floor of the structure. Geofoam is an expanded polystyrene product typically sold in block form that is used as lightweight fill for geotechnical applications. For this application, at least the upper 3 feet of the existing slope should be excavated and replaced with Geofoam in a stepped pattern and extending above the existing site grades up to a level of approximately 2 feet below the structure finished floor. To reduce the required quantity of Geofoam, consideration may be given to constructing a basement below the finished floor and utilizing this space as a storage room. This may allow the reduction of the overall footprint or height of the proposed structure by relocating the southern storage area or lofted fitness room to below the garage floor. Screenshots of our slope stability analyses, including the existing conditions, the proposed conditions with the placement of Geofoam, and the proposed conditions with construction of a basement level below the garage floor, are included as Appendix E. 5.2 Foundation Support The site soils are capable of supporting the proposed structure through the use of conventional shallow foundations. The foundation bearing grades will need to be stepped in elevation along the sides of the structure running parallel to the slope and will need to be established 4 feet below the final exterior grades along the other two sides of the structure. No grade increases should be made on the slope adjacent to the structure so as to maintain the existing slope's factor of safety against failure. Along all sides of the structure, the foundations must bear below the bottom of the Geofoam to be placed within its footprint. Underdrains should be installed -below the Geofoam and alongside the structure's perimeter walls prior to placement of any -4- C.T. MALE ASSOCIATES foundation backfill or Geofoam to address potential seasonal high groundwater levels and prevent the Geofoam from being subjected to uplift forces. Excavated site soils, provided they are granular in nature and culled of any deleterious components, may be used as foundation backfill. ® Site Class Assessment Our firm has previously completed shear wave velocity testing at a nearby site within the same geologic profile as the soils encountered on this property. Based on our previous testing,, we expect these site soils to exhibit shear wave velocities in excess of 600 feet per second. As such, we judge the profile to fit the Site Class D designation as identified in Section 1613.2.2 of the 2020 Building Code of New York State and Chapter 20 of ASCE 7- 16. ® GEOTECHNICAL RECOMMENDATIONS 6®1 Site Preparation Site preparation should begin with stripping the structure area of pavements and vegetation. Any utilities that are present in the structure area should be removed and rerouted around the same. These may include electric, stormwater,, sewer,, water, and/or telecommunications. Within foundation bearing areas, existing soils should be excavated to the planned foundation bearing grade. If fill material is present at this elevation, it should be further excavated until native soils are encountered. This excavated subgrade should be proofolled and compacted through several (6 or more) overlapping passes of a smooth drum vibratory roller with a static weight of at least 1 ton. Any areas which become unstable,, as evident by "pumping or weaving," of the undercut or under the passing roller, should be further undercut and backfilled with well compacted Imported Structural Fill. The Imported Structural Fill should consist of a run -of -bank sand or sand gravel blend which conforms to the gradation requirements listed in Table 1. Table 1 Imported Structural Fill Structural Fill should be placed in maximum loose lift thicknesses of 12 inches and compacted to a dry density equal to at least 95 percent of the material's maximum dry density as it is defined by ASTM Designation D-1557, "Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Modified Effort". -5- C.T. MALE ASSOCIATES Within the anticipated depths of foundation excavation, the overburden soils may be considered to be "Type C" soils as defined by the Occupational Safety and Hazard Administration (OSHA) excavation standards. According to these guidelines, Type C soils require excavation slopes no steeper than 1.5 horizontal to 1 vertical (1.5H:1V). In the event groundwater is found to seep into excavations, the excavation side slopes should be laid back to inclinations flatter than what is recommended above. Temporary excavation support, if required to limit areas required for excavation slopes, should be designed and stamped by a New York State licensed professional engineer. The geotechnical engineer of record should be afforded the opportunity to review such designs. The foundations used to support the proposed structure should bear on proofrolled and compacted native site soils or on compacted Imported Structural Fill placed over the same. The foundations should be seated at least 4 feet below the lowest adjacent exterior grade to afford their frost protection. Along the sides of the structure running down the slope, the foundation bearing grades should be incrementally stepped down to provide such embedment/ frost protection. Foundations may be proportioned for a maximum net allowable bearing pressure of 1,500 pounds per square foot. The maximum allowable edge pressure for foundations that function as retaining walls may be increased to 2,000 pounds per square foot. So constructed, foundations are not expected to experience a total settlement greater than one-half of an inch.. X, WZ; Geofoam blocks, EPS Type 19, such as Shelterfoam Geofoam 19, are recommended for use. The blocks may be placed directly on the existing slope after it has been cleared of trees,, vegetation and topsoil, and the foundation walls of the structure constructed. The blocks should be placed in a stepped pattern against the excavated slope. The Geofoam should be separated from its cover material through the placement of a geomembrane, such as Seaman XR-5 8130, to prevent inadvertent contact with petroleum -based contamination, which may compromise the structural integrity of the material. The geomembrane should be placed over the top layer of Geofoam and fastened to the sounding foundation walls. Geofoam blocks should be placed to fit together tightly with no vertical discontinuities and in a "running bond" pattern. The joints between Geofoam should be staggered to eliminate continuity of vertical joints between layers of the Geofoam. If field alteration of Geofoam blocks is required during construction,, they should be trimmed using a hot wire cutting device. A minimum of four manufacturer - approved barbed connection plates should be placed between full-size Geofoam blocks during placement. No equipment should travel on Geofoam block prior to placement of the floor slab subbase above the blocks. The geotechnical engineer should be afforded the opportunity to review structural drawings and Geofoam manufacturer submittal with respect to Geofoam placement and details. 6- C.T. MALE ASSOCIATES The parameters listed in Table 2 are recommended for use in designing the below grade foundation walls to support the structure. At -rest earth pressures are provided assuming the walls will be braced prior to their backfilling. Table 2 Design Parameters for Foundation Walls Des ign Parameter alue Moist Unit Weight of Soil Backfill 120 pcf Friction Angle of Soil Backfill 32" Earth Pressure Coefficients for Soil Backfill * Active with 2H:1V Backfill 0.46 * At -Rest with Level Backfill 0.47 * At -Rest with Sloping 2H:1V Grade behind Downhill Wall 0.68 * Passive with Sloping 2H:1V Grade in front of Downhill Wall 2.00 Coefficient of Sliding Friction, (C.I.P. Concrete on Native Soil) 0.35 L_ Minimum Factor of Safety Against Sliding and Overturning 1.50 Light compaction equipment such as walk -behind vibratory roller should be used within 3 feet of foundation walls. Geofoam,, if placed to the extents described in this report, will not induce lateral pressure on the foundation walls. Surcharge loads from the floor load within the garage will also not induce lateral loads on the walls within Geofoam depth. 6.4 Garage Floor Slab The floor slab should be at least 4 inches thick and provided a subbase of at least 6 inches of crusher -run stone, a NYSDOT Type 2 Subbase material. This subbase material should be placed over the Imported Structural Fill and geomembrane placed over the Geofoam blocks. The top of the Geofoam blocks should be at least 2 feet below the finished floor surface. Both the Imported Structural Fill and Type 2 Subbase may be placed as single lifts and compacted to the aforementioned 95 percent density requirement. So constructed, the slab may be designed assuming a modulus of subgrade reaction of 200 pounds per cubic inch. -7- C.T. MALE ASSOCIATES 7.0 CLOSURE This report has been prepared to assist in the design and construction of the proposed structure to be located at 5 Tomahawk Lane, Saratoga Springs, New York. The recommendations have been developed from our interpretation of the project site's subsurface conditions disclosed through subsurface explorations and our understanding of the basis of design as it was described herein. No other warranties, expressed or implied, are made. Respectfully Submitted, C.T. MALE ASSOCIATES John Scheetz, P.E. Geotechnical Engineer j.scheetz@ctmale.com Joshua Blake, E.I.T. Geotechnical Engineer j.blake@ctmale.com