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Home My WebLink About20260708 269 Broadway Site Plan Review SDL Foundation & Groundwater NarrativeSPRING LINE DESIGN │ 73 troy road, east greenbush, ny 12061 │ telephone: 518.487.4755 │ www.springlinedesign.com FOUNDATION NARRATIVE 29 October 2026 269 Broadway Condominiums 269 Broadway, Saratoga Springs, NY 12866 (Tax Parcel 165.75-1-7) Purpose and Background. This narrative summarizes the foundation system proposed for 269 Broadway and its coordination with the geotechnical recommendations of Terracon Consultants – NY, Inc. (Terracon), in support of the Site Plan Review application. SLD served as structural engineer of record for a previous design on this parcel in 2022–2023 of a six-story steel- framed office building over two levels of below -grade parking, which was advanced through City building permit review but not constructed. The current project is a six -story cast-in-place concrete residential building over ground -floor retail, lobby and amenity space, with a single level of below -grade parking. The approach below carries forward the earlier design and review process, including the comments of the City’s designated review engineer and building code consultant, and is tai lored to the current, shallower program. Subsurface and Groundwater Conditions. Terracon’s 2022 geotechnical report, based on eight test borings completed on the parcel in 2019, characterizes the site as follows. Existing grades are highest along Broadway (approximately El. 298 to 3 02) and slope down to Hamilton Street (El. 293 to 295). Beneath 3 to 9 feet of urb an fill, the profile consists of very loose to loose native sand, very soft to soft lacustrine silt and clay extending to roughly 25 to 31 feet below grade, a thin layer of glaci al till, and limestone bedrock at approximately El. 264 to 269 (lightly weathe red, medium strong, Rock Quality Designation (RQD) 80%). Groundwater was observed 5 to 10 feet below grade (El. 289 to 292), and Terracon recommends a design groundwater elevation of 290 feet. The seismic Site Class is D. Proposed Foundation System. The lower parking slab is currently planned at approximately El. 279, about two feet higher than the 2022 scheme, placing the garage floor roughly 11 feet below the design groundwater level rather than the 15 to 20 feet of the previous two-level garage. The foundation consists of the following elements, each following a specific recommendation of the Terracon report. Mat foundation. The building will bear on a reinforced concrete mat foundation seated in the native silt and clay, over a minimum two-foot-thick stabilizing and drainage base of clean crushed stone (ASTM C33 No. 57) on a drainage geotextile, as recommended by Terracon. Th e mat will be designed for Terracon’s maximum net allowable bearing pressure of 1,500 psf and modulus of subgrade reaction of 100 pci. A mat distributes the column loads uniformly over the soft clay and limits differential settlement, and the weight of soil removed from the excavation offsets a portion of the applied building load. Terracon anticipates subgrade rebound during excavation and subsequent settlement under load each on the order of one inch; the mat thickness, bottom-of-mat elevation and settlement estimates will be finalized as building loads are developed and reviewed by Terracon. Subgrade preparation, observation and approval of bearing grades will be assured during construction following recommendations in the Terracon report. Secant pile wall. The building footprint extends to the property lines, so the excavation must be supported on all four sides with no room to lay back slopes or to install tiebacks beneath adjacent properties and streets. A drilled secant pile wall so cketed into bedrock is proposed at the perimeter of the garage and will serve three functions: temporary excavation support, groundwater cutoff, and permanent foundation wall. Terracon recommends a drilled -in system and advises against driven or vibrated systems because of the urban setting, the historic buildings in the area, and the difficulty of seating sheets in the glacial till and bedrock. Terracon’s report specifically anticipates that a secant wall socketed into bedrock will substantially dimi nish both dewatering demands and buoyancy considerations. Temporary and permanent wall conditions. With a single parking level, the retained height is materially less than the nearly 30 feet of the 2022 scheme, which required two tiers of internal bracing. The wall will be designed to cantilever from its ro ck socket for the temporary excavation conditio n; where the retained height is greatest, along Broadway, a single tier of internal bracing may be required and will be confirmed in design. No tiebacks or other elements extending beyond the property lines ar e proposed. The design team is also evaluating locating the garage car lifts along the Hamilton Street side, where grade is lower, and ramping the garage floor up toward Broadway to further reduce excavation depth and retained height. In the permanent SPRING LINE DESIGN │ 73 troy road, east greenbush, ny 12061 │ telephone: 518.487.4755 │ www.springlinedesign.com condition the top of the wall is braced by the first-floor structural slab and the base by the mat, and the wall will be designed for at-rest earth pressure using Terracon’s soil parameters, hydrostatic pressure to the design groundwater elevation, and surcharge from adjacent streets and buildings. Passive resistance in the thin overburden below the mat will be disregarded, and lateral resistance will be developed in the rock socket at Terracon’s recommended 400 psf per foot of socket. Watertightness. The below-grade structure will be designed as a watertight enclosure. The secant wall provides the perimeter cutoff, and the mat and perimeter walls will be designed as water-retaining concrete with waterstops at construction joints and waterproofing meeting Section 1805.3 of the Building Code of New York State, as requested in the City’s 2023 permit review. The waterproofing assembly at the secant wall (interior liner wall, membrane and/or crystalline admixtur e) will be detailed in design. No permanent drainage relief or pumping beneath the mat is proposed, so there will be no long -term discharge of groundwater to the City system. Buoyancy. The structure will be designed for uplift at the design groundwater elevation under the code load combinations. The primary resistance is the self-weight of the building: the cast-in-place concrete columns, two-way flat plate slabs and mat are substantially heavier than the steel-framed building previously designed. If required based on the uplift analysis, permanent rock anchors or tension micropiles socketed into bedrock will be provided beneath the mat, designed and tested to Terracon’s preliminary parameters (50 psi allowable bond, 10 -foot minimum bond length in sound rock, double corrosion protection, performance and proof testing per PTI), with the supplemental rock investigation at anchor locations that Terracon recommends. Dewatering and Protection of Adjacent Properties . Because the secant wall is socketed into rock and forms a cutoff, construction dewatering is limited to the water contained within the enclosed footprint and precipitation entering the excavation, using sumps, well points or relief wells as the contractor’s dewatering plan requires. The groundwater table outside the wall is expected to remain essentially unchanged, which is the principal safeguard against the drawdown -related settlement of adjacent buildings, streets and utilities raised in the 2022 review comments. The contractor’s dewatering plan, including sediment control and the point and rate of discharge, will be reviewed by the design team and coordinated with the City. Dewatering and excavation support will be performed by contractors with demonstrated experience in similar conditions. The temporary excavation support design will be prepared and stamped by a New York State licensed professional engineer and submitted for review by SLD and Terracon, including assumed soil parameters, lateral pressure diagrams, surcharge loads, deflection analyses and the construction monitoring program. Consistent with the recommendations of the City’s review engineer and of the Terracon report, Terracon will be retained through final design and construction to confirm that its recommendations are implemented, observe wall installation, evaluate the mat bearing grades and review the monitoring data. A pre-construction condition survey of adjacent structures and an instrumentation and monitoring program will be in place before any drilling or excavation begins, following the program Terracon prepared for the previous design: inclinometers embedded in bedrock, vibrating wire piezometers, crack monitors and structure monitoring points on adjacent buildings, and continuous vibration and noise monitoring with alert and action limits, real -time notification, and remedial action requirements if a limit is approached. Drilled installation generates far lower vibration than driven or vibrated systems; the monitoring program verifies vibrations in the field and provides the basis to stop work for evaluation if a threshold is exceeded. In summary, a mat foundation within a rock-socketed secant pile wall is the system Terracon recommends for this site and the system developed through City permit review for the previous design ; the current single-level garage reduces excavation depth, retained height and the extent of structure below groundwater relative to that design, and the concrete superstructure adds weight to resist buoyancy. Final elevations, wall design and anchor requirements will continue to be coordinated with Terracon as the design progresses. Very truly yours, Christopher D. Snyder Christopher D. Snyder, PE Principal Engineer | Partner SPRING LINE DESIGN Architecture + Engineering, LLP PE License No. 097832 Registration Expires: 2028-11-30