Authored by SK&A Sr. Principal Hakan Onel, PE, SE, and Project Manager Naina Bhanot, EIT, LEED AP.
Turning existing foundation systems into project value
Adaptive reuse projects often begin with a fundamental set of questions that shape the direction of the entire effort: What elements of the existing structure can be reused? What must be demolished? And what can be feasibly renovated to support the building’s next service life? Owners raise these questions, as the cost of remediation can in some cases exceed the cost of a new construction project.
Early involvement of a structural engineer is critical in these discussions. Engineering input during conceptual planning helps distinguish between assumed limitations and actual structural constraints. One of the most overlooked early decisions is whether the existing basement walls can be retained and repurposed for the new occupancy. When conditions allow, keeping these walls can result in substantial cost and schedule savings, reduced risk, and lower environmental impact while still meeting modern building code requirements.
This article outlines the construction methods and primary drivers behind retaining existing basement walls and highlights the key engineering considerations that guide this decision-making process.
ENGINEER’S TAKEAWAY
Retaining existing basement walls is not a compromise—it is a performance-based engineering decision.
When supported by thorough evaluation, material testing, and targeted strengthening, these systems can meet modern building codes while reducing risk, cost, and environmental impact.
Early structural engineering involvement is an effective driver of successful reuse outcomes.
From a sequencing standpoint, demolition begins with the complete removal of the superstructure before any work can occur below grade. Once the superstructure is fully demolished, the support-of-excavation (SOE) engineer can fully mobilize to the site to begin installing soldier piles around the perimeter. From there, the sequencing becomes equally rigid. The ground floor slab is demolished, followed by selective demolition of the existing foundation walls. As sections of the wall are removed, lagging and tiebacks (or internal rakers) are installed in increments, continuing downward level by level until final foundation depth is reached and all SOE elements are in place.
However, this sequence is inherently challenging from a constructability standpoint and the likelihood of encountering below-grade obstructions which interfere with the installation of new soldier piles and lagging systems is very high, rendering this an impractical approach that is rarely used in practice.
Key Decision Drivers
Retaining and reusing existing foundation walls presents a compelling alternative to full demolition and rebuild by transforming what could be treated as a demolition item into a structural and logistical advantage. Rather than defaulting to complete removal and reliance on temporary SOE systems, project teams can evaluate if the existing walls can be safely integrated into the permanent structure reducing demolition scope, minimizing temporary structures, and improving overall project efficiency.
- Existing Conditions: Thorough testing and inspection of the existing basement walls including thickness, material strength, signs of leakage and deterioration, cracking patterns, reinforcement, etc. is important to assess the service condition of the walls. A structurally sound wall provides evidence of adequately supporting the loads imposed during its service life and complements the analytical evaluation for the new programmed occupancy.
- Existing Design: A detailed review of the existing drawings and the proposed elevations of the new slabs is a critical item to evaluate the feasibility of reusing the existing basement walls. Based on engineering judgment, where the proposed program alters the elevations to within +/- 1’-0” of the existing slab elevations, the existing walls can structurally support the anticipated loads imposed. The new slab can be designed assuming a pinned condition at the wall supports and the forces can be adequately transferred through post-installed slab rebar dowels into the existing walls. Where slab elevations differ significantly and/or the loading conditions are revised, further investigations are often necessary.
- Cost and Schedule Efficiency: The costs associated with shoring-of-excavation, dewatering, temporary support structure, and various direct and indirect costs related to schedule can be significantly reduced by keeping the existing walls.
- Waterproofing: Architectural and Building Envelope Consulting teams need to evaluate the proposed scheme. The waterproofing strategies that complement this approach often include interior drainage, interior side waterproofing, and localized repairs.
Once it is determined that the existing walls can be adequately re-used for the new program, there are engineering considerations which impact the final design of the walls which can range from localized repairs or localized strengthening to a complete retrofitting phase, the cost of which needs to be evaluated.
ENGINEER’S TAKEAWAY
With proper evaluation of the existing conditions, cost and schedule, sequencing, and waterproofing approach, project teams can effectively limit demolition scope and meet economic and sustainability goals.
From Liability to Leverage: The Case of Retaining Existing Foundation Walls
Adaptive reuse of existing foundation walls is not a one‑size‑fits‑all solution. The appropriate strategy depends on a range of factors, including existing slab elevations, wall geometry, soil conditions, site constraints, and programmatic goals. The following case studies illustrate three distinct approaches that demonstrate how existing walls can be evaluated, supported, and integrated into permanent construction, each tailored to different structural and logistical conditions commonly encountered in redevelopment projects.
Maximizing Efficiency Through Wall Retention at Existing Slab Elevations
Figure 2: SOE Strategy at 400 Army Navy
The most straightforward and efficient opportunity for reusing existing foundation walls occurs when the adaptive reuse program maintains slab elevations that align with the existing structure or where variations remain within acceptable tolerances. In these scenarios, basement wall heights remain mostly unchanged, allowing the existing walls to function with minimal structural modification.
One effective strategy under these conditions, used on the 400 Army Navy project, is the installation of soldier piles on the interior of the building by drilling openings in the existing floor slabs. Outriggers and walers are then installed at the underside of the slab soffits, utilizing the existing structure as a working platform. Once the temporary SOE elements and tiebacks are installed and fully stressed to provide lateral support, the existing slabs can be safely and completely demolished.
In this configuration, the inherent capacity of the existing foundation wall significantly reduces reliance on the temporary SOE system. Rather than replacing a functioning structure with temporary structure, the existing wall actively participates in maintaining stability. New floor slabs can then be poured, with leave-outs at pile locations. These leave-outs may be infilled after the piles are removed or cut off, or, in some cases, the piles may remain permanently encased within the slab.
The new slabs are doweled into the existing foundation wall using drilled rebar dowels, integrating old and new construction into a continuous structural system. This approach not only reduces SOE costs and demolition scope, but also extends the service life of the existing walls by incorporating them into the permanent structure which aligns strongly with both economic and sustainability objectives.
Strategic Wall Modification to Unlock Additional Underground Space
Figure 3: SOE Strategy at 5500 Wisconsin Ave
Owners and architects frequently seek to increase usable square footage by lowering foundation walls to create new underground levels, while simultaneously adjusting the top-of-wall elevations to accommodate revised grading plans. When soil conditions, wall geometry, and existing construction allow, these objectives can be achieved without defaulting to a full temporary SOE system.
Under the right conditions, lateral support of the existing walls can be provided using tiebacks alone, installed at or near existing slab elevations. By leveraging existing slab locations that already provide access and restraint, this approach can eliminate the need for temporary SOE piles altogether.
This strategy is being implemented at 5500 Wisconsin Ave, where a hybrid system was developed combining underpinning piers and strategically placed tiebacks. The existing foundation wall, originally supported on caissons, will be laterally restrained with tiebacks to
permit slab demolition. In locations where the wall depth is increased, two levels of tiebacks will be installed to support the wall during excavation.
The existing caissons will then be demolished down to below the elevation of the new planned footing. New underpinning piers, poured using the shotcrete method, will be constructed and are designed to resist both vertical and lateral soil loads, with reinforcing doweled directly into the bottom of the existing wall. These dowels are designed to transfer the imposed loads at the interface between old and new construction.
A similar approach is applied at the top of wall elevations, where a new vertical wall extension is doweled into the existing wall and designed to resist lateral earth pressures associated with final grading. Because final grading operations occur only after the new ground floor slab is in place, no temporary SOE is required to support the completed foundation walls, resulting in a permanent solution with fewer temporary systems, reduced site congestion, and improved constructability.
Hybrid Internal Support Systems Under Site and Utility Constraints
Figure 4: SOE Strategy at Century Center
In some projects, physical site constraints, such as existing utilities, limit the feasibility of installing tiebacks at critical elevations. At the Century Center project, conflicts at the uppermost support level required an alternative approach to support increased cantilever lengths of the existing foundation wall once slabs are demolished.
To address these constraints, the proposed SOE system consists of installing soldier piles inside the building footprint, combined with outriggers and continuous walers to provide internal lateral support. This system is designed to resist the temporary cantilevered condition of the wall during demolition while working around existing utility conflicts.
A unique element on this project consists of a new concrete wall proposed along the interior face of the existing foundation wall for waterproofing concerns. The proposed SOE design intends for the soldier piles to be embedded within this new wall. To ensure continuity of reinforcement and load transfer to the concrete walls, horizontal reinforcing will be drilled through the webs of the soldier piles.
In the final configuration, the soldier piles will remain embedded and abandoned. This hybrid solution demonstrates how adaptive reuse strategies can be tailored to site‑specific constraints, turning limitations into opportunities for integrated, long‑term structural performance.
In conclusion, the decision to retain and reuse existing basement walls highlights an informed shift away from a demolition-first approach and towards a more resource-conscious structural design. The coordination and engagement across the full design team to discuss this design approach early in the project timeline is critical to the success of an adaptive reuse project. As demonstrated by various strategies, ranging from interior soldier piles to hybrid solutions leveraging the lateral stability of the existing walls, these methods can be adapted to site-specific constraints and performance requirements. When supported by thorough investigation, proper analysis, engineering judgement, and carefully planned construction sequencing, the existing basement walls can meet current building code requirements while significantly reducing the costs and risks associated with full demolition.
Beyond technical feasibility, this approach offers clear structural and constructability advantages. By maintaining and integrating existing basement walls into the final system, reliance on temporary systems and the potential for unforeseen conditions can be reduced. More broadly, this strategy reflects a disciplined structural approach that prioritizes performance, stability, and constructability. When properly executed, teams can deliver safe, code-compliant solutions and extend the service life of existing structural systems.
Want to bring this conversation to your team? SK&A offers a 1 LU|HSW AIA course focused on evaluating and strengthening existing buildings for adaptive reuse. Learn more.
