In urban settings, construction sites for new buildings are commonly within dense building zones, with adjacent structures close by. New buildings that feature below-grade spaces require excavations with support of excavation (SOE) systems to retain soils around the site's perimeter. To limit movement of the SOE system, and thereby minimize the potential for damage to adjacent structures, SOE designers have to select appropriate thresholds for lateral and vertical SOE movement. This paper discusses common SOE systems used in adjacent construc- tion projects, the risk of damage to adjacent structures from movement of SOE systems, and a methodology for determining a threshold of SOE movement. Support of Excavation Systems The following three common types of SOE systems are typically con- sidered for construction of new buildings with below-grade spaces: • Cantilevered systems, such as sheet piles, soldier piles with lagging, secant or tangent piles, or traditional underpinning. • Anchored systems, using tiebacks or deadman with sheet piles or soldier piles and lagging. • Strutted systems using cross-lot, corner, or raker braces. ADJACENT CONSTRUCTION PROJECTS – METHODOLOGY FOR ALLOWABLE SUPPORT OF EXCAVATION MOVEMENT By T.J. Uveges, P.E., R.S. Silvester, P.E. , and B.P. Strohman, P.E., G.E., P.Eng
Many relevant role players drive the decisions behind the design and implementation of an SOE system; we call them the Construction Team. This team typically comprises such players as the new building owner, developer, architect, structural engineer, geotechnical engineer, SOE designer, SOE provider/installer, preconstruction condition sur- veyor, and the monitoring consultant. Successful execution of such a project requires aligning expectations between the Construction Team and adjacent property owners, who will likely have their own consul- tants to help them understand the scope and potential complications of the work the Construction Team proposes. Engineers design SOE systems to withstand horizontal earth pressures from the retained soils and surcharges they support. As part of the design process, the engineer should perform both serviceability and strength checks of the system to determine that the system can withstand the design pressures and will have movement within acceptable limits. The engineer uses several parameters, which will vary across the site, to determine the lateral earth and surcharge pressures acting on the SOE system. Parameters include, but are not limited to, the soil properties, groundwater level, excavation depth, SOE geometry, and the surcharge type and location relative to the SOE system configuration. Some SOE designers assume the lateral movements of SOE systems as a percentage of the exposed SOE system wall height rather than using an analytical approach. For example, we have seen the lateral and vertical movements behind soldier pile and lagging walls assumed to be approximately 0.2 percent to 0.5 percent and approximately 0.15 percent to 0.5 percent of the SOE exposed wall height, respectively. Applying these approximations to a 40 ft deep excavation, the lateral and vertical movements of an SOE wall are expected to be about 1 to 2.4 in. and 0.7 to 2.4 in., respectively. These lateral movements often result in corresponding lateral and vertical movement of the soils be- hind them. Damage Criterion for Adjacent Structures Predicting and limiting movements of SOE systems is particularly crucial when nearby structures are supported by retained soils, which is common in urban environments. The movement of SOE systems can
Photo 1: Typical SOE System with Soldier Piles, Timber Lagging, Tiebacks, and Raker Braces.
Figure 1: Summary of Soil Settlements Behind Insitu SOE Walls
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september 2021
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