C+S September 2021 Vol. 7 Issue 9 (web)

Damage to an adjacent structure resulting from SOE system move - ment will often result in costs due to repairs, construction delays, project redesigns, temporary loss of use of the building, or a potential reduction in resale value. These costs can lead to disputes between the adjacent property owner and the Construction Team. To minimize the risk of painful disputes and unexpected costs, adjacent building owners and the Construction Team should align expectations through collaboratively determining an acceptable threshold for SOE system movement, movements of the adjacent structure, and risk before be - ginning the project. Determining an Acceptable Threshold for SOE System and Adjacent Building Movement The Construction Team and adjacent building owners can limit the risk of damage to the adjacent building by determining an acceptable threshold for SOE and adjacent building movement. The acceptable threshold should consider tolerance for risk, the contractor’s estimated budget for cost of repairs, the cost of increasing the stiffness of the SOE system to limit movement, and the occupied use of the building. We present below a general procedure to establish this threshold: 1. Review the original structural drawings for the adjacent building to deter- mine the type and elevation of the foundations, the type of building construc- tion, and the building configuration. Where drawings are not available, field investigations may be required to determine the aforementioned information and could include visual surveys, exploratory openings, or test pits. 2. Determine the soil types both for the soils retained by the SOE system and those supporting the adjacent building. 3. Determine the distance between adjacent building foundation elements and the excavation and the elevation of these elements relative to the bottom of excavation. 4. Use published relationships between soil type, adjacent building construc- tion, and height of the excavation. Consider the fragility of the structure when determining what types of distress may result from movement. 5. Select an acceptable damage category using the criterion described above. 6. Estimate the vertical and lateral movement of the SOE system that cor- responds to an angular distortion and lateral strain within the acceptable category of damage. 7. Perform analyses of the SOE system demonstrating that the system lim- its movements to the acceptable threshold. This often requires performing analytical sequential modeling of the excavation and bracing process us- ing commercially available software to predict the SOE movement in lieu of relying exclusively on the empirical charts developed by Clough, W. and T. O’Rourke (1990). This often includes consideration of construction means and methods being utilized by the Construction Team. The procedure for establishing an acceptable threshold is often it - erative and requires input from members of the Construction Team. The Construction Team and adjacent building owner can perform a cost-benefit analysis to aid in selecting an acceptable threshold for movement by comparing the cost to stiffen the SOE system to limit movements to different damage categories, the cost for repairs at dif - ferent damage categories, and the cost for losing functionality or resale value of the building due to damage or to facilitate repairs. While many adjacent building owners will prefer a negligible amount of damage to their building, selecting a desired damage limit is not trivial. Limit -

ing SOE and adjacent building movement to the amount required for negligible damage will often require very stiff systems, which while possible, are often cost prohibitive. On the other hand, the benefit of even costly modifications to SOE systems to limit adjacent building movements to an agreed upon damage category can outweigh the difficult-to-predict cost—and often contentious process—of repairing an adjacent structure. A few ways to modify an anchored SOE system to increase the stiffness include increasing the size of soldier piles or sheeting, decreasing the spacing between soldier piles, and increasing the number of tiebacks. Other options that we do not address herein are also available. Importance of Movement Monitoring during Construction Prior to the start of construction, the adjacent property owner will often engage a consultant to perform a pre-construction survey of the adjacent property. The intent of the survey is to establish baseline conditions to allow for comparison during and after the construction activities. Additionally, SOE designers or a monitoring consultant will often develop a monitoring plan prior to the start of construction to record and measure movements of the SOE system and adjacent structures. This allows the team to determine whether the SOE system is performing as predicted and whether it approaches or exceeds the acceptable threshold of movement, which should be carefully selected based on the analysis described above. The monitoring plan should include a contingency plan for movement of the system and of the adjacent building or below-grade structures at defined percentages of the acceptable movement threshold. Example contingencies include increasing the monitoring frequency, installing additional tiebacks, installing raker braces, backfilling portions of the excavation, and un - derpinning adjacent structures. Closing Remarks Construction in urban areas frequently requires deep excavations adja - cent to existing structures. SOE systems are designed and constructed to facilitate these excavations, and movement of these SOE systems can result in damage to adjacent structures. SOE designers frequently assume a range of movement of the systems they design based on prior experience. These movement ranges are often large enough to result in damage to adjacent structures. Repairing the damage and resolving dis - putes can be costly, time consuming, and frustrating for the Construc - tion Team and adjacent building owner. The Construction Team and adjacent building owner can use an informed approach to manage risk by selecting an acceptable threshold for SOE movement that considers the Construction Team’s tolerance for risk, estimated budget for cost of repairs, the cost of stiffening the SOE system to limit movements, and the occupied use of the adjacent building, all in collaboration with the adjacent property owner. The authors have successfully applied this approach at multiple building sites. T.J. UVEGES, P.E. is Consulting Engineer at Simpson Gumpertz & Heger Inc. They can be contacted at tjuveges@sgh.com. R.S. SILVESTER, P.E., is Principal at Simpson Gumpertz & Heger Inc. They can be contacted at rssilvester@sgh.com. B.P. STROHMAN, P.E., G.E., P.ENG is Associate Principal at Simpson Gumpertz & Heger Inc. They can be contacted at bpstrohman@sgh.com.



september 2021

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