By T.J. Uveges, P.E., R.S. Silvester, P.E. , and B.P. Strohman, P.E., G.E., P.Eng
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 construction 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 considered 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.
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 surveyor, 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 consultants 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 behind 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 result in movement of the adjacent soils within the zone of influence of the excavation. While the soil type and strength determine the zone of influence, it is generally defined within a lateral distance of up to three times the depth of the excavation away from the face of the excavation (Figure 1). Similar charts are available for lateral movements of the SOE system.
Where portions of buildings or below-grade structures are supported on soils within the zone of influence, such as slabs-on-grade and shallow foundations, these structures are susceptible to differential vertical and horizontal movements. The relative movement of structures can be quantified using the following two terms:
- Angular Distortion: Sometimes referred to as relative rotation, this is a measure of the shearing distortion and is often approximated as the rotation, due to differential settlement, of the straight line joining two reference points on the structure, such as the foundation elements.
- Horizontal Strain: Sometimes referred to as lateral distortion, this is a measure of the relative horizontal movement of two reference points on the structure.
Angular distortion and horizontal strain can cause damage to structures, and different types of building construction are inherently more susceptible to distress. Less flexible systems like masonry bearing wall buildings are such an example. Damage can range from cosmetric distress to structural distress to loss of building functions. Examples of damage include cracks in walls, slabs, or finishes, racking of doors or windows, and loss of weathertightness.
Two notable research papers (References 1 and 2) compare the relationships between soil type, distance from the face of the SOE system, and depth of excavation to damage to adjacent structures. These papers summarize predictions for vertical and lateral movements for various soil types based on the depth of excavation and distance from the earth support system. They also predict the building damage severity based on soil type, the amount of angular distortion, and amount of lateral strain of a building or below-grade structure during SOE movement.
Boscardin and Cording (1989) provide a visible damage classification system that relates severity, damage type, and approximate crack widths. The visible damage classification system is characterized into the following categories based on the ease of repair of the damage: negligible, very slight, slight, moderate, severe, and very severe. The general required repairs for each classification and crack width magnitudes are based on analysis of empirical measurements and observations.
- Negligible – “Hairline cracks.” Approximate crack widths are less than 0.1 mm.
- Very Slight – “Fine cracks easily treated during normal redecoration. Perhaps isolated light fracture in building. Cracks in exterior brickwork visible upon close inspection.” Approximate crack widths are less than 1 mm.
- Slight – “Cracks easily filled. Re-decoration probably required. Several slight fractures inside building. Exterior cracks visible, some repointing may be required for weathertightness. Doors and windows may stick slightly.” The approximate crack width is less than 5 mm.
- Moderate – “Cracks may require cutting out and patching. Recurrent cracks can be masked by suitable linings. Tuck-pointing and possible replacement of a small amount of exterior brickwork may be required. Doors and windows sticking. Utility service may be interrupted. Weathertightness often impaired.” The approximate crack widths are 5 mm to 15 mm or several cracks greater than 3 mm.
- Severe – “Extensive repair involving removal replacement of sections of walls, especially over doors and windows required. Windows and door frames distorted, floor slopes noticeably. Walls lean or bulge noticeably, some loss of bearing in beams. Utility service disrupted.” The approximate crack widths are 15 mm to 25 mm, but the classification also depends on the number of cracks.
- Very Severe – “Major repair required involving partial or complete re-construction. Beams lose bearing, walls lean badly and require shoring. Windows broken by distortion. Danger of instability.” The approximate crack widths are usually greater than 25 mm, but the classification also depends on the number of cracks.
Further, Clough, W. and T. O’Rourke (1990) include charts that relate angular distortion and horizontal strain for each of the different visible damage categories and excavation soil type (Figure 2). Charts for both cohesive and cohesionless soils are available. These charts are based on analytical models and data collected over years for a variety of SOE systems, including cross- lot struts supporting sheet pile, soldier pile and lagging, and concrete diaphragm walls the authors consider to be of “average workmanship.” These charts enable an engineer to correlate a category of visible building damage, as described above, to quantifiable measures of building movement.
Damage to an adjacent structure resulting from SOE system movement 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 beginning 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:
- Review the original structural drawings for the adjacent building to determine the type and elevation of the foundations, the type of building construction, 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.
- Determine the soil types both for the soils retained by the SOE system and those supporting the adjacent building.
- Determine the distance between adjacent building foundation elements and the excavation and the elevation of these elements relative to the bottom of excavation.
- Use published relationships between soil type, adjacent building construction, and height of the excavation. Consider the fragility of the structure when determining what types of distress may result from movement.
- Select an acceptable damage category using the criterion described above.
- Estimate the vertical and lateral movement of the SOE system that corresponds to an angular distortion and lateral strain within the acceptable category of damage.
- Perform analyses of the SOE system demonstrating that the system limits movements to the acceptable threshold. This often requires performing analytical sequential modeling of the excavation and bracing process using 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 iterative 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 different 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. Limiting 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 underpinning adjacent structures.
Construction in urban areas frequently requires deep excavations adjacent 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 disputes can be costly, time consuming, and frustrating for the Construction 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.
- Boscardin, M., and E. Cording, “Building Response to Excavation-Induced Settlement,” Journal of Geotechnical Engineering, Vol. 155, No. 1, Jan. 1989.
- Clough, W., and T. O’Rourke, “Construction Induced Movements of In Situ Walls,” Proceedings of the American Society of Civil Engineers, Design and Performance of Earth Retaining Structures, Jun. 1990.
T.J. Uveges, P.E. is Consulting Engineer at Simpson Gumpertz & Heger Inc. They can be contacted at firstname.lastname@example.org.
R.S. Silvester, P.E., is Principal at Simpson Gumpertz & Heger Inc. They can be contacted at email@example.com.
B.P. Strohman, P.E., G.E., P.Eng is Associate Principal at Simpson Gumpertz & Heger Inc. They can be contacted at firstname.lastname@example.org.