Braced Excavations / Cross-Lot
- Oct 4, 2023
- 4 min read
Excavations with Internal Bracing: Information & Construction Sequence
Cross-lot bracing, also known as internal bracing, transfers lateral earth and groundwater pressures between opposing retaining walls through compression struts. Raker braces bearing on a foundation mat or competent rock provide an alternative form of internal bracing.
The struts are typically fabricated from steel pipe sections or wide-flange (I-beam) sections and are commonly preloaded to create a stiff support system and minimize wall movements.
Installation of the bracing system is generally carried out by excavating locally around each strut location and proceeding with further excavation only after the struts have been installed and preloaded. A typical excavation sequence for a cross-lot braced excavation is illustrated in Figure 1.
The struts bear against wale beams, which distribute the strut forces along the diaphragm wall and transfer the loads to the retaining system.
Preloading establishes firm contact between all interacting structural elements and reduces subsequent deformation of the support system. This is typically achieved by placing hydraulic jacks at both ends of an individual pipe strut, between the wale beam and a specially fabricated jacking plate welded to the strut (Xanthakos, 1994).
Strut loads can be monitored directly using strain gauges or estimated from elastic deformation measurements by determining the increase in separation between the wale beam and the strut. Figure 2 illustrates the basic arrangements for the wedging and telescopic preloading methods.
In some early excavation projects, struts were installed without preloading. As excavation progressed to greater depths, significant wall and ground movements were observed as a result of the initial lack of stiffness in the support system (C1).
Consequently, preloading of struts has become standard practice in modern braced excavations to reduce wall deflections, limit ground movements, and improve overall excavation performance.
DeepEX Software can design and optimize any braced excavation in Minutes!
Cross-lot bracing is particularly well suited for relatively narrow excavations, typically ranging from 60 ft to 120 ft (18 m to 36 m) in width, where the installation of tiebacks is impractical or prohibited due to property line restrictions, underground utilities, or adjacent structures. As excavation widths increase, the struts become longer and may experience significant flexural deflections under their own self-weight, making their use less economical and less efficient.
In addition, the design of cross-lot bracing systems must account for thermal expansion and contraction of the struts, as temperature variations can induce significant changes in axial forces and affect overall system performance.
Typical strut spacing is approximately 15 ft (4.5 m) in both the vertical and horizontal directions, although the exact spacing depends on project-specific loading conditions and wall stiffness requirements. This spacing is generally larger than that used for tieback systems because struts are commonly preloaded to substantially higher load levels, resulting in a stiffer support system and improved control of wall movements.
A significant advantage of cross-lot bracing is that it eliminates the need for tieback penetrations through the diaphragm wall. As a result, one potential source of groundwater leakage is removed, simplifying waterproofing considerations and improving the overall watertightness of the excavation support system.
Figure 1: Typical excavation sequence in cross-lot excavations: (A) V-cut initial cantilever excavation, (B) Strut installation and pre-loading in small trenches in soil berms, (C) V-cut excavation to next level and strut installation, (B) Final grade.
Figure 2: (a) preloading arrangement, and (b) measured brace stiffness (Xanthakos, 1994)
Figure 3: Methods of preloading struts; Wedging (top), Telescoping pipe (bottom)
Figure 4 Cross-lot supported excavation NYU Medical Center, New York City
Training Videos - Diaphragm Walls Braced with Struts - 2D and 3D Model Analysis with DeepEX:
Summary
Cross-lot bracing is a widely used support system for deep excavations, particularly in urban environments where tieback installation is restricted or impractical. By transferring lateral earth and groundwater pressures between opposing retaining walls through compression struts, cross-lot bracing provides an effective means of controlling wall deflections and maintaining excavation stability.
The performance of a strut-supported excavation depends not only on the structural capacity of the struts and wale beams but also on proper installation and preloading procedures. Preloading minimizes system deformations by establishing firm contact between structural components before significant excavation-induced movements occur. Experience from numerous projects has demonstrated that inadequate or absent preloading can lead to excessive wall movements and increased ground settlements.
Although cross-lot bracing offers advantages such as high system stiffness and the elimination of tieback penetrations through diaphragm walls, its application is generally limited to relatively narrow excavations. The design must also account for practical considerations such as strut buckling, self-weight deflections, thermal effects, construction sequencing, and interference with excavation operations.
When properly designed, installed, and monitored, cross-lot bracing systems provide a reliable and economical solution for supporting deep excavations in challenging urban and geotechnical conditions.
References
Xanthakos, P. P. (1994). Ground Anchors and Anchored Structures. John Wiley & Sons, New York.
Xanthakos, P. P. (1991). Slurry Walls as Structural Systems. McGraw-Hill, New York.
Peck, R. B., Hanson, W. E., & Thornburn, T. H. (1974). Foundation Engineering. 2nd Edition, John Wiley & Sons, New York.
Clough, G. W., & O'Rourke, T. D. (1990). "Construction Induced Movements of In Situ Walls." In Design and Performance of Earth Retaining Structures, ASCE Geotechnical Special Publication No. 25, pp. 439–470.
FHWA (1999). Geotechnical Engineering Circular No. 4: Ground Anchors and Anchored Systems (FHWA-IF-99-015). Federal Highway Administration, Washington, D.C.
FHWA (2003). Geotechnical Engineering Circular No. 5: Evaluation of Soil and Rock Properties (FHWA-IF-02-034). Federal Highway Administration, Washington, D.C.
Book A free web presentation:
