Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
When to Consider Anchors or Braces for Lateral Support of Deep Foundations
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Soldier Rev B
| 6.7.4 When to Consider Anchors or Braces for Lateral Support of Deep Foundations Except for retaining wall and slope stabilization designs that are based on failure of the soil, the design of laterally loaded deep foundations is based on empirical soil-structure interaction that relies on limiting displacement as a design criterion for both Strength and Service Limit States. For conventional bridge or wall foundations, the size of the foundation element (diameter or width) can be increased, the length increased, and/or the number of foundation elements within a foundation increased until the limiting deflection criterion is met. However, there are some applications, in particular non-gravity cantilevered walls (soldier pile and lagging walls, where these alternatives may not be able to reduce the deflections sufficiently or may not be the most cost effective approach to meeting the design criterion. 90 Cantilevered soldier pile and lagging walls typically consist of a single row of foundation elements. Adding a second row of foundations and a cap to connect the two rows significantly impacts the design, constructability and cost of the wall and effectively changes the wall type to a conventional retaining wall on piles typically, this type of construction is not feasible where a non-gravity cantilevered wall is required. Reducing the spacing between the soldier piles has a limited ability to increase the capacity of the wall because the zones of influence will overlap more, thus reducing the amount of passive soil resistance available to each pile (although closer pile spacing will increase the structural stiffness of the wall system. Increasing the size of the soldier pile, either the pile itself or the diameter of a pre-augered hole that it is set in, also reaches a point of diminishing return because of the increasing cost of each element (and therefore the cost per unit length of the wall) begins to increase significantly. In these situations, anchors or external braces may offer a feasible and cost effective alternative to limit the deflections of the foundation. Such anchors or braces provide a second point of support for the walls, in addition to the passive resistance on the foundation elements, and therefore reduce the amount of load that is to be resisted along the embedded portion of the soldier piles. Typically, external anchors or braces are needed for cantilevered walls that are greater than 8 to 15 feet in height, depending on the external loads due to soil conditions and surcharges, the soldier pile type and size, the available passive resistance, and the magnitude of the limiting deflections. The design of anchored, tied-back, or braced walls is beyond the scope of this manual. Refer to the LRFD Bridge Design Specifications by AASHTO (2014) for detailed design guidance of the design of anchored or braced walls in the LRFD design platform. Download 6.03 Mb. Share with your friends:
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