A well-designed project must be evaluated with constructability and cost effectiveness in mind. Some design engineers have a keen eye for constructability and are sensitive to project costs. Their designs reflect this understanding. In the same manner, a limited understanding of the installation process may very well result in a design that drives up the project cost.
One of the more common value engineering issues on small- and medium-sized residential and commercial projects is site retaining walls that require temporary shoring. A prudent retaining wall designer should fully consider the site conditions, terrain, access, proximity to property lines, and required excavations. On such projects, the use of a conventional retaining wall may result in the need for temporary shoring, which can often be an unforeseen cost in the owner's budget.
Some owners may not realize that shoring is required even after they get a set of plans, collect bids, and award contracts. It is usually the contractor that ends up being the bearer of bad news as they consider the constructability of the final design. The owner is then left to weigh the cost and time impact of slot cutting or temporary shoring.
To avoid unforeseen shoring issues, owners can include an experienced shoring and excavation contractor in the design/development phase. Shoring contractors can help design engineers save time by looking at the constructability issues upfront rather than revising plans after permits are pulled and delaying the start of the project with redesign.
A conventional retaining wall with a "toe-out" or "heel under" footing and a shear key (if necessary) is the most common retaining wall design. Engineering firms typically have a database of acceptable conventional wall details and designs at their disposal. However, when the site conditions will not allow a layback of soil, and it appears that temporary shoring will be required, a practical designer should consider using a permanent pile wall.
Drilled piles function as the retaining wall footing and provide support for the installation of lagging that will act as shoring during construction. When you add the cost of the conventional wall to the cost of temporary shoring, a permanent pile wall with a structural facing may be the most economical solution.
Even if a layback of soil is possible, the additional time and cost of excavation and soil recompaction must be added to the cost of the conventional wall construction. Depending on the soil conditions, it may make sense to eliminate this grading work altogether with piles and lagging.
Another cost to consider is the possible removal of temporary shoring. Often this is required in the upper soil cap zone of 2 to 5 feet below grade. Sometimes complete shoring removal is required if it is installed in a public right-of-way, temporary easement, or in an environmentally restricted area. So the potential time and cost savings to eliminate some site grading work, temporary shoring work, and the potential costs to remove temporary structures and restore that area by encompassing those trades with one permanent structure all add to the attractiveness of the pile wall option.
In hillside applications or where retaining wall footings are near descending slopes, the pile wall approach may be the most economical way to satisfy the "distance to daylight" restriction. This setback requirement can be the governing factor when debating earth-retained solutions. Sometimes during the design process it becomes apparent that the footing and shear key grow in size such that the enormity of its construction will be impractical. Again, a pile footing may indeed be the solution. The piles eliminate the necessity for a deepened shear key common in conventional retaining wall footings.
Certainly there are additional costs to consider when designing a permanent pile retaining wall that will also be used for shoring during construction. You can't just stick a shotcrete wall facing over temporary shoring and call it a permanent structure. A permanent pile wall will certainly be more expensive than temporary shoring, but it is the combination of shoring and the conventional wall that must be considered when determining if the permanent pile wall is the better solution. So if you are comparing temporary shoring (pile and lagging) with a permanent pile wall, remember to consider the additional design constraints when designing the permanent system.
There was a time when designers who engineered permanent structures would exclude the temporary shoring from their design, often because their insurance policies excluded construction shoring from their coverage. In recent years, there has been an increase in structural engineers taking a stab at temporary shoring designs. But unless they are familiar with the many "rule of thumb" aspects of temporary shoring, the design can range from exceedingly conservative and costly to deficient and unsafe.
Because many approving jurisdictions do not review or issue permits for shoring, these designs are sent out to bid and may bust the project budget, or worse, endanger the public. If temporary shoring is unavoidable, it is important to hire the proper consultant or design-build contractor to engineer and perform this work. Specialty contractors and consulting firms that specialize in shoring design will usually result in a more practical, safe, and cost-effective excavation.
Dale Scheffler is founder and president of D.J. Scheffler Inc. (www.djscheffler.com), a foundation drilling and shoring contractor specializing in temporary and permanent earth retention systems. Scheffler has more than 30 years of experience in the industry, providing design-build services to both public- and private-sector clients. Marshall Mead, S.E., P.E., is a project engineer for D.J. Scheffler Inc. Mead has more than 15 years experience as a structural engineer.
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