Category Archives: Secant Pile Wall

Considering Movement of Excavation Shoring Systems

Considering Movement of Excavation Shoring Systems

Are you shoring to limit excavation into a tree protection area?  Are you shoring to excavate near an historic masonry church?  Of course, we would all be more concerned about the shoring system chosen at the church, but which system?  The first step is to choose the right system.  Soil nail walls move 0.1% to 0.4% of the height of the wall or ¼” to a 1” for a 20-ft cut.  Anchored piles will move less as anchors are tensioned for the load the wall will encounter by the time you excavate to the bottom of the cut.  For projects where movements are especially critical, shoring walls that eliminate shotcrete lifts or wood lagging such as a secant pile wall may be considered.

But it is still difficult to assess the amount of movement that may occur as construction methods may affect movement more than design.  What is the height of each lift?  Is each lift installed quickly after excavation?  Are the lagging boards placed tight to the soil?  Did a large storm pass halting construction for days? Construction methods have a large effect on movement.

Most shoring systems are designed to a minimum factor of safety.  Permanent retaining wall structures are often designed with a factor of safety of 1.5 and temporary structures with a factor of safety of 1.35.  Higher factors of safety result in a wall system under less stress and therefore less strain or movement.  Sometimes walls are designed to performance requirements.  This requires the designer to model the excavation and shoring system using a finite element analysis software such as Plaxis to estimate movement.


Finite Element Model of Anchored Secant Pile Wall Near a Heavy Building

Whether designed using a factor of safety or performance method, measuring movement as the excavation progresses will alert the contractor to any concerns prior to reaching the bottom of the excavation.  One effective way to measure is using an inclinometer.  Inclinometers track the movement (angle changes) from the bottom to the top of a casing over time.   The casing can be installed in or behind the shoring wall. Contractors can monitor the movement as the excavation and wall installation progress.  Figure 2 shows that the top of the wall leaned out after the first 5-ft cut, then the top was pushed back by the anchor, and as expected the largest movement outward occurred between the anchor and the bottom of the excavation (in this case less than 1/8”).


Wall Movements as Excavation Progresses

Ultimately the successful performance of a retaining wall system depends on selection of the appropriate retaining wall system, competent design, high quality construction techniques,  and monitoring of the progress.

 

Virginia War Memorial Shoring Support

VA War Memorial featured in Foundation Drilling Magazine

Subsurface Construction is honored to have the Virginia War Memorial as a Technical Featured article in Foundation Drilling Magazine July 2018, a publication of the Association of Drilled Shaft Contractors (ADSC). Subsurface designed and constructed a permanent anchored secant pile wall and a temporary soil nail wall for the expansion project.

For the secant pile wall, Subsurface teamed up with the experts at Equipment Corporation of America (ECA) to supplement its own experience with this construction method. From the initial bidding process to the wall’s construction, Subsurface worked closely with ECA’s Gordian Ulrich and Jeff Harmston to assess the appropriate drill rig and tooling needs. Casing was required due to the soft ground, presence of groundwater, and accuracy of the alignment of the piles. Subsurface used a Bauer BG 20 rig with 35-inch sectional casing and an earth auger for secant pile installation. The process alternated between 1-2 days of primary pile construction and 1-2 days of secondary pile construction spaced at 6 ft. on center. The construction of the secant pile wall as a success and was completed two weeks ahead of schedule.

For the temporary shoring, Subsurface designed and installed a hybrid pipe pile and conventional shotcrete soil nail wall had an overall retained height of approximately 35 ft. A primary challenge with design and installation was coordinating the location of soil nails (+/-45 ft. long) to miss existing utilities and soil nails on the opposing side of an outside corner. Subsurface was concerned that despite engineering the nails to miss, slight drilling deviations would inevitably cause damage to previously installed nails. To mitigate this risk, significant effort was spent on the layout of each
soil nail while continuously monitoring during drilling to ensure the original alignment was maintained. In addition, the shotcrete facing was not applied until all drilling within the zone of opposing nails was complete for each lift.

To learn more about the installation, project challenges and modeling/measuring of deflections please read more:

Foundation Drilling Magazine
Duke University Bed Tower Secant Wall

Secant Pile Wall

Subsurface completed a permanent secant pile wall at the VA War Memorial.  The wall serves as a permanent basement wall and supports vertical load from the structure.