Lessons from below

Subsurface construction presents a unique and challenging risk. Owners often consider it a temporary expense that takes place out of sight and adds little value to a development. For example, when looking at the Burj Khalifa in Dubai, few people would marvel at the shoring walls for the foundation of what is presently considered the tallest building in the world.

All subsurface construction, whether a basement, underground parking facility or utility trench, requires the completion of an excavation. Although routine, even minor excavation operations carry inherent risk. When a failure does occur, the financial costs are significant, with remediation costs alone often exceeding the million dollar mark.

This article is an introduction to shoring walls, and the mechanical support systems used to reinforce the walls of large excavations. These subsurface support systems are both essential to safe and successful excavation and vulnerable to numerous failure scenarios. Hopefully, this will remove some of the mystery surrounding the world below the surface.

Shoring Walls

Shoring walls are essential in providing support to the walls of an excavation and preventing excavation collapse. Excavation and shoring wall installation can be difficult even in competent soils due to the natural variability and complexity of soils. The risks are increased when excavating in ground containing soft, highly compressible clays, a high groundwater table or when excavating close to adjacent buildings/structures and utility or transportation corridors. Although these constraints can be accounted for through proper shoring wall design and construction, failures can still occur as a result of human error.

Depending on the amount of deflection tolerated by shoring walls, they are classified as either flexible or rigid. Flexible walls experience some deflection while rigid walls generally experience little to no deflection. The selection of a type of wall will depend on the ground conditions and the project requirements. Examples of flexible retaining walls include steel sheet pile walls with post-tensioned tieback anchors (Photograph 1), and soldier beam walls with wood lagging and anchors (Photograph 2). More rigid retaining walls include reinforced concrete slurry walls and contiguous caisson walls, with additional support from bracing or tieback anchors.

Lateral Earth Pressure

All retaining walls, whether flexible or rigid, are designed to resist various applied pressures without experiencing excessive deformation or catastrophic failure. Retaining walls experience earth pressure and may also experience hydrostatic (water) pressure, seepage pressure (if there is groundwater flow behind the wall), surcharge pressure (due to surface load acting at the top of the excavation), frost pressure during sub-zero environments, and earthquake pressure in seismically active zones.

Depending on the type of retaining wall and the soil conditions, the lateral earth pressure on a wall can vary significantly. Various pressure distributions have been developed for different walls and soil conditions. Rigid walls, such as concrete walls, are commonly designed for a triangular lateral earth pressure distribution, wherein the lateral earth pressure on the wall increases linearly with depth. Flexible retaining walls, like temporarily supported braced walls, are commonly designed for trapezoidal distributions, and termed ‘apparent lateral earth pressure distributions.

The pressure distribution on a wall is determined using published formulations, which have been developed in part through pressure measurements on actual walls. When the appropriate formulation is selected and correctly applied, it should provide a conservative estimation of the pressure applied to a retaining wall. Correct application of these formulations is essential, as errors may lead to the underestimation of the actual lateral earth pressure on the wall, which can result in the wall being critically under-designed.

Giffin Koerth has investigated several shoring wall collapses wherein the lateral earth and water pressure was underestimated (Figure 1). In Figure 1, three pressure distributions are shown, with each pressure distribution normalized to represent a percentage of what should have been the design pressure. For these three distributions, the average pressure initially considered in the design represented between about 50% and 60% of the actual pressure that should have been considered.

Frost Pressure

The Canadian climate presents unique challenges to subsurface construction. In most parts of the country, winter temperatures result in freezing of the near-surface ground for several months each year. This ground freezing results in volumetric expansion of the moisture within the soil, which can apply pressure to adjacent structures, like shoring walls. These pressures can be significant, measuring several times greater than earth and hydrostatic (water) pressures applied to the wall.

Mitigation measures for a shallow retaining wall may include backfilling the wall with granular fill, a non-frost susceptible backfill, and ensuring sufficient drainage and surface barriers to isolate the wall and backfill from sources of water and moisture. For temporary rigid or flexible retaining walls, measures could involve dewatering to lower the water table and lessen the moisture content of the soil, in combination with applying insulation to the face of the wall, and heating the face of the wall with propane heaters during the subzero winter months.

Frost-protection measures are often recommended by geotechnical engineers, but sometimes are not implemented in the field due to cost and schedule impacts during construction. However, the cost to insulate and heat a temporary rigid or flexible retaining wall could represent a fraction of the remediation cost should a failure occur due to the development of excessive frost pressures.

Installation Errors and Construction Deficiencies

Even if a shoring wall is properly designed, and frost-protection measures are implemented, a failure may still occur if the wall is improperly installed or if construction practices do not take into consideration the seriousness of maintaining the integrity of the excavation.

Issues often occur when bracing is removed to permit foundation wall construction as the building construction advances upward. While removal of bracing is typical, premature removal of bracing can lead to lateral deformation of the shoring wall. This can damage adjacent buildings, as soil behind the wall upon which the buildings are founded shifts downward and laterally.

Additionally, many municipalities require that tiebacks be de-stressed (load released by severing) as construction within an excavation progresses upward. This requirement is intended to eliminate safety concerns for workers who may be excavating in the future and potential adverse impacts to existing infrastructure, such as gas pipelines or underground fiber optic lines. However, like bracing removal, destressing may impact adjacent structures, as it allows for the relaxation of the shoring wall and the movement of the retained soil behind the wall.

Due to the lateral wall deformation that can result from tieback de-stressing and bracing removal, it is important that such practices be done in a progressive and controlled fashion. Condition assessments of adjacent buildings should always be completed prior to any excavation or construction activity, as the possibility will always exist for this activity to damage adjacent buildings.

Conclusion

Excavations are challenging due to the natural variability and complex nature of soil. However, the risk of failure and collateral damage can be greater in areas where soft, highly compressible clays are present, the groundwater table is high or when excavating near buildings/structures and utility or transportation corridor
s. Under these circumstances, it is particularly important that proper design is executed and sound construction methods are followed.

If properly designed and installed, a shoring wall should provide continuous support to the walls of an excavation with minimal wall deflection and adjacent ground surface settlement. However, when a failure does occur, the result is often catastrophic, with the potential for personal injury, significant financial costs associated with remediation, and possible litigation.

Geoff Lay is a technical engineering associate with Giffin Koerth.