Shoring is the action taken to retain soil so that it does not collapse. During excavations, the side walls of a pit may become unstable, causing a risk to people working in the pit. Foundations of surrounding buildings can even be compromised due to the unstable ground.
The Occupational Health and Safety Administration's (OSHA) requirements for shoring states that excavation deeper than 5-feet must have shoring implemented into its project. If an excavation is to be deeper than 20-feet, then the additional requirement of an engineered design for shoring is specified.
In trenchless projects, the main need for shoring is in the establishment of entry and exit pits for the trenchless operation. Entry pits contain the trenchless equipment, like micro tunnel boring machines MTBM, and segments of pipes or pipe casings for installation. The exit pit is normally smaller and is used to extract equipment from the end point of the installation.
The following are different types of shoring methods that can be used for trenchless projects:
Soil Nail Shoring
Soil nail shoring is a process whereby a horizontal or slightly angled hole is drilled into the side wall of the excavation. Steel bars are inserted into the holes and then grouted into place. Shotcrete or precast panels are used to seal the side wall face. The soil nails act as anchors holding the soil in place.
Sheet Pile Shoring
Sheet piles are inserted into the ground before an excavation begins. A sheet pile is a steel sheet with an interlocking edge so that each pile can be joined to the next one in sequence. They are sized to reach the specified depth.
Vibratory hammers or impact hammers are used to force the sheet piles into the ground. Their function is to support the side walls and also to prevent groundwater ingress into the excavation while it is in process.
Soldier Piles and Lagging
Soldier piles and lagging is a technique where vertical steel piles are inserted into the ground around the perimeter of the excavation before it begins. As the excavation proceeds, horizontal lagging sheets are installed behind the piles to act as a resistance barrier to the soil behind.
The weight of soil against the lagging is, thus transferred to the steel piles. If an excavation is very deep or the soil is very unstable, additional support by means of anchors or braces can be added to the structure to provide extra strength. Lagging sheets can be made of wood, steel or concrete.
Secant or Tangent Piles
Tangent or secant piles are columns constructed in the ground around the perimeter of an excavation in order to provide a sealed wall against the side wall of the excavation.
Secant piles overlap with each other while tangent piles fit alongside each other. A hole is drilled into the ground, re-enforcing (e.g. steel) is added if necessary and then concrete is poured to form the column. Secant or tangent piles create an effective barrier against water entering the excavation, eliminating the need for dewatering.
Anchoring is a similar process to soil nailing where holes are drilled through the shoring wall (e.g. Sheet piles) and into a stable, rocky zone of the ground.
During anchoring, a steel bar is inserted into the hole and grouted into position. An anchor head is threaded onto the exposed end of the steel bar and then threaded tightened to anchor the shoring wall securely in the ground.
When engineers design shoring walls for excavations, they must take into account the calculated soil pressure that the wall must resist as well as groundwater levels and potential water pressure. The type of soil is also significant in terms of its natural stability. The deeper the excavation, the more attention must be given to extra internal and external bracing of the basic shoring structure like rakers, struts and tiebacks. (Learn more about ground stabilization in "When Ground Improvement is Needed During Trenchless Rehabilitation.")
Rakers are angled supports that lean up from the base of the excavation against the side wall of the shoring structure. While rakers provide excellent support for shoring, they take up space inside the excavation and therefore are not suitable for many applications.
Tiebacks are installed by drilling through the lagging of a soldier pile wall and inserting a steel cable, which is concreted into position. The cable is then attached to a soldier pile and tensioned so that it is actively pulling the pile into the soil that is being retained.
Struts are connecting beams that provide support between opposite sides of an excavation. In other words, they are hydraulic braces or solid steel beams that use the retained soil force of the one side to hold up the other side of the excavation and vice versa.
Constructing shoring for trenchless projects is a critical component of the operation in order to ensure the safety of workers and the protection of equipment in the entry and exit pits. OSHA requirements must be met and engineering calculations must be carried out in order to ensure the right design for the specific application.