Shotcrete Shoring
/By now you have probably seen the partial collapse of the shotcrete shoring wall at an excavation site in Coquitlam, BC. If you haven’t, check it out here.
Unless you are active in civil construction or building excavations, you may have no idea what this type of shoring is or what shoring even means. Shoring is a way of securing material – temporarily or permanently – that is weak, unstable, or susceptible to movement from falling or collapsing down. Remember when you were a kid digging in the sand at the beach, and the sand kept collapsing back in? Shoring would prevent this. In construction, this allows project sites to go deeper by providing a safe access for workers to an excavation while maximizing building footprint by protecting the neighbouring structures from loss of supporting soils. Shotcrete shoring, specifically, is a versatile shoring system combining soil anchors and shotcrete that is commonly used in building excavations (especially common in the lower mainland of BC) but is also used in rock and slope stabilization projects around Canada. In this blog we will offer insight into what a shotcrete shoring system is and what situations it is suited for.
Understanding Shotcrete and Soil Anchors
Shotcrete is “a method of applying concrete projected at high velocity primarily onto a vertical or overhead surface. The impact created by the application consolidates the concrete. Although the hardened properties of shotcrete are similar to those of conventional cast-in-place concrete, the nature of the placement process results in an excellent bond with most substrates, and rapid or instant capabilities, particularly on complex forms or shapes.” (Source) (See my blog about concrete here).
Soil anchors (also referred to as tiebacks in a shotcrete shoring system) are a structural element designed to anchor and stabilize the wall. Soil anchors consist of a tendon which is either a steel bar or steel strands that is drilled and grouted into the soil, and a bearing plate and anchor head that facilitates testing and stressing of the anchor. The anchor lengths and angles need to be designed by a specialized engineer based on soil conditions, structures and equipment above, underground services, and property limits/agreements.
Together, these two products forge a robust shoring solution that is versatile and stands strong against the challenges of excavations.
When and When Not to Use Shotcrete Shoring
Shotcrete shoring is an excellent choice for loose and unstable soils (which is common in the Lower Mainland) that need to be kept in a vertical or steep fashion. Additionally, if time and costs are priorities a shotcrete shoring system can be used as your exterior wall with a blindside waterproofing system. For these two reasons, it is commonly used on tight city lots in the Lower Mainland. Further, it is a great option for rocky formations with irregular surfaces or for slope stabilization where there may be future concern of a landslide.
One solution does not fit all and when highly saturated or waterlogged soils are discovered, other shoring systems should be considered. Additionally, in regions with expansive clays prone to shrink-swell cycles (like in Alberta), shotcrete does not perform well and secant piles or other shoring methods may perform better.
Benefits of Shotcrete Shoring
Speed, Efficiency and Cost: Offers a rapid and efficient shoring solution for excavation sites, reducing construction time without compromising on stability. As mentioned above, this system can double as the permanent wall for a below grade structure (like a parkade) by using a blindside waterproofing system. This approach eliminates the need for extensive formwork, saving on materials and labour costs.
Versatile/Flexibility in Design: Allows for flexible design options, accommodating the unique requirements of each shoring project.
Durability and Longevity: The durability of shotcrete, coupled with the anchoring strength of soil anchors, results in a robust shoring system that withstands the test of time.
Conclusion
Shotcrete shoring is versatile, fast, and durable shoring system with many benefits. It works well in a variety of soil and rock conditions but meets its match against clays and waterlogged soils. It is used safely all over the country without incidence. Regarding the Coquitlam incident, this is a strong reminder of the importance of QAQC as there are countless reasons why the shoring system could have failed. There could have been higher than expected water pressure, soil conditions that were different than designed for, incorrect concrete mix was specified or placed, anchors weren’t tested, or many more. Thankfully the site was safely evacuated, no one was hurt, and a follow up report will remind everyone about the importance of due diligence around shoring design and construction and will contribute to ongoing improvements in the industry. Despite the incident bringing shotcrete shoring to the forefront of construction conversations, it is a tried-and-true system used regularly with no issues.