Shotcrete is pneumatically applied concrete and is an alternate means and method that offers cost savings in the construction of basement foundation walls compared to cast-in-place (CIP) concrete construction. CIP concrete construction requires two-sided forming to along the exterior and interior foundation wall. In comparison shotcrete construction only requires one-sided forming, which is also termed “back forming”.
Shotcrete offers even greater construction cost savings compared to CIP concrete for projects that also require temporary excavation shoring because the temporary excavation shoring system serves as the back form for shotcrete application of the permanent foundation wall. Temporary excavation shoring is required if the depth of excavation of the planned development relative to the proximity of existing structures, utilities, property or easement boundaries is less than the distance required to safely slope or bench the basement excavation. Sometimes shotcrete foundation construction is the only feasible method when there is limited clearance between the temporary excavation shoring system and planned foundation wall.
For projects that also require temporary excavation shoring, the following must be considered to properly assesses the construction cost savings of a shotcrete foundation system to ensure original design intent is preserved and constructability is optimized:
1. Deep foundations and pile caps:
a. If drilled shafts or piers are part of building foundation support system, the piers along the foundation wall perimeter must be offset from the face of the temporary excavation shoring system to provide minimum clearance and space required of equipment to install piers. The resulting offset in alignment between the column above and the pier below creates eccentricity within the pile cap which is commonly resolved by increasing the pile cap thickness/depth and increasing the steel reinforcement within the pile cap.
2. Wall & column footings: in order to take advantage of the temporary shoring as the back form for shotcrete application the foundation wall and/or column footings at the building perimeter must be ‘L’ type footings so that the outside face of perimeter wall and outside face of perimeter footings are in the same vertical plane.
3. Evaluate required offset clearance between temporary excavation shoring system and adjacent structures, subgrade utilities and property or easement boundaries and minimum clearance and space required of equipment to install temporary excavation shoring system.
4. Evaluate required offset clearance between temporary excavation shoring system and foundation wall:
a. Once alignment of temporary excavation shoring system is determined per step 3, evaluate space requirements of foundation wall drainage matt, insulation and foundation drain or any other component outside the planed foundation wall such as utility tie-ins, storm water detention vaults, grease traps or sand/oil separators.
5. Exterior perimeter foundation drain: must be either eliminated or placed beneath wall footings and pier caps along perimeter with consideration to tie-in detail of vertical drainage matt into the perimeter foundation drain. The drainage matt provides drainage of water that collects between temporary excavation shoring face and outside face of waterproofing membrane.
6. Waterproofing installation on temporary shoring surface (blindside application):
a. A waterproofing membrane is traditionally applied on the exterior face of basement foundations. There are a variety of waterproofing options available. Selection of the most appropriate waterproofing system must consider the means and methods of foundation wall construction and also the temporary excavation shoring material components. b. Waterproofing membrane manufactures require that surfaces upon which the waterproofing membrane is applied meet minimum acceptable standards in terms of smoothness and maximum gaps the material can bridge. These standards vary widely among products and manufactures.
ie. Waterproofing installation contractors will not install waterproofing on surfaces that do not conform to manufacturer’s guidelines for obvious liability reasons.
c. Waterproofing membrane must be durable and able to withstand shotcrete application.
d. Waterproofing attachment to temporary excavation shoring system:
ie. Fastener type must be adequate for temporary shoring material and conform to waterproof material manufacturer’s requirements.
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Our team has the resources and technical knowledge needed to install shotcrete safely and effectively. Interested in learning more about the benefits of working with shotcrete for your next concrete project? We are here to help answer any questions you may have and are happy to provide preconstruction service during the design phase of your project to ensure the economy shotcrete offers is fully realized.