Concrete contractors face many demanding and increasing requirements including construction schedules, stormwater runoff regulations, erosion control, health and safety, difficult environments, quality control, and site access.

One phase that is extremely important but not often acknowledged is installation of complex foundations to support structures. Foundations are broken into two categories: shallow and deep.

Shallow foundations include:

  • Grillage foundations
  • Inverted arch foundations
  • Raft foundations
  • Spread footing or open trench foundations
  • Stepped foundations

Deep foundations include:

  • Caisson foundations
  • Pile foundations
  • Well foundations

The method selected depends on soil conditions, groundwater table, dead loads (weight of total structure), and live loads (wind loads, earthquake loads, vehicle loads and others), but they all have two things in common: The surface must be excavated to a typical depth and the hole must remain open and safe during installation of rebar, anchor bolts, concrete placement and curing, and, in some cases, waterproofing. The excavation must be carefully planned and executed to protect crews, minimize disruption to other construction activities and the surrounding area, and maintain access to the jobsite.

There are three ways to stabilize the excavation so it stays open and safe: stair stepping (passive), shoring (mechanical), or some combination thereof. Most contractors prefer stair stepping because it’s less expensive, faster, and has minimal impact on other construction activities.

The contractor must ensure the slope doesn’t crumble and slide. Failure is a common risk when working in silt or clay, which tend to dry out from the sun or wind and crack. This might not be an issue for the actual stabilization of the excavation. However, if the rebar is already installed and the slope crumbles and soil falls around the rebar, the soil must be removed. This is a major setback if the entire repair needs to be disassembled. Rain can further destabilize an excavation.

Heading Disaster Off at the Pass

Because of the range of mud layers, groundwater table, and earthquake considerations, the San Francisco Bay area is particularly tricky to work in vis-à-vis foundation construction. Fortunately, Truebeck Construction in Redwood City, California, is used to dealing with these challenges.

A full-service general contractor that specializes in preconstruction, alternative delivery methods, progressive building, and innovative construction methods, the company strives to have contingency plans in place to correct issues immediately because at any moment conditions can change. To that end, it’s evaluated excavation stabilization methods to minimize delays due to factors such as weather, changing soil conditions, and construction sequence and schedule.

Muller Construction Supply of San Jose, California, supplies Truebeck with concrete forming, patching/repair products, and accessories. One stabilization product the company has recommended is a geosynthetic cementitious composite mat (GCCM) made by Milliken Infrastructure Solutions of Spartanburg, South Carolina. Introduced in 2005, Concrete Cloth is like a very thin sandwich: a layer of reinforcing fibers impregnated with high-early-strength cement with fabric on one side and impermeable PVC coating on the other. It comes in giant rolls that are rolled out, drenched with water to conform to the shape of whatever surface it’s applied to, and joined via screws and/or adhesive. It can’t be overhydrated and cures in 24 hours. In 2018, ASTM International released D8173 - 18, a standard governing GCCM site preparation, layout, installation, and hydration.

“The material has ideal characteristics to be used in unconventional ways to successfully complete projects,” says Muller Construction Supply President Brian McGovern.

“When our permit date changed, we needed a product that would withstand winter weather,” says Truebeck Superintendent Mike Currier. “Brian suggested armoring the excavation walls to minimize the risk of cave-ins due to rain, so we visited some jobsites that had used the material and decided to try it on this project.”

Truebeck’s jobsite had shallow and deep foundations, so the crew used both stair stepping and mechanical shoring. In addition to minimizing rain delays, the company wanted to enhance crew safety, exceed stormwater runoff quality requirements, and prevent overexcavation that would require extra concrete. The company decided to use a combination approach for armoring protection with geotextiles, slurry concrete (mud mats), and Concrete Cloth.

Some contractors may think such protection adds unnecessary costs, but Truebeck wanted the peace of mind of being able to meet construction schedules during the rainy season. More importantly, QA/QC verification is easier and better-documented when foundation construction can be reviewed at any stage of the installation until that last yard of concrete is poured.

Truebeck developed a procedure to determine when the geotextile, Concrete Cloth, mud mats, steel sheet pile, and other shoring devices would be used to stabilize each excavation for the foundation construction as well as the installation methods. For Concrete Cloth, the crew precut the sizes needed per wall, hydrated the material in a tub, and then hung and secured it to the excavation wall with spikes.

“We used it for the grade beams and pile caps to minimize slough after rebar was placed,” says Currier. “It kept the foundation from caving in and we didn’t have to buy extra concrete. The material worked well with no failures. We’d use it again.”