In order to comply with handicap accessibility laws, the City of Ventnor, NJ, planned to install an elevator shaft in the historic City Hall building. Located just one block from the ocean, the 76 year-old structure is founded on shallow spread footings bearing on fine to medium sands, with the groundwater table at two to three ft below the basement floor. The elevator pit was to be installed in one corner of the building, adjacent to two load-bearing walls. Since the base of the excavation would extend approximately 2.5 feet into the water bearing soils, geotechnical engineers Lippincott, Jacobs, Gouda of Riverside, NJ specified that the ground be grouted ahead of the excavation to mitigate ground loss beneath the footings during excavation, and prevent settlement.
Value Engineered Approach
The initial project design required the grouting of the bottom of the excavation as well as all four sides. Thus, the grouted soil would function as structural support and would also prevent water from entering the excavation. Moretrench offered a cost-saving design alternative that reduced the area to be grouted by utilizing a higher-strength permeation grout than specified, coupled with dewatering of the excavation with deep wells.
The intent of the program was to form a contiguous grouted perimeter wall approximately 3.0 to 3.5 feet wide. To accomplish this, target grout volumes would be injected at designated grout pipe locations to propagate through the pore spaces in the treatment zone and form a full grouted mass by the merging and overlapping of the zones of grouted soil. This would serve two distinct purposes: it would transfer the load placed on the soil by the shallow footing to below the extent of the pit excavation, and would add strength and cohesion to the soils to prevent sloughing during subsequent excavation.
Grouting and Dewatering Program
Prior to the grouting work, three deep wells were installed outside the building and activated to draw the groundwater down below bottom of the proposed elevator pit. This reduced the loading on the sides of the excavation, and provided a dry, stable subgrade for the excavation contractor.
Since all of the grouting work had to be done inside the building, conventional drilling equipment could not be used to install the grout pipes. Moretrench therefore elected to hand drive short open-ended lengths of steel pipe (grout needles), at primary/secondary locations surrounding the excavation. Additional lengths of pipe were added as driving continued in order to reach the targeted depth. A calculated quantity of sodium silicate grout was then injected at each needle location. When target volume was reached, or refusal occurred, the needle was raised to the next vertical stage and the process was repeated.
Quality Control and Quality Assurance
Moretrenchs grouting QA/QC program included:
- Real time monitoring and recording of injection parameters.
- Permeability testing to verify the presence and continuity of grout.
- Preparation of a grouted soil sample for unconfined Compressive Strength testing.
- Slab monitoring for movement during grouting.
In addition, settlement monitoring was conducted by the General Contractor. The grouting and dewatering program resulted in a completely stable and dry excavation, eliminating the need for shoring prior to rebar installation.