Project Features:

• Cement-Bentonite Slurry Wall
• 25,745 Linear Feet. 1,600,000 Square Feet. Maximum of 65-feet deep
• Target Average Permeability of 9 x 10-6 cm/s
• Target Average Unconfined Compression Strength 10 psi

Background:

The Homestead levee is located in Miami-Dade County approximately 13 miles North of Homestead, FL. This property is part of the Biscayne Aquifer which contains a highly permeable limestone layer. Located on the property is a levee formerly constructed by the Army Corp. The levee acts as a border for the residential areas of Miami Dade County and the Everglades.

Project Objectives:

To construct a 5-mile-long Cement-Bentonite (CB) cutoff wall along the center of the existing levee. Complete excavation with depths up to 65 feet below the existing ground surface reaching the soils under the porous limestone layer. Installation of the cutoff wall with a design that will slow or stop the plume of water intrusion toward the residential areas.

Description of Work:

At completion the cutoff wall measured 25,745 feet in length and consisted of 1,600,000 vertical square feet (vsf) of cement-bentonite (CB) slurry wall. Wall installation has depths ranged from 58 feet to 65 feet below ground surface, and keyed in a minimum of three feet into the underlying soils of the Tamiami sand layer.

The cutoff was installed using a Komatsu PC1250 excavator and GSI’s long-reach boom and stick attachment. A Komatsu PC240 excavator was used to excavate the first 5-8 feet of trench. Cement Bentonite slurry was used for trenching to maintain stability during excavation and minimize seepage through the porous limestone sidewalls. The bentonite slurry was mixed onsite in GSI’s custom-made batch plant and pumped to trench.

The completion of this cut off wall was conducted in multiple phases that required simultaneous advancement. First, H.L. Chapman would trench 57’ deep through the limestone layer with a 32” inch wide custom boom attached to a 1860HD Trencher. Geo Solutions would then back fill the open trench with limestone generated from the trenching to comply with safety standards. Following, Geo-solutions excavated down to the Tamiami Layer while simultaneously backfilling with a CB mix. A middle trench between the PC1250 and trencher was excavated with a PC490 to backfill CB spoils and generate clean limestone. When excavating down, bucket samples (visual verification of Tamiami sand) and depth measurements were done to ensure proper tie in. Additionally, samples were collected and tested per specifications for permeability with a minimum target of 9×10-6 and an unconfined compression strength of 10 psi.

Maintaining the production schedule was a key factor of this project because of the 3 open trenches simultaneously advancing and codependent upon each other. Staying within the open trench limit and maintaining access throughout the jobsite proved to be big obstacles throughout the project. While keeping commitment to safety, GSI worked through the extreme heat conditions of south Florida and around rain events. GSI safely worked through obstacles and kept communication open with the client to keep the project on schedule.