With a long-term objective of providing 200-year flood protection to the northwestern Sacramento, California region, the Natomas Levee Improvement Program (NLIP) includes a wide variety of construction projects: excavation, slurry cutoff walls, drainage canals, removal of unsuitable levee materials, and levee construction & maintenance.
NLIP’s cohesive network of levees, channels and dams are designed to protect the area from flooding. With over $100,000,000 in construction contracts, about half of the 42-mile-long Natomas Basin perimeter levee system has been completed to date.
Originally, the levees were constructed with inadequate materials, which proved to be extremely porous when exposed to sustained flows. Great Lakes Environmental & Infrastructure (Great Lakes E&I) team members have participated in four phases of the project thus far – correcting seepage issues and bringing the structures to a compliant level.
All of our projects were completed on-time, within budget, and without incident, leading to recognition in the form of the 2011 Outstanding Flood Management Project Award from ASCE Region 9.
Team members utilized many different techniques, safety precautions, and construction materials to as part of the NLIP:
Excavation & Slurry Walls
The slurry cutoff walls for the NLIP were constructed using two methods – the conventional excavation method, commonly referred to as the slurry trench, and the DSM method, and commonly referred to as in-situ soil mixing. The conventional excavation method involves excavating a narrow trench while pumping slurry in the trench for excavation support and maintaining its level at or near the top of the trench during the excavation process.
Due to equipment limitations on the dimensions of the boom and stick, the minimum constructed trench width for the noted equipment was 36 inches. This was determined by the width of the excavator bucket. In most cases, and which is true for the NLIP, the minimum specified width of the slurry cutoff wall is also 36 inches, thus eliminating any design concerns with regard to equipment limitations. The advantage is that the excavator can excavate the trench to the full required depth in a single pass – saving both time and manpower.
In addition to the excavation equipment, another major component of the conventional excavation method is the backfill mixing equipment. Unlike the excavation equipment, this equipment is standard size and includes, at a minimum, having one excavator and one dozer (e.g. PC 200 excavator and CAT D6 LGP dozer) for mixing the backfill materials on the surface of the work pad. The soils can be pre-mixed by the excavator, to break down large clumps of clay or soil, prior to track walking through the material with the dozer. This was the case for the NLIP.
Batch Plant Equipment
The final component of the conventional excavation method is the batch plant equipment that is utilized to mix the slurry on site. The slurry is typically a mixture of bentonite clay and water which produces a colloidal mix that is capable of supporting the trench during excavation. In addition, the bentonite slurry is utilized in the backfill materials to produce the specified mixture. The slurry is mixed onsite in a temporary pond, using a jet shear mixer and pumps for circulation and conveyance to the trench.
Although several factors such as wall depth and thickness are an important part of the project standards for acceptance, permeability is the primary acceptance criteria element and key function of the slurry cutoff wall. Permeability testing is performed by collecting wet bulk samples of the backfill mixture prior to placement in the trench. In the case of the NLIP, all permeability samples achieved passing results, with no record on any section of the cutoff wall.
The final completion of the Natomas Levee Improvement Program will bring the Natomas Basin perimeter levee system into compliance with Federal and State standards for levee protecting urban areas. The slurry cutoff wall is a key design component that will address the confirmed under seepage deficiencies of the existing levee system.