The City of Santa Cruz, California owns and operates a Class III landfill that was constructed in canyons close to the Pacific Ocean. For several years, the landfill acted as a barrier to fresh storm water drainage in these canyons. Surface runoff ended up flowing into the landfill and becoming polluted by landfill contaminants. The State of California Regional Water Quality Control Board imposed a Cease and Desist Order on the landfill until the city constructed a system to intercept fresh water flow above the landfill, conduct it around the landfill and discharge it downstream of the landfill back into the creek bed that the landfill occupies.

The solution that PB provided for the city included:

• Two 0.7-m to 0.9-m (26-inch to 36-inch) -diameter microtunnels to convey the fresh water around the
  landfill
• Soil-cement-bentonite slurry cutoff walls to prevent subsurface migration of fresh water into the landfill.

Project History

Preliminary studies for a tunnel solution were first prepared by others in 1988. Geologic conditions along two tunnel alignments were evaluated and geotechnical considerations for tunnel construction, potential geologic hazards and tunnel design characteristics were addressed. At that time, two 305-m- (1,000-foot-) long, 2.1-m- (7-foot-) wide, horseshoe-shaped bypass tunnels were recommended to be excavated using drill-and-blast methods through ridges upstream and west of the landfill.

A 1990 environmental impact report (EIR) addressed the environmental aspects of two possible solutions for a freshwater bypass:

• A system of bypass tunnels as proposed in 1988
• A permanent pipeline and pumping system along the east and west canyon ridges.

After approval of the EIR, prepared by others, PB made further studies to review available data, prepare hydrologic and hydraulic calculations, evaluate fresh water bypass alternatives using pipelines and tunnels, and recommend a preferred alternative for further study—two small-diameter bored tunnels (microtunnels) constructed within landfill property. These microtunnels would provide the benefits of the larger tunnels proposed in 1988, but with fewer of the environmental and permitting consequences associated with constructing tunnels on state park land next to the landfill.

Figure 1: Geologic Cross Section of the Slurry Cut-off Walls

Figure 2: Soil Fines Content Effect On Slurry Wall Permeability

Figure 3: Schematic Section of Slurry Cutoff Wall Construction

Figure 4:Constricted Construction Site

Several Issues to Consider

Once the option of a tunnel from the east canyon to the west canyon and a second tunnel from the west canyon to a creek under the middle of the landfill was selected, the task was how to install tunnels under the landfill cost-effectively and prevent flow of fresh storm water into the landfill. The issues considered when designing the tunnels included:

• Locating where the rock-landfill interfaces occurred
• Evaluating how to mine through the landfill materials, if encountered
• Determining how to handle contaminated groundwater from the landfill, if encountered,
• Evaluating whether there would be methane gas present and at what concentrations.

In addition to the tunnel studies, PB’s design considered ways to prevent subsurface groundwater flow from entering into the landfill and direct it into the freshwater bypass tunnel system.

The geotechnical exploration program undertaken to resolve these issues included geologic mapping, soil borings through refuse, rock core borings, laboratory testing, and geophysical testing. Because the site was contaminated, personnel had to be trained in health and safety and all of the requirements for a contaminated site had to be followed, including physical examinations of all personnel, appropriate health and safety equipment, training, and monitoring. PB developed contract documents for the exploration and actual execution of the work including operation of monitoring equipment and groundwater sampling. Figure 1 on the following page indicates the in situ character and extent of subsurface conditions at the slurry wall locations.

Slurry Wall Cutoff Design and Construction

Several alternatives were evaluated for cutting off subsurface groundwater flow through the valley alluvium overlying bedrock upstream of the tunnels. Steel sheet pile cutoffs, concrete wall cutoffs, clay blankets, and soil-cement-bentonite slurry walls were considered. After an exhaustive alternatives analysis, a slurry wall cutoff solution was selected based on cost. On August 21, 1995, construction of the tunnels and walls began.

The project design included the requirement for a mixture of soil, cement, and bentonite in accordance with the contractor’s approved mix design. The specified requirements for the slurry cutoff walls included criteria for strength and permeability as indicated in Table 1. These criteria were based on published literature and case histories.

The contractor who constructed the slurry cutoff walls first drilled additional borings at the wall sites to obtain soil samples that would be incorporated into the mix design trials. Fines content of the soils has a dominant effect on the in situ permeability of the cutoff walls as indicated in Figure 2. The cement content has a dominant effect on the in situ strength of the cutoff walls.

The method of construction of soil-cement-bentonite cutoff walls (Figure 3) consists of:

• Excavating a trench with addition of bentonite-water slurry to maintain stability of the trench
• Keying the trench into bedrock
• Backfilling the trench with soil-cement-bentonite slurry mixed on-site, which displaces the bentonite-water slurry
• Capping the slurry cutoff wall.

The trenches were excavated in stages because the valley walls constricted the site (Figure 4). The backhoe straddled each wall and excavated one half at a time. Once one half of the wall was completed, the backhoe would turn around and construct the other half.

Table 1: Criteria for Strength and Permeability

As indicated in Table 1, the mix designs were achieved in the field and they met the specified criteria. It should be noted that quality control in the field required intense scrutiny of field operations and follow-through on laboratory testing of field samples.

Conclusions

Valley landfills are common on the West Coast. Changing regulations have had a drastic impact on how landfill operators deal with storm water in and around their facilities. In the case of the Santa Cruz Landfill, where a pump and pipeline system was installed as a temporary solution, the City determined that drainage tunnels would be more cost effective over the life of the facility (100 years) because of the high operations and maintenance costs of the pumps. Subsurface cutoff of groundwater flow with soil-cement-bentonite slurry walls was an important feature of this project.

This project used the best of PB’s full-service resources. Some of our environmental specialists provided unique services related to landfill contamination and our water resource specialists provided instrumental guidance during design. These specialists were in addition to other PB professionals who provided their construction and civil engineering expertise. With four PB entities involved, we can say that we had a PB “Grand Slam” on this small, but very important project along the coast of northern California.