|Southern California's Cleveland National Forest is being considered for drilling an 11.5-mile traffic tunnel system.|
The 91 Freeway corridor between Orange County and the Inland Empire has some of the most congested traffic in the U.S. Two hundred and fifty thousand motorists per day can travel at a crawl during peak drive times or frequent accident occurrences on the 30-mile stretch of concrete roadway. Imported goods trucked from the Port of Los Angeles to new warehousing hubs further inland add to the problem. Years of lane widening and toll road construction have done little to bring relief.
Southern California's Inland Empire counties of Riverside and San Bernardino to the east continue to grow. Although new housing has temporarily ground to a near stop thanks to the sub-prime mortgage crisis, business development continues — especially concrete tilt-up warehousing.
Traffic along the 91 corridor is projected to skyrocket. If trends continue, by 2030 the current 12-lane freeway would need to expand to an impossible 22 lanes to handle increased traffic
Orange and Riverside County Transportation Commissions are trying to figure out ways to cope with this issue. But topography is a problem:
"The Santa Ana Mountains are a major geographical barrier that rises 2,500 to 5,000 feet above the population centers east and west," says Paul Guptill, engineering geologist with Kleinfelder, the company providing preliminary geologic testing. "Surface roads across the mountains are not an option due to the extreme topography and the foothill-to-foothill extent of the Cleveland National Forest and Nature Conservancy that both preclude any new roads."
"The only other option is to squeeze more surface roads through the Santa Ana Canyon, where the Santa Ana River, public recreational uses, homes, rail right-of-ways, and massive landslides in the hills restrict possibilities for more surface roads. Even a viaduct for double-decking the roadway is high risk due to the high seismic shaking potential of the area and the intersection of the Elsinore and Whittier faults with the SR-91 in the Santa Ana Canyon."
So, transportation authorities are looking into whether boring two 50-foot-wide tunnels under these mountains would be a viable alternative traffic venue — diverting some 70,000 vehicles per day. But there would be an estimated $8.5 billion price tag for the tunnels and significant impact on the existing groundwater system as well as other environmental issues stated by the United States Forest Service (USFS).
- Ground Water
Kleinfelder team is gathering information on the groundwater system by 1) drilling holes and measuring groundwater pressures at the tunnel depth and shallower and 2) collecting data on regional springs, stream flows, water wells and water chemistry.
"The deepest core hole is 1,502 feet below ground. The shallowest is 1,191 feet below ground. The tunnel depth may be as deep as these core holes. If needed, possible construction solutions could include gaskets between tunnel lining segments, grouting ahead of the tunneling operations, and supplementing water resources artificially within the forest." Kleinfelder is installing pressure transducers measuring hydrostatic pressure at tunnel depth and lower.
- Earthquake Fault Lines
The tunnel alignment avoids the Elsinore fault line, but the Kleinfelder team must identify other fault lines, if any, that intersect the tunnels.
"Generally, earthquakes do not damage tunnels," Guptill said. "Simply stated, this is because the ground motions induced by an earthquake are not felt as strongly below ground as at the surface. Thus, below ground transportation roads (i.e. tunnels) are as safe as or safer than surface roadways.
"The most severe damage potential from an earthquake on a tunnel could be where an active fault line intersects a tunnel. In such a case, the shift of the ground on either side of a fault that experiences offset can rip apart the tunnel lining and displace the alignment of the tunnel across the fault — up to tens of feet."
- Evaluating Rock Strength
Rock stability through which a tunnel is bored is a crucial component of the construction. According to Guptill, Kleinfelder engineers are finding the rock strength to be highly variable along this tunnel alignment.
- Helicopter Access Only
Working at five remote, protected sites accessible only by helicopter, it took Kleinfelder and the United States Forest Service 1-1/2 years to reach an agreement before the field investigations could begin.
Kleinfelder's $6-million testing study began in April and concluded in October. The final report is due in 2009. All testing sights will be brought back to their original state in the Cleveland National Forest.