Houston, We Have a ... Solution!
It started off as a typical urban pothole on Almeda Road in Houston, Texas, but soon grew big enough to swallow a full-size car, about 60 feet by 40 feet. It closed the road for months, disrupted local businesses and cost about $10 million to repair.
With infrastructure continuing to age, failures of existing systems and even complete collapse of deteriorated pipe and structures are not hard to find. With over 100 miles of large diameter monolithically cast in place (MCIP) pipe, the City of Houston, Texas, is no stranger to this phenomenon. The recent development of a sinkhole led to an intensive investigation of the existing conditions to develop a design that would provide a long-term solution to the ongoing problems.
Background
The sewer line along Almeda Road was originally installed in a tunnel around 1978. Repairs were made in 1991 when 330 feet of HOBAS centrifugally cast, fiberglass-reinforced, polymer mortar (CCFRPM) pipe was used to fix a damaged area. During 1993 and 1994, much of the line was internally or externally grouted in an effort to extend the life of the line. This was done to seal the line and combat soil compaction.
Depth of the problem
Even though the line was 40 to 43 feet below the ground surface and 30 feet below the groundwater table, evidence of the problem was reaching the surface. The damage to the pavement and surrounding area was proof that the problems were getting worse. Another indication of an interceptor leak was erratic flow to the Almeda Sims Wastewater Treatment Plant.
On November 5, 2002, a sinkhole appeared and emergency repairs were made. But damage continued to escalate as a nearby waterline was broken by the loss of support as the soil strength continued to weaken. Ultimately, the affected area was so large that two lanes of Almeda Road had to be closed and traffic rerouted.
Team effort
The City of Houston took quick action to assemble and coordinate a team to repair the Almeda Road cave-in. The city chose Pate Engineers as consulting civil engineer, Tolunay-Wong as geotechnical consultant, BRH Garver as contractor and HOBAS as pipe supplier. They were brought together to find a solution. "The City specified HOBAS for the repair and it was the perfect choice for this large diameter project," said Peck Boswell, president of BRH Garver.
J.E. Pate, Jr., principal at Pate Engineers, explained that years of groundwater infiltration had carried fine soils through small cracks in the 84-inch MCIP pipe and this weakened and compromised the native soil to the point of failure. As the embedment worsened, additional cracks developed causing more infiltration, continuing the vicious cycle. "The cyclical failure process deteriorates bedding strength," Pate explained.
"HOBAS pipe was chosen for several reasons," Pate continued. "Given the existing site conditions, we needed a zero leak pipe. We also wanted to insure the pipe could withstand the hydrostatic conditions. HOBAS met both of these criteria."
Construction sequence
The area adjacent to the collapse was stabilized, dewatered and a liner plate shaft was constructed. Next, the flow was rerouted using 3,000 gpm by-pass pumps. To assess the extent of the problem, the 28 thousand feet of existing interceptor was evaluated and inspected by closed circuit TV. Areas that were in need of sliplining were subsequently cleaned in preparation for rehabilitation. HOBAS sliplining pipe, 72-inches in diameter with a pipe stiffness of 46 psi, was used in several locations adjacent to the sinkhole and the liner plate shaft. The sliplined section included lengths of 1,700 feet, 360 feet and 370 feet in the problem areas.
Ground penetrating radar was used to determine where the soil strength was compromised and future settlement might be expected. Soil grouting and underpinning were required in this area to give the embedment adequate strength to support the pipe. In the location of the sinkhole itself, HOBAS 72 stiffness FWC coupling pipes were also installed by direct bury. The total repairs included one-half mile of CCFRPM pipe.
Results
"We had an opportunity to inspect the HOBAS pipe that was installed in 1991 by sliplining, and it still looked great. There wasn't any evidence of leaks, and this added confidence to our choice in pipe material for the recent repairs," commented Mark Stendahl, senior project manager with Pate Engineers. The new pipe installation was also inspected with remote TV cameras and again no evidence of leaking was found.
In less than a year after the first sinkhole appeared, the team was assembled, the situation evaluated, designed, repaired and all four lanes of Almeda Road were back in service. "We appreciate the efforts of everyone involved that allowed for the timely completion of the project," said Stendahl. "HOBAS did their part by insuring timely delivery."
HOBAS pipe is manufactured in sizes from 18 inches to 110 inches in pressure and non-pressure classes. For more information, please contact HOBAS at 800-856-7473, 281-821-2200 or e-mail at [email protected]. Facts are also available at www.hobaspipe.com.
