That Sinking Feeling
As infrastructure comes of age, failures and complete collapses of deteriorated pipe are not hard to find
It started off as a typical urban pothole on Almeda Road in Houston but soon grew big enough to swallow a full-size car, about 60 ft by 40 ft. It closed the road for months, disrupted local businesses and cost about $10 million to repair.
As infrastructure comes of age, the failure of existing systems and even the complete collapse of deteriorated pipes are not hard to find. With more than 100 miles of large diameter monolithically cast-in-place (MCIP) pipe, the city of Houston is no stranger to this phenomenon. The recent development of a sinkhole led to an investigation of the existing conditions to develop a design that would provide a long-term solution to the city’s ongoing problems.
The sewer line along Almeda Road was originally installed in a tunnel around 1978. Repairs were made in 1991 when 330 ft 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 problem
Even though the line was 40 ft by 43 ft below the ground surface and 30 ft below the groundwater table, evidence of the problem was reaching the surface. The damage to the pavement and surrounding area was proof the problems were getting worse. Another indication of an interceptor leak involved erratic flow to the Almeda Sims Wastewater Treatment Plant.
On Nov. 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 were closed and traffic rerouted.
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 Graver as contractor and Hobas as the pipe supplier.
“The city specified Hobas for the repair and it was the perfect choice for the large-diameter project,” said Peck Boswell, president of BRH Graver.
J.E. Pate, Jr., principal of Pate Engineers, explained that years of groundwater infiltration had carried fine soils through small cracks in the 84-in. 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,” explained Pate.
“Given the existing site conditions, we needed a zero leak pipe,” Pate continued. “We also wanted to ensure the pipe could withstand the hydrostatic conditions.”
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 bypass pumps. To assess the extent to the problem, the 28,000-ft of existing interceptor was evaluated and inspected by closed circuit television. Areas that were in need of sliplining were subsequently cleaned in preparation for rehabilitation. Hobas sliplining pipe, 72-in. 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 ft, 360 ft and 370 ft 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, 72 stiffness coupling pipes were also installed by direct bury. The total repairs included one-half mile of CCFRPM pipe.
“We had an opportunity to inspect the Hobas pipe that was installed in 1991 by sliplining and it still looked great,” said Mark Stendahl, senior project manager with Pate Engineers. “There wasn’t any evidence of leaks and this added confidence to our choice in pipe material for recent repairs.”
The new pipe installation also was inspected with remote TV cameras and no evidence of leaking was found.
In less than a year after the first sinkhole appeared, the team was assembled; the situation was evaluated, designed and 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.