When the engineers at Molzen-Corbin & Associates realized that the soil at Albuquerque’s Lift Station No. 20 was too weak to build on and that contaminated water plumes surrounded the area, it was clear that they would have to do something more than normal soil stabilization. They turned to a unique procedure: jet grouting.
Perfected by Hayward Baker Inc., a Fort Worth, TX-based company specializing in ground modification, jet grouting is a process used to reduce erosion and control groundwater in soils. Using a specialized drilling rig, the drill plunges deep in to the soil and sprays a pressurized mixture of water and grout in a circular pattern, forming columns. When the grout solidifies, it creates a soil and cement mixture that Hayward Baker calls Soilcrete. The Soilcrete forms hard columns that are impervious to water and stabilize the soil. This is exactly what the engineers decided to do at Lift Station No. 20.
The project’s first phase had been completed some three years before. Now in its second phase, the city of Albuquerque planned to build a bar screen (a screen to separate sewage) and link an existing sewer interceptor line into a Lift Station across the river. The problems first began when the wet soil was discovered. It was intensified by the presence of contaminated water plumes. The best option was to try jet grouting to stabilize the soil. RMCI, the general contractor on this $3.2 million-project, contacted Hayward Baker to help with the process.
Their portion of the task was very involved. The project required the construction of more than 800 overlapping jet-grouted columns in the project area.
“Imagine a bathtub,” said RMCI Vice President and Project Manager Jeffery Mohns. “Then excavate and build a building inside that bathtub.”
Under the leadership of project superintendent Dennis Boehme, Hayward Baker began creating the jet-grouted columns. The subcontractor marked off column sites before beginning, then drilled the columns one at a time. Using a C-7 hydraulic rotary drill rig, a National Oil Well 640-5 triplex plunger pump and a Quincy QST-25 rotary screw air compressor, they poured first the even columns, then the odd. Since the columns eventually overlap, the driller must pour every other one, then return to fill in the missing sections. Creating almost 800 columns that range from 1.2 to 1.8 meters (4 to 6 ft.) in diameter and 10.6 meters (35 ft.) deep can cause significant difficulty.
“It’s not a perfect type of construction,” said Mohns. Because the rig operator is unable to see beneath to surface to view the precise area that has been grouted, it’s easy to miss spots, which allows groundwater to seep back in. In the case of Lift Station No. 20, the crew did have some leaks after construction. Under contract, RMCI was permitted to have up to 10 gallons per minute of water leakage, which would be automatically evacuated with an in-place pump. When the leaks exceeded this amount, the crew was required to fix the problem with a special grouting process.
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