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Soil Stabilization With Grout
“For this project,” said Dickes, “ground stabilization prior to tunneling was the only option. Ground stabilization grouting in advance of tunneling is a proven technique and has been used for many years. The primary goal was to control water and prevent subsidence of the active railroad tracks passing over the planned tunnel, and grouting was used to prevent uncontrolled soil erosion into the tunnel bore.”
Dickes said Constellation Group provided consulting services so that Northeast Remsco could self-perform the grouting operation. Working closely with NER Project Manager Giovanni Scotto, these services included overall grout submittals, operations planning, equipment and material selection and operation, quality control and full-time grouting supervision. Extensive surveying was done prior to, during and after grouting and tunneling operations.
The project engineering team selected sodium silicate grout for the project.
“Some of its advantages,” Dickes explained, “are low cost compared to other chemical grouts; its ability to strengthen the soil; low environmental impact; and very low viscosity enabling it to be pumped long distances and to permeate into fine sands and silts.
“Sodium silicate is a highly predictable and proven grout that can be controlled as to strength and set time. It produces excellent results in sandy/gravelly soils. It is readily available in large quantities from numerous sources and also environmentally friendly.”
Dickes said prior to tunneling it was necessary to install 11 horizontally located sleeve port pipes (SPPs), also called tube-a-manchettes (TAMs). The pipes were installed approximately 18-22 inches outside the planned tunnel alignment and parallel to it.
An Akkerman Model 339A guided boring machine (GBM) was used to advance a 4½-inch pilot hole. After exiting the opposite pit, the SPP was attached and pulled back to the launch pit. As the SPP was installed, a casing grout was pumped to fill the annular space left by the drill. These 11 pipes were installed radially along the tunnel alignment from the 7 o’clock position to the 5 o’clock position. This annular grout is designed to fracture easily, but fill the bore hole so sodium silicate grout does not follow the SPP bore, but goes into the ground.
The upper sleeve port pipes were constructed of 1½-inch PVC, the lower ones SPPs were steel. Port spacing was every 15 inches.