Drill and blast installation of the three deep shafts through highly variable karstic conditions was the challenge facing the design and construction teams for Phase 2 of a 23,000 foot-long sewer tunnel in Columbus, OH. A specially designed pre-grouting program developed by Moretrench was key to successful excavation in the dry.
The OSIS Augmentation and Relief Sewer (OARS) is designed to augment the existing Olentangy Scioto Interceptor Sewer. The tunnel itself would be mined approximately 170 feet below the surface through karstic rock with solution features ranging from small fissures to large voids. During the site investigation, a test well that was drilled to design shaft depth was pumped at 490 gpm, with just four feet of ground- water drawdown in the well. With the water table 20 feet below the surface, and the high hydraulic conduc- tivity of the rock evident from the pumping test, it was estimated that inflows of thousands of gallons per minute could be anticipated during shaft excavation under hydrostatic head of up to 150 feet. Pre-treatment of the rock prior to excavation of the Phase 2 shafts was therefore paramount and required by the bid documents.
The shafts extended through shale underlain by three distinct strata of limestone, and the grouting program design would have to cater to the highly variable conditions present throughout. Given a hydrostatic head of up to 150 feet, even small fissures could result in significant inflow. The grout needed to be fluid enough to penetrate these, yet sufficiently viscous to fill large voids efficiently without excessive grout loss out- side the target area. A suite of balanced-stable grouts was therefore developed by Moretrench to achieve these goals.
The grout hole pattern consisted of a double row of holes outside the shaft perimeter as well as vertical holes drilled within the secant pile excavation support to construct a central plug. Perimeter holes were battered to ensure intersection with near-vertical joints and fissures. A grouting program was established using a primary, secondary and tertiary sequence, with successive stages within each hole isolated by pneumatic packers to maintain control over the grouting program.
During grouting, Moretrench utilized an automated data acquisition system to monitor progress. In addition to recording duration, flow rates and pressures, the system also plotted the apparent Lugeon value at each stage, allowing the grouting crew to make informed decisions on which of the suite of four grout mixes to be used at any one time, and to determine when refusal was reached.
A low viscosity grout was injected first, with progressively thicker mixes used as warranted for the more open features. A high viscosity, low-flow grout containing sand addressed wide open features and large voids. Grout quality was maintained by means of a computer-controlled batch plant, with daily viscosity checks made and periodic independent laboratory testing conducted.
With the grouting program as designed successfully addressing the complex and challenging subsurface conditions, the general contractor was able to safely excavate to design depth, with minimal sumping required to handle residual infiltration.