Zagros Water Conveyance tunnel in western Iran crosses a broad and vast aquifer. This TBM burrowed tunnel has long drained the region's groundwater at an average rate of 416 lit/s. The groundwater is rich in hydrogen sulfide (H2S) gas. The gas reacts with the tunnel humidity and produces an acidic fume that penetrates the tunnel lining and decomposes its concrete and steel reinforcement, as well. The exact locations of these discharging conduits and their morphology are not known, since the TBM segmental lining conceals them. An elaborate post-grouting plan is on the drawing board to reclaim the aquifer but the cost of a systematic grouting is particularly high and at best scenario not conclusive.
The scope of this paper is to discuss a series of carefully controlled field experiments in a 100m pilot study area along the tunnel. It manipulates on the TBM performance parameters that were registered during the tunnel excavation stage. Based on this information, suitable field models are established that may be interpreted as being associated with either water or air-filled solution channels and open joints. The results are supported by in-situ permeability tests, where the suspected joints were theatrically identified and were cement grouted after construction phase.
Zagros water conveyance tunnel in Kermanshah province of Iran is near the town of Pole Zahab. This part of the project is regarded as the second lot of a broader plan, which is schemed approx. 26 km long and is 6.73m in diameter. It has been under construction using a Herrenknecht hard rock double-shield TBM since March 2005. So far, 15 km (58%) of this tunnel has been completed.
In the course of tunnelling, the machine encountered nearly many extraordinary situations related to high groundwater and H2S gas intrusion, all of which resulted in a significant reduction in TBM utilization rate and an increase in construction delays, as well as excavation costs.
The water seepage in low amounts was first experienced at TM 3700. A significant water ingress in the range of Q>110 lit/s was later intruded at TM 4157, which rapidly accumulated to Q~315 lit/s with further advancement to TM 4435. Advancing deeper into the core of aquifer at TM 8256, the accumulative water seepage totaled Q~730 lit/s, with further advancement to TM 13846, one hundred fifty five more liters of gas bearing water seeped into the tunnel, totaling the tunnel discharge flow at the portal outlet to 900 lit/s. This amount of water released 700 ppm hydrogen sulfide gas into tunnel atmosphere. Figure 1 shows the water discharge rate and the H2S gas concentration at the pilot study area where is marked as Ezgelleh Anticline on the graph. One of the major factors affecting the performance of tunnel boring machines is the degree of fracturing of the rock. During excavation, machine performance parameters are continuously displayed and recorded on the control cabin monitors. Figure 2 shows the TBM display screens. The screens display TBM performance parameters and register TBM mechanical behavior against the excavated material. These screens disclose an assorted set of data; e.g., penetration rate, boring time, total thrust, torque power, cutter speed etc… Based on the careful analysis of these parameters, uniform patterns were established as a model to identify concealed joints and to delineate water-carrying conduits along the tunnel path. The objective is to employ a model to locate water conduits and fill in the leaks and cracks by post-curtain-grouting. This is in contrast to a systematic grouting approach where grouting is done in a continuum fan pattern.