Cairo Metro Construction

Construction of Africa’s first metro is under way in the shadows of the ancient pyramids of the Giza Plateau in Cairo.
This portion of the Greater Cairo Metro system, called Line 2/Phase 1, includes considerable underground work:

• 1.4 km (0.87 mile) of cut-and-cover tunnel
• 5.8 km (3.0 miles) of single bored tunnel with an excavated diameter of 9.5 m (31.2 feet)
• 7 cut-and-cover stations.

Tunneling Made Difficult by Centuries of Flooding

Cairo is at the apex of the Nile delta where, until completion of the High Aswan Dam in the 1960s, seasonal flooding of the Nile created thick alluvial flood deposits of clays and fine sands, materials that are increasingly coarse and pervious with depth. Tunneling in this area is particularly challenging because of the presence of the upper alluvial clays and fine sands in combination with a high groundwater level.

These conditions presented difficult construction conditions for tunneling in general, but particularly for the two most sensitive construction aspects of the project:

• Tunnel boring machine (TBM) station break-ins/outs
• Special hand-mined gallery connections between the bored tunnel and tunnel annex structures (deep shafts located about midpoint between stations along the bored tunnel route that provide for tunnel ventilation, dewatering, and the system power supply).

Jet Grout Treatment Selected

The treatment zones at these critical locations extended through both the upper clays and fine sands, and the lower more pervious sand aquifer. Effective permeation of the finer soil types of the upper alluvium is not normally possible with chemical or mineral grouts. The design-construct contractor, therefore, envisioned the ground treatment to be performed using the jet grout method only.

Jet grouting, applicable for all soil types, was expected to provide both the required soil consolidation and ground water control to safely complete these works (Figure 1). The required depth of the treatment at these locations, however-30 m to 40 m (100 feet to 130 feet)—was considered to be near the limits of the current jet grout technology, particularly when it is intended to be a watertight treatment.

Figure 1: Early TBM Break-in/out and Tunnel Annex Connection

Erosion Problems Surface

It became clear during the execution of the early break-ins/outs that the jet grout treatment would provide the required soil consolidation, but could not be relied upon for ground water control. Small discontinuities of untreated soil were occasionally encountered between the jet columns at depths of about 15 m or 20 m (50 feet to 65 feet).

The clean and uniform lower sand deposits have no natural cohesion so they are particularly susceptible to erosion when subjected to a hydraulic gradient. These discontinuities were exposed during the demolition of the station end wall for the early TBM break-outs, causing rapid and progressive failure by forcing soil and water under pressure through the discontinuities and into the station. This ultimately required the flooding of two stations for stabilization, causing significant construction delays. Similar leakage through the jet grout columns was observed during execution of the first two tunnel annex connections, again causing construction delays.

As a result of the early problems at these areas, we had the contractor review the ground treatment design for subsequent break-ins/outs and annex connections.

Figure 2: Revised TBM Break-in/out and Tunnel Annex Connection

Slurry Wall is the Solution

After some trial and error, the final solution adopted was an unreinforced cement/bentonite slurry perimeter wall box with an impermeable chemical grouted hard gel base plug beneath the jet grout treatment (Figure 2). Using the slurry wall as a supplemental ground water control concept proved to be the solution to completing the construction of these critical areas, despite the difficult ground conditions within the Nile valley. In addition, it allowed for:

• Using an internal pumping test to confirm the watertightness of the closed box before the demolition of the concrete diaphragm wall began
• Lowering of the groundwater level inside the box, thus reducing the water pressure and increasing the safety
  factor.

It was clear that this more conservative design approach to the ground treatment of these critical areas effectively reduced the risk and minimized further construction delays.