Valuable lessons on the watertightness of deep jet grouted blocks have been learned from ground treatment work for the latest phase of the Cairo Metro.
When Regional Line 1 of the Cairo Metro was completed in 1989, it was the first of its kind in Africa or the Middle East. The 42.5km long line - 4.5km of it running beneath the centre of the city in cut and cover - links Helwan in the south with Al-Marg, near Heliopolis, in the north. This is just the first stage of a much greater scheme that includes two urban lines to link the densely populated districts of Shubra, Giza and Embaba.
Line 2, now under construction, also runs under the capital, with 13km of its total 19km underground - 9.3km in bored tunnels, the rest in cut and cover.
As part of construction, over 80,000m3 of ground treatment has been needed at 40 sites to modify the ground's behaviour. This aspect of the three year contract was awarded to Bachy and Soletanche of France, which joined forces to create Soletanche Bachy Egypt (a local joint venture which subsequently prompted the two companies' permanent worldwide merger in 1997).
Overlapping jet grouted columns were used to form strengthened blocks of ground for the end walls of the 10 cut and cover underground stations along the new line, forming 22 departure and arrival blocks for the TBMs. The method was also used at other risky elements of the project, such as the cross-passages between the bored tunnels, for protecting major utilities and buildings above the alignment, and also for ground support at connections between diaphragm walls and existing structures.
A double jet grouting system - high pressure grout jet enhanced by an envelope of compressed air - was chosen because of its ability to form large diameter columns (up to 2500mm diameter) in dense fine grained sand to the required strength of 3MPa at 28 days.
Cairo sits in the River Nile valley at the apex of the Nile delta. Its geology consists of 2m to 4m of fill over 8m to 12m of silty clay, and clayey silt over deep deposits of fine to coarse alluvial sand. The sand is medium dense, becoming very dense below 20m. It contains layers of clayey silt that vary in thickness from a few centimetres to over a metre. Below 25m to 30m, there are also lenses of gravel and cobbles, up to several metres thick. Groundwater level in the Nile sands is linked to the river, varying between 3m and 4m below ground level.
A major aspect of the ground treatment contract was the requirement for complete watertightness at depths greater than 15m or 20m, sometimes up to 40m, in sand under several bars of water pressure. In these conditions, a small untreated area left within the jet grouted mass could immediately cause major collapse by piping through the untreated zone.
However, Soletanche Bachy soon found that watertightness could not be ensured, with 'windows' of untreated soil occurring within jet grouted blocks. These untreated windows started to appear in the alluvial sand below 15m, and became more common with depth. Their presence caused concern because the alluvial sands are particularly susceptible to erosion, and it was found that even windows as small as 100mm wide caused significant problems when connected to the water bearing sands surrounding the jet grouted blocks.
Column deviation from verticality with depth was found to be the main cause of the problem. Even with tight tolerances of 1% on horizontal deviation, two adjacent 40m long columns can be up to 800mm apart at their bases. The problem was made worse by a variation in column diameters due to the variation in grain size of the sand.
Despite changes to the design, such as tightening the drilling pattern and quality control operations, the untreated windows continued to occur, so it was decided that the watertightness of the method could not be relied on below 15m.
The windows of untreated soil caused significant problems at the departure and arrival blocks for the tunnel boring machines at the stations. Initially, simple blocks of jet grouting between 10m and 30m depth behind the station endwalls were used. But small windows were found at the interface between the jet grouted block and the diaphragm wall.
At the Masarra station, one such untreated zone extended vertically to the base of the jet treatment, resulting in sudden and rapid ingress of soil and water into the station box. To stabilise the piping, the station had to be flooded to balance the hydrostatic pressure, and remedial grout injection used to seal the leak. A similar problem, although not as serious, occurred at the Rod El Farag station arrival block, and was controlled by emergency injection grouting from the surface.
To combat this problem, a more conservative approach was taken using a combination of jet grouting with a slurry wall enclosure box and an injected hard gel sodium silicate base plug. This allowed dewatering of the untreated ground above the injected plug, and allowed hydrostatic pressure relief should a critical leakage path remain in the jet grouted block. A leak detection system was also installed, comprising a series of 3m long horizontal and inclined test drains drilled around the perimeter of the diaphragm wall opening before demolition.
Even with this new design, some minor leaks occasionally occurred, but these were easily handled by local injection of bentonite/cement grout. Nevertheless, emergency pumping was needed to control groundwater between the jet grouted block and the injected plug inflow at the El Behoos arrival block.
A similar solution was used for the 12 shafts used for tunnel ventilation, dewatering and train traction power supply. These were built using deep diaphragm walls located within 5m of the bored tunnels. Connection was made through horizontal galleries excavated manually. Again, simple jet grouting blocks between 25m and 40m deep were initially used, but test drains installed to check for leakage before demolition of the opening revealed water and running sand under high pressure. This indicated occasional leakage paths near the base of the jet grouting block, so the more conservative design was used.
Despite the problems, the ground treatment works were finished without significant incident. As a result, Soletanche Bachy is now warning against use of the method in isolation for watertight treatment in waterbearing and erosion susceptible sands at depth. The company feels that until more accurate measurement of the location and diameter of jet grouted columns is made, a combination of permeation grouting and jet grouting with slurry wall enclosures provides a less risky, but still economical, ground improvement solution.