Musaimeer pump station and outfall project is located directly south of the Hamad International Airport in Doha, Qatar. The project is owned by Ashghal (Public Works Authority) and is designed to receive both ground and storm water from 270 km2 of southern Doha.
Qatar although a predominately dessert country can experience torrential rain fall in the rainy season which occurs from September to March. The quantity of rainfall received in a short period can cause severe disruption to the transport systems for days at a time.
Groundwater level is high in the Doha area and in general is rising in the Middle East, this possesses problems for urbanisation and increases the cost of construction.
Ashghal has a policy of reducing groundwater to –3m and maintain this level by dewatering, the resulting ground water will be collected and processed by this project.
From the pump station, the outfall tunnel extends 10.2km offshore and connects via a riser shaft to a diffuser field. This arrangement will allow the safe and environmentally compliant discharging of storm and ground water flows into the Gulf (see Figure 1).
TUNNEL BORING MACHINED (TBM) SELECTION
The Contractor for this project was a Joint venture between HBK (Qatar) and PORR (Austria). The TBM selected to bore the outfall tunnel was manufactured by CREG/Wirth as a 4.3m diameter earth pressure balance (EPB) machine, designed to support a working pressure of 4.5 bar (full hydrostatic). See Figure 2.
The configuration of the EPB machine had a cutterhead, excavation chamber, main bearing, screw conveyor, front, middle and tail shields, personnel locks, thrust system, articulation, erector, and brushes as part of the tail shield seal system.
In addition, there were 18 gantries, 160m long, accommodating all electromechanical parts, additive tanks, and rescue chambers. The TBM was designed to install a universal segmental ring lining with a width of 1.3m.
The TBM was equipped with:
- Optimised cutting tool combination for hard rock and soft rock Open mode or closed mode operation
- Materials and equipment to support a maximum face working pressure of 4.5 bar
- Eliminate the risk uncontrolled water inflow into the tunnel during construction
- The screw conveyor had two sets of gates, for emergency purpose in high pressure
- Long screw conveyor to dissipate the maximum pressure of 4.5 bar
- Extra row of wire brush seals
- Removable probe drilling machine
- Real time probing system using BEAM (The Boretunnelling Electrical Ahead Monitoring).
GEOTECHNICAL CONDITIONS
Ashghal undertook a significant geotechnical investigation programme for this project prior to tender stage. The investigation programme consisted of drilling 22 offshore boreholes at 500m centres, which provided the following geotechnical data:
- Cores for inspection
- In-situ packer tests.
- Pressure meter tests at each location.
- Geophysical surveys, including Sonar Bathymetry, Magnetometer, Seismic Reflection and Seismic Refraction.
- Laboratory mechanical and chemical tests on soil, rock, and water samples.
This investigation data was combined on a long geotechnical profile and became the basis for selection of the selection of TBM to bore the offshore outfall tunnel and the type of concrete segmental lining.
Although there was significant TBM tunnelling experience in the Doha area, there was no previous experience of subsea tunnelling to draw upon in preparing for the offshore tunnelling work. The geological data, shown in the profile, indicated the rock quality to be competent along the tunnel alignment.
However, there were two major tunnelling risks to be considered by the contractor and their designer.
Karstic Features
The first challenge for the TBM boring of the offshore outfall tunnel was the possibility of the machine encountering karstic features along the alignment.
Although the tender geotechnical investigation showed no significant evidence of such features, the TBM was equipped with grouting ports ahead and around the shield plus associated drilling facilities to deal with such geological features should they be encountered.
Hydraulic Connection to Seabed
The second risk was associated with the connection between the seabed and the ground ahead of the TBM as it bored the outfall tunnel. In this case, full hydrostatic water pressure would be directed to the advancing EPB cutterhead, requiring increased face pressure and thrust to advance the tunnel.
The EPB TBM was designed to withstand the 4.5 bar pressure. There was significant evidence from the existing geotechnical investigation that in 85% of tunnel alignment the geology comprises of competent and/ or low permeability rock, at least one tunnel diameter above the crown of the tunnel.
Geological Formations Expected in Tunnel Alignment
Referring to the borehole information and additional geophysical investigation, the TBM would drive through three general geological formations: Rus Formation (RF), Midra-shale (MS), and Simsima Limestone (SL). As the TBM passes from one to another the excavation face will included both strata. Water pressure both Low (LP) and high (HP) were expected but the extent was not known.
The TBM excavated through seven zones, as follows:
- Rus Formation (RF) – full face
- Rus Formation with Midra-shale (MS)
- Midra-shale – full face
- Midra-shale with Simsima Limestone (SL)
- Simsima Limestone – full face (Low Pressure (LP) and High Pressure (HP))
- Simsima Limestone with Midra-shale
- Simsima Limestone – full face (Low Pressure (LP) and High Pressure (HP))
The tunnelling works for construction of the 10.2kmlong outfall tunnel were discussed in the first paper on the project. The tunnelling was completed in two years and the TBM reached its endpoint, at the riser shaft, 56 days early, according to the baseline programme.
The riser shaft, though, had to be prepared in advance of the TBM arrival. The offshore works involved a number of complex steps being executed, highly accurately, as part of the preparations and then the receival of the TBM, and then letting it pass beyond, even if only for a short distance farther.
The preparations included:
- Excavation of the shaft below the seabed from a platform rig
- Priming the shaft at depth to receive the TBM
- Then, ensuring accurate and safe penetration and pass-through of the shaft structure by the TBM
- After the transit, the shield would then bore a short distance farther. The TBM equipment would then mostly be dismantled and removed and the TBM shield buried
- The stub tunnel would then be backfilled
- The riser shaft would have a temporary cap installed, upon removal of which the system could be flooded for the start of its operational life.
OFFSHORE DIFFUSER FIELD STRUCTURE
To disperse the outfall flows into the sea, the project uses a marine diffuser field structure. The discharge from the diffuser bed, located just above the seabed, is via 84 duck bill valves located with equal spacing along six radial arm structures.
The key linking structure from the diffuser field structure down to the outfall tunnel, under the seabed, is the riser shaft. Connecting both is the central manifold, located directly on top of the riser shaft and is a concrete structure measuring 4m x 4m x 4m and weighing 120 tonnes.
The radial arm structures with the duck bill valves are either directly connected to the manifold or are connected by short secondary manifolds. Each arm is 147m long and is separated from the adjacent arm by 20m. This makes the entire structure outline 294m x 40m. The secondary manifolds and six arms are manufactured out of high density polyethylene (HDPE) pipe and are of various diameters (2000mm, 1400mm, 1200mm and 710mm to ensure an even flow throughout the various arms and varying flow rates from the pump station.
RISER SHAFT STRUCTURE AND CONSTRUCTION
The riser shaft connects the TBM-bored, segmentally lined, outfall tunnel to the marine diffuser structure which sits on the seabed. The invert of the outfall tunnel is 14.5m below the seabed and the depth of the sea at the location of the diffuser field structure is 15.5m.
The specialist contractor engaged by HBK PORR JV for the section of the work involving construction of the riser shaft was MIC (W.L.L).
First stage of construction was carried out by a dredger, which removed an average depth of 2.5m of loose sediments from the planned area of the seabed around the location of the diffuser structure. A further 2m of the seabed was excavated directly over the location of the riser shaft to expose competent strata for shaft drilling operations.
With the dredging complete, the next stage of construction was carried out from a jack-up rig that was located adjacent to the centre line of the riser shaft. On the side of the rig, a cantilever frame was installed to house the drilling equipment, directly over the site of the riser shaft. The drilling rig or Bottom Hole Assembly (BHA) is shown in Figures 3 and 5.
The BHA was lowered to the precise location where the riser shaft was to be excavated and it commenced drilling. It could drill various diameter holes by use of extending hydraulically activated arms to suit the desired diameter of a shaft. For this project, the first section of the riser shaft was drilled to 4m diameter, and then the arms were extended out to enable a 5.3m diameter excavation for the next, deeper, excavation.
The 5.3m diameter section was needed for the riser shaft to be wide enough for the approach of the 4.3m diameter EPB TBM. The TBM would approach, penetrate into and then pass through the shaft with only small clearances of approximately 500mm, either side, to the shaft walls (see Figures 4 and 10).
On completion of drilling for the riser shaft, the BHA was withdrawn. A 3D internal survey of the shaft was then carried out to determine the exact dimensions and position of the riser shaft (see Figure 6i).
The next stage was to install a steel casing into the top 3m of the shaft, to provide stability and prevent inward collapse of the shaft. Four concrete manifold foundation blocks were then positioned and levelled to the correct requirements and connected to the steel casing. A manifold concrete base slab (see Figure 6ii) was put on the blocks.
The internal lining of the riser shaft would be made from glass reinforced plastic (GRP) and projects downwards from the base of the concrete manifold to 500mm above the TBM outfall tunnel. The GRP lining is cylindrical in shape with a solid end at the lowest level (closest to the TBM tunnel) and a removable flange at the other end. To prepare it for insertion into the bored shaft, the upper flanged end was removed and a temporary steel support frame was installed within the GRP cylinder. The assembly was then lowered into the riser shaft (see Figure 7).
The GRP cylinder was positioned centrally within the shaft and connected to the base slab. At this point, the riser shaft was still full of seawater. Temporary kentledge was placed on top of the whole structure to prevent uplift during the grout placement.
The void between the GRP cylinder and shaft walls, and also below, in the whole of the lower portion of the wider shaft, was filled with a grout mixture. The operation was performed in a controlled manner, the grout fill slowly displacing the seawater. The grouting equipment and operations are shown in Figure 8. When the grout was cured the kentledge could be removed, followed by the internal steel support frame (see Figure 9). The upper flange of the GRP cylinder was secured and sealed.
The top and bottom ends of the GRP cylinder had a series of valves fixed to them to allow venting of air/water. The upper valves are then connected to compressed air to remove all the seawater within the GRP cylinder and leave it, secured in the grout envelope, with only air inside. The grout envelope in the annulus around the GRP cylinder, and also in the wider shaft below, shown as blue colour infill in Figures 4 and 10. The central manifold which collects the combined ground and storm water from the outfall tunnel and then distributes evenly to the diffuser structure, was constructed onshore. Taken offshore, the heavy concrete manifold block was lowered into position on top of the riser shaft and connected to the manifold base slab (see Figure 6iii).
Piece by piece, the arms of the diffuser field structure would be connected to the central manifold, along with the 84 duck bill valves. The arms were then covered with backfill material followed by installation of a layer of scour protection, covering the entire diffuser bed structure.
With the riser shaft structure constructed and sealed, it was secure and ready to receive the TBM that has been boring the outfall tunnel.
OUTFALL TUNNEL TO RISER SHAFT CONNECTION
In preparing to meet the riser shaft, the TBM excavated up to within 5m of the shaft location (see Figure 4iv). The EPB cutterhead was then depressurised.. The situation was monitored for any water inflow over 8 hours.
With no water inflow detected, the TBM then advanced to excavate through the lower, wide and grout-filled, portion of the riser shaft (see Figure 10 i-iii). Slowly, the EPB machine bored through the grout plug and erected more segmental ring lining. Then the TBM advanced to begin to leave the grouted shaft zone. The machine advanced beyond the riser shaft for a short distance more and stopped 25m beyond and would go no farther.
The backup train was withdrawn. Some parts of the shield machine were dismantled and removed (see Figure 10 iv).
In the shaft area, the tunnel was prepared to take further, fixed, lining elements needed to link to the riser zone which was sealed above and beyond the tunnel. The tunnel crown was opened, and short probe holes drilled into the bottom of the riser shaft. Minimal water was encountered and as such the next phase of connecting fully the outfall tunnel and riser shaft was completed safely.
The remainder of the stub tunnel with the last parts of the TBM was buried (see Figure 10 v-vi).
With the tunnel now ending below the riser shaft, the final infill and profile lining was performed to create a vertical hydraulic bend to suit the future flow needs of the full outfall tunnel, when in operation.
For the final stages of the works, the air was vented from the combined outfall tunnel and riser shaft. The venting took place in a stepped process to enable the tunnel to be filled with inland water, ready for operation (see Figure 11).
CONCLUSIONS
At the early stage of the project this key element of the project was designed, planned and the methodology developed, analysed, and refined several times to arrive at the optimal solution. Also considered and evaluated were all the possible issues that might affect the tunnel transportation system for the removal of the TBM and the construction work required for a successful connection.
Identifying the key risk items early in the process of project development for Musaimeer sea outfall and devising mitigation measures with backup plans were vital preparatory steps to ensure completion of the work without delay.
The Authors would like to thank all parties involved in the completion of this challenging project and in particular the completion of this critical activity of the riser shaft construction and connection to the outfall tunnel