Completing cut-off wall construction

The Bagatelle Dam, a newly built embankment dam, crossing over the Terre Rouge River, in the west of the Mauritian island, will form a reservoir containing 25 million m³ of water. Due to the demanding geology (basalt in different stages of weathering) an underground barrier is required to cut off potential seepages underneath the embankment dam.

In March 2012 construction of the Bagatelle Dam began. Initially grouting of the subsoil to form a permanent barrier was studied. Experts advising the government on the best option agreed on the design of a barrier wall. The installation of a permanent plastic concrete cut-off wall was accordingly designed and awarded.

As part of its work, Bauer Spezialtiefbau GmbH constructed a 2.4km long diaphragm cut-off wall for the dam, which is executed in three sections: the left and right bank constructed as earth-fill sections (height approximately < 30 m) with a clay core and random fill body, whereas the central part of the dam is a rock-fill section with central clay core (height approximately = 47 m) in the centre part of the dam. Five crawler cranes of the type MC 64, MC 96 and MC 128 with three BC 40 trench cutter, as well as two grabs, were used to install the cut-off wall to the specified depth.

The project is being developed for the Ministry of Energy and Public Utilities of Mauritius, with China International Water & Electric Corporation (CWE) as main contractor.

The last panel of the wall was completed on 9 May 2015, almost a year after work commenced on the project.

The geology

Mauritius was formed by successive volcanic episodes which started in Middle Miocene, about 10 million years ago.
Two magmatic and structural periods are significant:

• The first period breccias and ancient volcanics.
• The second period intermediate and recent volcanics.

Each period comprises of successive cycles of volcanic activity (breccia eruptions, lava flows, pyroclastic emissions) followed by cycles of relative dormancy, with weathering of present layers.

The terrain in the area of the Bagatelle Dam project is generally flat, with very small overall slope towards the coast (West). The Terre Rouge and Cascade Rivers are the main drainages. They form 15 to 20 m deep incisions in residual soils, with steep to very steep slopes. The basalts in different stages of weathering forming the foundation of the new dam are assignable to the Intermediate Series lava. This formation is deeply weathered, the bedrock comprising of sequences of basaltic lavas and pyroclastic interlayers. The thickness of individual basalt layers amounts up to about 10 m.

At the dam the geologists described/divided the basalt in four different units: completely weathered (CW), highly weathered (HW), moderately weathered (MW) and slightly weathered (SW) with unconfined compressive strength of well above 200 MPa. Due to the process described above, very hard basalt layers are underlined by highly or even completely weathered layers, some of which with cracks and fissures carrying ground water.

Design

The aim was to achieve a watertight barrier in the foundation of the dam.

As the permeability of the upper residual soil was not consistent due to the varying degree of weathering of the rock and various soil gradations and pore no systematic distribution within the residual soil was given. Different rock layers with different stages of weathering additionally increased the complexity of geology.

Following the expert assessments a plastic concrete cut-off wall as a permanent durable barrier was planned and designed.
According to the preliminary design the total length amounts to 2.4km, the maximum depth reaches down to between 30 to 36 m, the total area comprises of 56,781m², and the nominal thickness is 800mm. About 20,000m2 of slightly to moderately weathered basalt had to be penetrated over the entire structure for the new dam.

The concrete specification

The specialist contractor had to design, and recommend for the approval by the engineer appropriate concrete mixes for the cut-off wall which meet the specified requirements. Two types of concrete had been identified - the mix #2 with higher compressive strength to be used in the central part under the rock-fill section for the upper part of the plastic concrete cut-off wall.

Requirements of hardened plastic concrete 1 and plastic concrete 2 as per specification are shown in the table.

It can be concluded that a certain mixing of two different concrete loads, or even different types of concrete with mix-specific flowability behaviour placed by tremie technique had to be accepted and allowed for in the design.

Execution of the diaphragm wall

The construction of the two-phase cut-off wall comprised of the following main components:

• Installation of working platforms at different elevations with storm water drainage strong enough to withstand heavy rainfall during the time of wall installation.
• Installation of ramps to reach the different platform elevations.
• Installation of guide walls.
• Pre-excavation and final excavation of the individual panels under slurry support.
• Continuous quality control during execution of the panels including cleaning and preparation for concreting.
• Tremie-concreting.

Challenges during execution

To successfully install the plastic concrete cut-off wall as a seepage barrier, heavy equipment was to be placed along the axis of the barrier wall. This was made possible by the installation of a high quality working platform on top of the upper soil layer of natural ground and the lower level layer of in-situ clay to weathered basalt. This platform had to be able to withstand extreme weather conditions.

Such an "all season working platform" with proper storm water drainage was designed enabling unhindered working throughout the project.

Regular maintenance and permanent drainage during heavy rainfall was essential.

Cutting through rock

The basalt, in the central part at foundation surface, was extremely heterogeneous. Weathering grades and strength of basalt varied significantly. Therefore it was difficult to predict on the basis of a limited number of samples and limited number of testing on such samples. The compression strength with a maximum of 236 MPa measured at samples in combination with the very high destruction effort (measured in kJ/m³) required to break the rock which was to be cut for the excavation of the trenches.

In addition intermingled layers of completely weathered basalt, clayish type of soil, posed additional challenges running the risk to completely block the cutting surface of the cutter wheels.

Due to actual progress for installation of the cut-off wall and milestones to be reached, at peak times three trench cutters and two grab units worked 24 hours and seven days a week.

In moderately weathered basalt an average excavation rate of approximately 1.5 m/hour could be achieved. Cutting times, due to the heterogenic soil-rock, differed significantly along the panel depths. Even neighbouring bites showed recognizable differences in cutting speed.

Keying into competent rock was successfully achieved. In some areas with less detailed upfront geological data significantly more competent (fresh) rock cutting was executed to reach the cut-off wall toe level as per engineers design.

Cut-off wall installation at different elevations assuring a continuous wall

At the central part the working platform elevation was at level 360 m whereas the platform at the adjacent left bank was at approximately level 372 m and at the right bank at approximately level 374 m.

After initial installation of the cut-off wall in the central part, working platforms and ramps had been installed to overcome the elevation change to the banks in certain steps. A particular platform design to assure safe working on backfilled parts was required. Particular care with extensive measuring and controlling was taken to assure the continuity of the cut-off wall at the joints of elements executed from different elevations.

Hardened concrete testing and quality control

To proof and ensure the required hardened concrete properties a site laboratory was equipped with six water permeability testing cells and one triaxial cell for confined compression strength (CCS) and deformation modulus testing.

As different testing conditions such as various confining pressure, back pressure, saturation time or deformation speed and the different methods of CCS testing (consolidated, unconsolidated, drained or undrained) showed significant influence on the confined compressive strength and deformation modulus results, it was obligatory to clarify the exact testing procedure for the particular specified and required concrete properties. Together with all parties involved this test series was executed successfully.

Conclusion

To install a plastic concrete cut-off wall as a positive, clearly defined seepage barrier in challenging ground conditions, the experience of the specialist contractor is essential. Mobilization, setting up on site, testing materials and method were the basis for the successful construction of the specified barrier wall. With good performance of the experienced staff and equipment with low downtime also working 24 hours and seven days, the plastic concrete cut-off wall through rock was installed in time and to the full satisfaction of the client and his representative.

Table: Requirements of hardened plastic concrete 1 and 2
#1 (generally for the cut-off wall)
Permeability: < 1x10-8 m/s after 28 days
Compressive strength (28 days): between 1 MPa and 1.5 MPa
Deformation modulus: 100 to 150 MPa
#2 (for the upper part of the cut-off wall in the central part only)
Permeability: < 1x10-8 m/s after 28 days
Compressive strength (28 days): up to 10 MPa
Deformation modulus: 1 to 5 GPa