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The Moliniere landslide

It is good to see that plans are underway to repair the Moliniere Landslide (“the Slide”). The repairs to the Slide proposes an extensive geotechnical investigation which will likely include the mobilisation of expensive drilling equipment in order to advance a number of soil borings to further evaluate the geology of the area and assess the slide plane along which the Slide is occurring.

Because of the need for specialised drilling equipment that is not available on the island, the geotechnical investigation will likely require approximately 6 months to complete and will cost over $100,000 EC.

In the meantime, there is evidence that the Slide continues to move and it is possible that prior to completing the geotechnical investigation the Slide could rupture and hence the initial geotechnical investigation could provide little data that could not otherwise be obtained during the excavation of the Slide.

The following is an approach using a series of steps that eliminates the initial geotechnical investigation thereby reducing the time required to repair the Slide and road with significant cost savings.

Step 1 – Secure and stabilise the Slide by preventing further movement above the escarpment and within the Slide. This can be done by installing sheet pile walls at strategic locations within the Slide to further reduce movement along the slide plane;

Step 2 – Create a surficial geologic map of the Slide. The map will identify all cracks, fissures, surficial materials and various features within the Slide. Surveyed elevation and markers within the Slide will be identified on the map as well as dimensions, including length and width of the Slide as well as vertical displacement and width of cracks and fissures;

Step 3 – Remove the existing vacant buildings within the Slide using the excavators;

Step 4 – Using excavators and bulldozers, already present on the island, begin removing soil from within the slide plane. This removal will begin at the top of the Slide to reduce the mass of the soil at the head scrap and further reduce the potential for additional sliding;

Step 5 – Remove the loose soil along the slope of the Slide creating a 3:1 (H:V) slope or less or as determined by the project engineer. The side slopes of the excavated Slide will also be reduced to 3:1 (H:V) or less, as determined by the project engineer and tracked walked to stabilize them; and

Step 6 – Once the excavated slopes have been further stabilised by removing the loose soil, further assess the geology within the Slide using a series of test pits or trenches, created using the excavator, to determine the potential depth of the slide plane. Determining the depth of the slide plane is critical to designing and building retaining walls that will be required to rebuild the slope and road that is so critical to the flow of traffic along the west coast of the island.

Patriotic Grenadian

Editor’s Note: The writer of the above is a Grenadian living in the Diaspora who is a qualified Geologist.

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