Dikes

DikesMining diamonds beneath Lac de Gras using the open pit method presented Diavik with a unique challenge. Open pit mining could only safely occur if the water of the lake was held at bay with certainty. A unique 3.9 kilometre rockfill dike, called the A154 rockfill dike, thus had to be built into the lake.

The design and construction of the dike was an enormous achievement in a frigid and forbidding setting. Not only did the arctic climate present significant construction challenges, Diavik had to ensure that the construction of the dikes did not blemish the purity of lake water. To determine the best methods for building the dike, Diavik mounted international research and developed new engineering technologies. A dike review board consisting of five distinguished independent dam engineers was appointed to oversee the development of the dike.

The A154 rockfill dike contains four million tonnes of rock of three different size fractions:

  • The lake side of the dike is large run-of-quarry size rock.
  • The central part of the dike is 50 millimetre minus.
  • The pit side of the dike is 200 millimetre minus.

Another two million tonnes make up the dike cap and toe berm.

The water barrier includes a flexible concrete cut-off-wall down the centre of the dike, overlapping jet-grout columns between the base of the cut-off-wall and the underlying bedrock, and pressure-grouted bedrock beneath the cut-off-wall. A piping system on the inside base of the dike collects expected seepage.

Thermosyphons (refrigeration systems) were installed where the dike crosses islands to maintain permafrost. They operate in passive mode in winter and active refrigeration mode in summer. In the dike, several hundred sensors monitor dike integrity, measuring temperature (thermistors), pressure (piezometers), and movement (inclinometers, extensometers, and survey pins). There are 1,600 data collection points in total along the dike.

The A154 dike was made watertight in July 2002. It was pumped dry three months later. Pre-stripping of the lakebed sediment and glacial till began as the water level was lowered. The A154 dike averages 10 metres high, is 28 metres at its deepest and consists of 3.5 million tonnes of rockfill. This dike received Canada's highest award for engineering excellence - from the Canadian Council of Professional Engineers - and the engineering model is now widely applicable across the Tundra.

A new dike, the A418 dike was completed in 2006. It joins the existing A154 dike with East Island. Construction methods were the same as those used in A154 dike - various sizes of rockfill, a central concrete cut-off-wall, grout, and monitoring instrumentation.  A smaller dike - only 1.3 kilometres long - the A418 dike required approximately 1.1 million tonnes of rockfill and is built in water up to 32 metres deep. A significant crushing facility prepared much of the rock before placement using rock from the A154 open pit. The rockfill portion of the A418 dike was completed in 2005, and the dike was made watertight in 2006. Like the A154 dike, fish within the embankment were returned to Lac de Gras and silty water was treated prior to its return to the lake.