Cut-off Wall complete ahead of schedule

In preparation for the commencement of the Hinze Dam Stage 3 Project, geological and record assessments were conducted. The assessments showed that Stage 2 grouting was not completely effective resulting in water seepage through the right abutment.   The Stage 3 upgrade means that the dam water level will increase by 12.3 metres and  the permeability of the hill at the right abutment would be unacceptable.  In order to rectify this issue three aspects were incorporated into the Hinze Dam Stage 3 design:

•  A plastic concrete cut-off wall (up to 54m deep) built through the high permeability zones in the right abutment and keyed into the clay core
• Selective grouting of zones at the eastern end of the cut-off wall
• A filter trench/blanket combination downstream of the core and below the embankment shoulder

The construction of the Cut-off Wall has been completed seven weeks ahead of schedule and involved the excavation of a 218m trench up to 54m in depth and 0.85m wide.  The trench was excavated in an alternating sequence of panels using a clamshell grab and a hydraulic trench cutter.  Panels were supported during the excavation process using bentonite slurry and backfilled with plastic concrete using a twin  tremmie placement process.  The excavation of material was completed in stages to ensure that the integrity of the main dam wall was never compromised and safety standards were continually met.

The Cut-off Wall at the Hinze Dam is an extraordinary piece of construction, being the deepest continuous cut-off wall addition to an existing storage reservoir in Australia to date.   

Crest Cut Down Commences

The wall of the Hinze Dam is not just a pile of rocks; it is constructed using a series of layers which keep the water contained. The central structure is a clay core which runs through the middle of the dam wall. The clay core acts as the key water retention element of the dam wall and approximately 190,000m³ of clay will be required. To protect the clay core layers of different sized rocks are placed over the top. The first layer is the filtration level which is made from rock quarried and crushed on-site to produce smaller grades for a Fine Filter and a Coarse Filter. The filtration level protects the clay core from being eroded by the water. The filtration level is then covered by rockfill which offers the final protection against erosion and keeps the clay core and filters in place, giving the dam its strength and assisting in keeping the water in.  

All of the clay and rock required for the clay core and embankment will be sourced on-site which minimises the amount of truck movements on the local roads surrounding the Hinze Dam site. A Dam Safety Management Plan has been established which outlines protocols for backfilling the excavation. If rain, severe weather, or a storm warning is issued by the Bureau of Meteorology the Dam Safety Management Plan will be implemented.