When constructing a new dam (or remediating an existing one) it is important to ensure that water does not flow below the foundation of the dam If it does, then a barrier must be constructed There are different techniques to create such barriers, and one of the most sophisticated, is the construction of a plastic diaphragm (slurry) wall The first step to build the wall, is to create what is called a concrete "guide wall" along the axis of the dam Then, excavation begins with a grab to remove the soils from the top, until a pre-determined depth is reached, or until the limit of the tool (normally 70 SPT)
Grabs are effective to dig through soils such as clays, silts or sands The hydraulic grab has verticality sensors and flaps on its body to ensure that the excavation is vertical While digging, bentonite slurry (or mud) is inserted at the top, so that the excavation does not collapse The panel being excavated is called a "primary" panel At this point, the grab equipment moves to another panel, while a trench cutter (hydromill) system is positioned in the pre-excavated panel The trench cutter can cut through both the soils and harder materials such as rock up to 200 MPa And it can reach depths of up to 250 meters
A pump which is located between the cutter wheels, pumps the material through hoses These hoses run all the way to the top The slurry (with cuttings) then flows to a slurry recycler system Here the material goes through shakers, desanders and desilters, which will then clean/separate the bentonite slurry from the cuttings (rock and soils) in a closed system, so that the clean slurry is then pumped back to the bentonite silos and then pumped back into excavation as the trench cutter advances with the excavation to keep the excavation always full of slurry This is important as to avoid any collapse of the recently excavated panel Once the target depth has been reached, the trench cutter is removed And the panel is prepared to be backfilled with a cement bentonite mixture which gives it the "plasticity" This is a low permeability and low strength mixture Once the first panel is completed, another "primary panel" is built in the same way Panel thickness normally varies between 06m and 1
5 meters The grab (also called clamshell) can reach depths of up to 80 meters Verticality tolerance is normally a function of the depth of the wall and its thickness to ensure that a continuous wall is built As a rule of thumb, a verticality of 1% is utilized even though verticalities of less than 0
5% can be achieved with the advance of the technology and controls in the equipment utilized Once the excavation with the grab is finalized, the trench cutter is positioned, and starts excavating As soon as the excavation of the primary panel is finished, it is backfilled with the cement bentonite (CB) mix (also called coulis) The CB mix is tested before the project starts in a laboratory setting on what is called a bench scale test Here the mix is verified if it obtains the design (or target) criteria for both permeability and strength Once the two primary panels are constructed, the construction of what is called a "secondary" panel begins The trench cutter is utilized to "overcut" the two primary panels so that it guarantees that a quality joint is built Overcuts lengths are also pre-determined, and again depends on thickness and depth of the wall, as to ensure that a continuous wall is built Once excavation of the secondary panel is finished, it is backfilled in a similar manner to the primaries The process is then repeated, primaries being built first, and then secondaries, until the complete wall is finalized This is just an example, of a cutoff wall build with a plastic diaphragm wall There is also another video in the Brasfond channel that shows a wall being constructed with another technique called "secant piles"
The two techniques can be combined to create "combi" walls (a secant pile wall within a slurry wall) In critical situations, where severe piping takes place, a grouting campaign precedes the construction of the wall, so that slurry loss is minimized while excavating the panel Whichever technique is chosen, it is important to create a seepage barrier as soon as possible, so that the dam or reservoir can operate safely Ideally, as shown in this video, seepage barriers are constructed before the construction of the new dam Thanks for watching this video For other videos, please subscribe to the Brasfond Channel