DAM/Specific/TBT-DAM-018

Dam Core and Cutoff Construction

Dams, Reservoirs & Flood Defence › Specific › Dam Core and Cutoff Construction

Presenter		Project
Location		Date

The dam core is the central impermeable zone that prevents water seeping through the embankment body.
Cutoff walls extend below the dam into impermeable strata to prevent water bypassing under the embankment.
Core materials such as puddle clay, bentonite, and concrete must be placed and compacted to strict specifications.
Cutoff construction methods include sheet piling, slurry trench, jet grouting, and deep soil mixing.
The quality of the core and cutoff determines the long-term safety and integrity of the entire dam.
Working in deep trenches for cutoff walls creates excavation collapse and confined space hazards.
Bentonite slurry used in cutoff trench construction is slippery and creates significant slip hazards on site.
Compaction testing of core material must verify density and moisture content at specified frequencies.
The Reservoirs Act 1975 requires dam construction to be supervised by a qualified civil engineer (Panel Engineer).
Weather conditions significantly affect core material placement — rain can render clay cores unsuitable.

Dam safety	A defective core or cutoff allows uncontrolled seepage that can lead to catastrophic dam failure and flooding.
Excavation hazards	Deep cutoff trenches create collapse, drowning, and confined space risks for workers below ground.
Legal duty	The Reservoirs Act 1975 and CDM 2015 require competent supervision and strict quality control throughout.

Don't

Place and compact core material in layers to the specification overseen by the Panel Engineer
Test core material density and moisture content at the frequencies specified in the design
Support deep cutoff trenches with sheet piling or slurry stabilisation before entry
Wear slip-resistant footwear when working near bentonite slurry operations on site
Monitor weather conditions and stop core placement when rain makes material unsuitable
Record all compaction test results, material sources, and layer depths for the dam record
Ensure the Panel Engineer inspects and approves each critical stage of core construction
Contain all bentonite slurry within bunded areas to prevent it reaching watercourses
Brief workers on the specific excavation, slurry, and quality control hazards each shift
Maintain safe access and egress from cutoff trench excavations at all times during work

DON'T place core material in conditions the Panel Engineer has not approved for compaction
DON'T enter deep cutoff trenches without excavation support and confined space controls
DON'T skip compaction testing — unverified layers may allow seepage through the dam core
DON'T allow bentonite slurry to spill onto walkways or enter surface water drains
DON'T continue core placement during rainfall that raises moisture above specification limits
DON'T use unapproved fill material in the core zone without the engineer's written consent
DON'T stand near the edge of open cutoff trenches filled with bentonite slurry
DON'T proceed past any hold point without the Panel Engineer's formal inspection and approval
DON'T compact with equipment that does not meet the specification for the core material type
DON'T treat dam core construction as routine earthworks — the quality standards are critical

See also: Dam and Reservoir Safety Awareness | Embankment Dam Construction

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