Digging straight down
A clamshell is a two-shell grab that closes vertically, which lets it excavate below the point where a conventional bucket-and-arm geometry runs out of useful reach. It is the standard tool for shafts, cofferdams, deep sumps and dredging at depth.
Mechanical or hydraulic
Hydraulic versions close under cylinder force, giving controlled and repeatable closing pressure at any arm position. They handle denser material and grip better on partial bites.
Mechanical versions close by rope or by their own weight. Simpler, fewer failure points, no auxiliary circuit needed, and better suited to deep repetitive lifts where hose management becomes the problem.
Working depth usually decides between them: past a certain depth, hydraulic hose runs become the limiting practical factor.
Points to confirm
- Closed and open dimensions against the excavation you need to fit into
- Shell weight against carrier capacity at the depth and radius involved
- Whether a rotator is required to orient the bite — usually yes for anything other than a simple vertical shaft
