Screw Anchor & Helical Pile FAQ
Short answers on how screw anchors hold, how they go in, how long they last, and what the permit file needs.
Frequently Asked Questions — Technical & Project Planning
Steel shafts with one or more helical plates, screwed into the ground by hydraulic torque instead of being dug in or driven. Once at depth, the plates bear against the soil and carry tension, compression and lateral load. They go by several names: screw anchors, helical anchors, screw piles, helical piers, torque anchors and earth anchors. Installation torque is recorded as each one goes in and converted to a calculated holding capacity (Qult = Kt × T).
A concrete block holds by its weight and its friction on the bottom; a steel anchor by being dragged until its fluke (the blade at its tip) digs in; a screw anchor by plates bearing on the soil at depth. The screw anchor holds as a fixed point at the coordinates you give, on a much shorter scope (about 2:1 instead of 5:1), with no ground chain. Its limit: it needs a seabed it can be screwed into, not rock.
From the installation torque. The final torque (T), averaged over the last three helix diameters of penetration into the soil, is multiplied by the published torque factor (Kt) for the anchor's shaft size to give its ultimate capacity: Qult = Kt × T. Every anchor gets a torque record with that calculated capacity before the crew leaves. Where the project calls for it, a pull test on selected anchors confirms the estimate. Your engineer of record decides what the file needs.
The same day. Once an anchor reaches its target installation torque it can take its design load: there is no concrete to cure and no grout to set. Your mooring, dock, farm structure or foundation connection can be made while the crew is still on site, which matters most when the work has to fit a seasonal window.
Typically 2 to 6 metres into the seabed, but there is no fixed depth. The anchor goes down until its plates reach soil firm enough for the required capacity, which the installation torque over the last three helix diameters shows: less in firm, dense soil, and in soft harbour mud shaft extensions take the plates deeper. The crew stops at the target torque, not at a set depth.
Yes, as long as firmer soil can be reached at depth. Shaft extensions carry the plates past the soft surface layers, larger plates bear on more soil, and extra plates along the shaft engage several depths at once. Capacity then depends on the strength of that deeper soil, which is why we review the soil data before choosing the configuration.
Well, when they are sized for it. Screw anchors recover from repeated wave and current loading when it stays well below their ultimate capacity (Qult). We size marine installations so design storm loads stay well below Qult, with safety factors from a site-specific load analysis, and the installation torque then gives each anchor's calculated capacity.
Service life is set mainly by corrosion. The anchors are hot-dip galvanized to ASTM A123/A153, and how long that coating lasts depends on its thickness and the exposure: splash zone, tidal zone, permanently submerged or below the mudline. Below the mudline, where the load-bearing plates sit, oxygen is scarce and corrosion is slow. Aggressive sites get thicker galvanizing or an added coating. Each project gets a site-specific corrosion design, not a blanket lifespan figure.
Very little. The anchor is turned in, not dug or hammered: no sediment is brought to the surface, turbidity is low, vibration is near zero, and there is none of the impulsive underwater noise of pile driving. That suits work near sensitive habitats and next to existing structures, though in-water work windows are set by the permitting authority for each site.
They leave far less behind than a block and chain. Nothing is dug out, only the anchor head stands at the seabed, and with a floating rope no chain sweeps the bottom, so a ring a chain kept bare is free to grow back. The galvanizing releases zinc slowly as it protects the steel; whether that matters at your site is a question for its environmental assessment, and we supply the full coating specification to answer it.
A mooring with no chain on the bottom. A screw anchor turned into the seabed replaces the concrete block, and a floating rope replaces the ground chain, so nothing sweeps the seabed between the anchor head and the buoy. The ring a chain kept bare is free to grow back with seagrass such as eelgrass or Posidonia. Whether an authority encourages or requires conservation moorings depends on the jurisdiction and the habitat.
Usually, yes. In most jurisdictions in-water mooring works need authorization; in the United States that is typically a Section 10/404 permit from the U.S. Army Corps of Engineers, plus state and local approvals. A conservation mooring has less habitat impact to document (no excavation, no spoil, no chain on the bottom), which makes the application simpler to prepare, though no outcome is guaranteed. We supply the installation records, torque logs and specification for the file.
The project, not a fixed price per anchor. The main drivers are the design load (which sets the shaft, plate size and number of plates), the soil, water depth and access, the number of anchors and the mobilisation, the corrosion protection, and how much documentation and permit support you need. Against a block mooring, there is no crane barge to hire and no curing to wait for. Send the site for an estimate.
Ask it about your own site.
Share your soil report, target loads and jurisdiction. An engineer replies within 24 hours with questions and a first read; the full proposal follows within two business days.