Rock armour placed along a coastal edge as a primary erosion-control measure

Rock armour breakwater. Source: Geograph / Wikimedia Commons (CC BY-SA 2.0)

Background and failure mechanisms

Coastal slopes in Italy fail through a range of mechanisms depending on the underlying geology. On the Ligurian coast, thin-bedded flysch sequences dip toward the sea; when rainfall saturates the weathered upper layer, translational slides occur along the contact between soil and intact rock. Along the Calabrian coast, highly fractured crystalline rocks can produce rapid block toppling near the cliff edge when wave action undercuts the toe.

In both settings, the factor of safety — the ratio of available shear strength to the driving forces acting on a potential failure surface — needs to be raised above a threshold typically around 1.3 for permanent works. Anchoring systems achieve this by applying a load that acts against the direction of potential movement.

The Italian standard NTC 2018 (Norme Tecniche per le Costruzioni) sets the design framework for geotechnical structures including ground anchors and retaining walls. Anchors on coastal slopes are also subject to verification under EN 1997-1 (Eurocode 7).

Ground anchors

A ground anchor transmits a tensile load from a structural element at the surface into a stable zone of soil or rock at depth. The basic components are a steel tendon (strands or bars), a grouted fixed length bonded to the surrounding ground, and a bearing plate or waler beam at the head.

Temporary versus permanent anchors

Anchors intended to remain in service for more than two years are classified as permanent and require corrosion protection. On the coast, where chloride concentrations in the soil are elevated by sea spray and tidal fluctuation, double-corrosion-protected strands — encased in a corrugated HDPE sheath with a grease-filled space — are standard for permanent applications.

Typical installation sequence on coastal slopes

  1. Drill a borehole at a pre-determined inclination (commonly 10–25° below horizontal to favour grout flow).
  2. Insert the tendon and inject cement grout from the bottom of the fixed length upward using a tremie tube.
  3. Allow grout to cure to the required strength before stressing (typically 7–14 days depending on mix design and temperature).
  4. Stress the tendon to the design load using a hydraulic jack and lock off with a wedge anchor or nut.
  5. Proof-test to verify load transfer to the fixed length; accept if creep under the test load remains within the acceptance criteria of EN 1537.

Soil nailing

Soil nails are passive inclusions — they are not post-tensioned. Installed at close spacing (typically 1.0–1.5 m in a grid), they mobilise tension and shear as the slope moves. The face between nails is usually protected by a shotcrete layer, sometimes reinforced with wire mesh or geogrids, to prevent surficial ravelling.

On coastal slopes, soil nailing is well suited to moderately steep faces (35–55° from horizontal) in cohesive soils or decomposed rock. In purely granular coastal sands, grout take can be unpredictable due to the open void structure; in such cases, micropile arrays may be preferred.

Gabion retaining wall at St Abbs Harbour showing wire mesh baskets filled with stone

Gabion retaining wall at a coastal harbour. Source: Geograph / Wikimedia Commons (CC BY-SA 2.0)

Gabion and gravity retaining structures

Gabion walls — wire mesh baskets filled with stone — are a common gravity-retaining solution on lower coastal slopes where the required retained height is modest (typically under 4–5 m). Their principal advantage on coasts is permeability: water moves freely through the fill, preventing hydrostatic pressure build-up behind the wall.

On the Tuscan and Ligurian coasts, gabions are often combined with geotextile filters on the soil-side face to prevent migration of fine particles through the mesh openings. Basalt or limestone fill is preferred for its durability in salt-spray environments; calcareous stones from local quarries can be used where they have demonstrated adequate abrasion resistance.

Monitoring anchor performance

Permanent anchors in critical locations are instrumented with load cells that allow the retained load to be checked over time. Load loss can indicate corrosion of the tendon, grout deterioration, or slow deformation of the slope. Italian practice typically specifies annual inspection for coastal anchors in service, with full re-testing if the measured load falls below 80% of the lock-off value.

Further reading

  • ISPRA — National landslide risk mapping and coastal dynamics reports
  • EN 1537:2013 — Execution of special geotechnical works: Ground anchors (European standard, publicly listed)
  • EN 1997-1 — Eurocode 7: Geotechnical design (BSI/CEN)
  • Consiglio Nazionale degli Ingegneri — NTC 2018 implementation guidance