Slope stabilization

The problem

Slope instability happens at two different scales, and confusing them leads to the wrong system. Shallow instability is the surface-parallel sliding of the soil cover or the weathered rock crust, the peeling of blocks and erosion by rain; its depth rarely exceeds a few metres. Deep instability concerns the whole mass: the slip surface passes below the anchor length and cannot be solved by covering the face with mesh. Highway cuts, railway slopes, mine benches and excavation faces can carry both at the same time.

The warning signs are read on the surface. A tension crack at the crest and bulging at the toe point to deep movement; curved tree trunks, a sagging shoulder, longitudinal cracks in the pavement and muddy water from drain holes belong to the same family. Shallow movement shows as fines collecting at the toe, rills and gullies, and crust fragments peeling off the weathered layer. Water is the trigger or the accelerator in almost every case; drainage is therefore not an add-on but part of the solution itself.

How we decide

The measurement sequence: slope geometry and height, orientation and spacing of the discontinuity sets, depth of weathering, thickness of the soil cover, an estimate of the slip surface depth, rock mass classification, groundwater level and its seasonal change, depth to sound rock. Seismic action enters as a horizontal coefficient under the applicable national code (TBDY 2018 in Türkiye).

The last row is the critical one. With a deep slip surface, covering the face with mesh does not stop the movement; it only hides it.

System family

Anchored mesh kits are assessed under EAD 230025-00-0106, which defines the resistances against puncturing, shearing under the plate and surface-parallel tension. Ground anchors fall under EN 1537, hexagonal double-twist mesh under EN 10223-3, wire under EN 10218-2, coatings under EN 10244-2 and EN 10245-3, shotcrete under EN 14487-1 and EN 14487-2.

Installation

The sequence starts with making the slope safe. Scaling and vegetation removal proceed from the top down; when this step is skipped, material that moves later remains under the mesh. Then come the setting-out of anchor axes, drilling, hole cleaning, bar placement and grouting. After the grout has set, the mesh is laid, plates are fitted and pre-tension is applied by torque. Where shotcrete is needed, it is applied after the mesh and the drainage weep holes are in place.

On steep faces, access is by experienced rope-access crews. Control points: drilling length and direction, grout volume compared with the theoretical volume, plate seating, torque value, mesh overlap and clip frequency, weep hole spacing, shotcrete thickness.

Acceptance and maintenance

The backbone of acceptance is the anchor pull-out test: on selected anchors a test load derived from the design load is applied, displacement is read, and if the criterion is not met the number or length of anchors is increased. The number and method of tests are written into the contract at the start. The handover file also carries the as-built anchor plan, grouting records, the pre-tension torque list, mesh overlap measurements, shotcrete thickness and core results.

Maintenance watches for settlement under plates, corrosion at anchor heads, tears and sagging in the mesh, clogged weep holes, cracks in the shotcrete and the width of the crest crack. Where measurable movement is expected, a remote monitoring system (Turkish page) reduces the inspection frequency.

Frequently asked

• Is scaling alone enough? Scaling removes loose material but does not stop weathering from producing new blocks. A lasting solution needs mesh, bolts or a barrier.
• Who decides the anchor length? The length is calculated from the slip surface depth and the bond length required in the sound zone, and the calculation is verified by the pull-out test.
• Does the specification have to name a brand? No. When performance values and the scope of the assessment document are written, competition is preserved and bids become comparable.