Landslide mitigation

[1] The proposed scheme was elaborated taking three principles into account: The measures that can be achieved to reduce the effects of water can be shallow or deep.

In rocks, the choice of drain spacing, slope, and length is dependent on the hillside geometry and, more importantly, the structural formation of the mass.

Features such as position, spacing and discontinuity opening persistence condition, apart from the mechanical characteristics of the rock, the water runoff mode inside the mass.

Sub horizontal drains are accompanied by surficial collectors which gather the water and take it away through networks of small surface channels.

The charge fuse times are established so that those at the outer edges explode first and the more internal ones successively, so that the area of the operation is delimited.

The protection of natural and quarry faces can have two different aims: Identification of the cause of alteration or the possibility of rockfall allows mitigation measures to be tailored to individual sites.

The most-used passive protection measures are boulder-gathering trenches at the foot of the hillside, metal containment nets, and boulder barriers.

This method is effective for correcting shallow forms of instability, where movement is limited to layers of ground near the surface and when the slopes are higher than 5m.

The process of infill at the foot of the slope may include construction of berms, gravitational structures such as gabions, or reinforced ground (i.e., concrete blocks).

When planning this type of work the stepping effect of the cuts and infill should be taken into account: their beneficial influence on the increase in safety factor will be reduced in relationship to the size of the landslide under examination.

An important aspect of stabilisation work that changes the morphology of the slope is that cuts and infill generate non-drained charge and discharge stresses.

These provisions will serve the purpose of avoiding penetration of the landslide body by circulating water or into any cracks or fissures, further decreasing ground shear strength.

This process tends to weaken the slope by removing material and triggering excess pore pressures due to the water flow.

The reduction in pore pressure by drainage can be achieved by shallow and/or deep drains, depending on hillside morphology, the kinematics of movement predicted and the depth of creep surfaces.

Traditional drainage trenches are cut in an unbroken length and filled with highly permeable, granular, draining material.

The draining elements are microdrains, perforated and positioned sub-horizontally and fanned out, oriented uphill to favour water discharge by gravity.

In this way the water discharge takes place passively, due to gravity by perforated pipes with mini-tubes, positioned at the bottom of the wells themselves.

The use of isolated wells with drainage pumps leads to high operational costs and imposes a very time-consuming level of control and maintenance.

Deep drainage trenches consist of unbroken cuts with a small cross-section that can be covered at the bottom with geofabric canvas having a primary filter function.

[2][3] The hollow part is filled with compacted, high-permeability gravels and can drain water via a vertical drain-pipe or sub-horizontal pipes connected to the slope surface.

This solution requires the installation of a series of micropiles that make up a three-dimensional grid, variably tilted and linked at the head by a rigid reinforced concrete mortise.

In practice, those piles in the most unstable area of the slope are positioned first, in order to reduce any possible lateral ground displacements.

Preliminary design methods for the micropiles, are entrusted to computer codes that carry out numerical simulations, but which are subject to simplifications in the models that necessitate characterizations of rather precise potential landslide material.

The soil nailing technique applied to temporarily and/or permanently stabilise natural slopes and artificial scarps is based on a fundamental principle in construction engineering: mobilizing the intrinsic mechanical characteristics of the ground, such as cohesion and the angle of internal friction, so that the ground actively collaborates with the stabilisation work.

Drainage is important to the CLOUJET method since the hydraulic regime, considered in the form of pore-pressure applied normally to the fractured surfaces, directly influences the characteristics of the system.

The drained water, both through fabric and by means of pipes embedded in the ground, flows together at the foot of the slope in a collector installed parallel to the direction of the face.

The phases of jet-grouting work are: (see[4]) The high energy jet produces a mixture of the ground and a continuous and systematic "claquage" with only a local effect within the radius of action without provoking deformations at the surface that could induce negative consequences on the stability of adjacent constructions.

Depending on the characteristics of the natural ground, the type of mixture used, and work parameters, compression strength from 1 to 500 kgf/cm² (100 kPa to 50 MPa) can be obtained in the treated area.

The realisation of massive consolidated ground elements of various shapes and sizes (buttresses and spurs) within the mass to be stabilised, is achieved by acting opportunely on the injection parameters.

Another method for improving the mechanical characteristics of the ground is thermal treatment of potentially unstable hillsides made up of clayey materials.