The Stabilization of a Landslide Using Trench Drains ... - Harpo spa

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EuroGeo4 Paper number 248Figure 2. Geological and morphological mapGEOTHECNICAL MODELOn the basis of geological and geotechnical research, lab tests and surface maps and monitoring, the instabilitywas defined as a surface roto-translational landslide. The sliding surface, started by saturation of soils characterized bypoor geomechanical properties, was identified at a 3 through 5 depth from the ground level. Two geotechnical unitsand their relative mechanical parameters were established (Table 1).Table 1. Summary of geotechnical parametersGeotechnical units Silty clay Clayey sandUnit weight (γ) (kN/m 3 ) 20 18Friction residual angle (ϕ r ) 15 23Cohesion (c r ) 0 0On the basis of such, a series of simulations with the Limit Equilibrium were carried out by adopting Bishop’s andSimplified Janbu’s theories in order to analyse safety factor values of total stability on water level depth changing.From such calculations it was observed that the critical condition (FS=1) took place at an average -2.5 m water depth.Furthermore, it was analysed that decreasing the water level to an average value of -3.5 m, the FS value was higherthan 1.15 which means the sufficient condition for an area to be stabilized.Figure 3. Geological section.INTERVENTION CHOICESOnce defined geotechnical mode and instability reasons, it was decided to stabilize the slope through thefollowing interventions:• carrying out of a trench drains system;• surface rainfall waters control;• carrying out of a gabionade at the slope bottom;• rebuilding of the wall parts which were heavily deformed and of the fence above.2
EuroGeo4 Paper number 248Figure 4. Plan of intervention.Design of trench drainsUsually the drainage system is plan to be carried out by placing a series of trench drains in a parallel way alongthe slope covering an area wider than the instability to reach the designed water control on the whole interested area.In particular cases, especially when referring to waterproofing substrate thickness, different geometries can be adoptedaccording to the specific case under studying. In fact, one of the main problems in predicting a drainage systemconcerns the placing choice as well as trenches spacing and depth. In the last decades, different studies have beencarried out on that subject in order to analyse both the transitory phase and that of steady motion through filteringmotion analysing. Some of these studies showed charts determining trench drains depth and spacing according toborder conditions and requested water effectiveness (η). Following these principles and predicting to decrease theslope down to an average depth of 3.5 m, a trench drains system was determined with a 2 through 5 m depth placingtrenches along the slope with i= 10 m spacing. To estimate the water quantity to be drained it was cautiouslyconsidered to have average horizontal “k” permeability equals to 1E-03 cm/s and corresponding to a “q” water supplyof 0.01 1/s sqm of excavation walls.Choice of materials for draining bodyThe most widespread traditional technique to carry out trench drains predicts the execution of soil excavation in arectangular or trapezoidal section which will be afterwards filled using permeable material. A first layer of gravel set,a draining pipe is placed to help waters flowing and finally the remaining draining material is laid. The upper part isthen “clogged” with low-permeable material or more often with the excavation material itself which is compressed.The traditional technique showed in Figure 5 predicts the following work programme:• transport of gravel to yard;• execution of excavation;• placing of no woven geotextiles as filter-separating element;• placing of drainage pipe (often carried out by workers going down into the excavation);• filling with gravel;• upper closing of gravel by no woven to avoid clogging;• placing of upper soil layer;• transport of excavated material to dump.Figure 5. Trenches schemeThe correct carrying out of that work programme usually requires relatively long time periods changing according totrench drain depth, yard entering conditions and mechanical properties of excavated soil (easy excavation andsidewalls stability). Taking into account that technical times and self-stability times of excavation sidewalls arerelatively short, the section must often be enlarged or strengthened in comparison with previous design planning in3
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