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

EuroGeo4 Paper number 248order to avoid collapses during materials placing. Collapses occurring, besides being dangerous for workers,invalidates system continuity and therefore its effectiveness because it can, for example, cause pipe clogging anddamaging.In the last decades, the use of geosynthetics in geotechnical field has progressively got to a more widespread diffusionboth because professionals and firms technically trust these materials and because of their unquestionable advantagesand benefits in comparison with some traditional techniques. In the specific case of trench drains, technology allows ageocomposite to be used as a draining body replacing usual inert material (Figure 5). Such structures, even if ofreduced thickness, are able to drain high water quantities because of their high percentage of core voids (approx.95%), high-permeable filters and reduced clogging. The use of quality materials guarantees a long-term drainagesystem effectiveness avoiding the risk of collapse phenomena, progressive clogging of structures or deteriorationagainst chemical aggressions or microorganisms. The characteristics must be suitably valued during designprogramme to guarantee the effectiveness of the whole drainage system. The use of geocomposite predicts thefollowing work programme (figure 6):• placing of géocomposite at the side of trench axis and connection to collector pipe;• execution of excavation;• filling with material excavation.Figure 6. Trenches construction. A trench path. B geocomposite laid. C geocomposite put near the trench and ready tobe laid. D digger to excavate. E digger to fill the trench.In comparing the work programme to the traditional method it is evident how many advantages there are ifconsidering work programme, economic benefits and quick execution.For such reasons it was chosen to carry out trench drains using draining geocomposites.Design and choice of drainage geocompositeDrainage geocomposite was chosen firstly determining following service life conditions:• highest horizontal pressure operating on geocomposite calculated on the famous Rankine’s formula:• Sa= 0.5 γ h k a = 63 kPa;• lowest water flow capacity of geocomposite at 63 kPa: T= 2q h = 0.10 l/s m;• work service life > 50 years;• test condition of flow capacity: Flexible/Flexible or Rigid/Flexible tests.To value the water flow capacity of geocomposite, particular attention has to be drawn to test choice. In fact, while theF/F tests (flexible/flexible) or R/F (rigid/flexible) tests allow simulating the mechanical geosynthetic/soil interaction,the use of R/R (Rigid/Rigid) test is valid in applications over pseudo-rigid surfaces. The use of R/R data for a trenchesdesign could overestimate the real long term flow capacity of the draining body.On the basis of design conditions it was decided to use a 5 m high geocomposite consisting of a three-dimensionaldrainage core made out of polypropylene monofilaments surrounded by two no woven filtering thermally bondedgeotextiles. In detail the choice of such a material was based on the examination of the lab test results analysing thechanging of draining capacity in time. The results (Figure 7) showed that for a service life of 1000 months (83 years)the drainage geocomposite is able to give a flow capacity which is six times higher than that requested.4