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

EuroGeo4 Paper number 248THE STABILIZATION OF A LANDSLDIE USING TRENCH DRAINS (COSENZA, ITALY)Giovanni Pietro Pinzani 1 , Francesco Senatore 2 & Antonio Senatore 31 SEIC Geotecnica, Division of HARPO s.p.a.,Trieste, Italy (e-mail: g.pinzani@seic.it)2 GEO.PRO.GET. Cava de’ Tirreni,Salerno, Italy (e-mail: francesco@geoproget.com)3 GEO.PRO.GET. Cava de’ Tirreni,Salerno, Italy. (e-mail: antonio@geoproget.com)Abstract: The execution of some excavations at the basis of a slope in Laino Borgo (Cosenza- Italy) started a 180m wide and approx. 150 m long landslide. On the basis of a series of simulations under different conditions ofsaturation, it was planned to carry out a system of trench drains as well as some works of surface rain waters controlfrom the top to the bottom of the main slope being stabilized. In order to reduce excavation times and to accelerate thework programme it was planned to make the draining bodies with geocomposites. For their design, lab tests were usedfor a service life longer than 50 years. Three years after the stabilization works the slope is stable even after criticalweather events. Therefore the validity of the adopted design choices can be confirmed and their medium-long-termeffectiveness is guaranteed by the adopted drainage system.Keywords: slope, stabilization, geocomposite drainage materials.INTRODUCTIONNowadays the use of drainage geocomposites has been widespread applied in trench drains works. In comparisonwith the traditional techniques using gravel, such geo-materials present many technical and economical advantages.Furthermore, due to the most recent studies on medium and long-term behaviour of geosynthetics, it is now possible todesign such works for a service life longer than 50 years. In this paper we report design programme and approach,works carried out and the results achieved in the stabilization of a landslide in Laino Borgo (Cosenza – Italy) after theexecution of some excavations to enlarge a power plant.Figure 1. View of landslide from the toeLANDSLIDE DESCRIPTIONThe instability area is a lightly leaning slope (8-15°) being 180 m long, 200 m wide and with a maximumdifference in height of approx. 40 m. The soils in situ are made up of a 3-4 m thick surface layer of silty sands restingon a layer of silty clays of equal thickness. Under these lithologies there is a reduced layer of gravels and sands abovebrown coal depots and clays. Already in 2002 a study reported the geologic risk of the slope characterized by erosionalmorphologies and heavy wave-motions and counterslopes. Notwithstanding such a risk being pointed out, excavationsat the basis of the slope and settlement works were carried out to enlarge the power plant. In detail, work programmeincluded: execution of a 3-5 m high and approx. 150 m long excavation followed by the carrying out of a support workconveying surface waters through gabions and of a support and fencing wall down at the bottom. Having these worksbeen finished, the slope began showing its first marks of instability forming tension cracks and restricted deformationsof structures. In the following months, the phenomenon got progressively increasing forming more tension cracks,showing heavy wave-motions and soil raising at the basis of the slope which led to deformation and partial upsettingof the existing works. Surface maps determined that the main instability was 150 long and 160 m wide. Furthermore, itwas pointed out that there were heavy tension cracks on the sides of the main instability being 180 m wide in total.1

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

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

EuroGeo4 Paper number 248Figure 7. Test results of variation flow capacity in the time at 50 kPa (Colbond).FURTHER WORKSBesides trench drains at the bottom of the slope, it was carried out a system of gabions and a perforated pipe topick up and convey waters from trench drains themselves. Above such structures and always in gabions a smallsupport wall and a channel drain were built. Two further surface waters ditches were out along middle and top slopethrough small drains waterproofed by LDPE geomembranes. In the end, it was predicted to rebuild the parts of thewall damaged by landslide in reinforced concrete.RESULTConsolidation and drainage works finished, the slope got stable without showing any slope motion. Surfacemorphologies of landslide were afterwards changed by agriculture and up till now there have not been any marks oftension cracks, store or motions pointed out. After three years from the intervention, such a condition of equilibrium isstill stable. The carried out structures have not undergone any damage or deformation and the unloading of drainagesystem has been working even in drought periods.Figure 8. Actual stage.CONCLUSIONSThe examined intervention has pointed out how the stabilization of a slope through trench drains can be carriedout using drainage geocomposites. The design and choice of geocomposite require the analysis of its short and longtermproperties particularly referring to the changing of its drainage capacity in time. Such a datum can be deducedfrom R/F or F/F tests in order to simulate real tensile conditions between soil and geocomposite. Finally, the carryingout of trench drains using geocomposites instead of the traditional gravel gives important technical and economicaladvantages.Acknowledgements: The authors wish to express special thanks to Mrs Annalisa Rondi for the English editingsupport.Corresponding author: Mr Giovanni Pietro Pinzani, harpo s.p.a., via torino, 34, trieste, italy, 34100, Italy. Tel:+390403186611. Email: g.pinzani@seic.it.REFERENCESBottcher, R.D. 2006. Long-term flow capacity of geocomposites. 8 th International Conference on Geosynthetics,Yokohama, Vol. 2 pp 426.Desideri, A., Miliziano, S., Rampello, S. 1997. Drenaggi a gravità per la stabilizzazione dei pendii. Hevelius Edizioni.Koerner, R.M. 1994. Designing with Geosynthetics. 3 rd edition. Prentice Hall.5