НАУЧНА КОНФЕРЕНЦИЯ С МЕЖДУНАРОДНО УЧАСТИЕ ВСУ`2007 INTERNATIONAL CONFERENCE VSU`2007

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1 GEOSYNTHETICS USED IN SOLID WASTE LANDFILL FOR GROUND WATER PROTECTION Olimpia Blagoi 1, Esmeralda Chiorescu 2 Gh. Asachi Technical University of Iassy, Faculty of Hydrotechnics, Romania Abstract: The complex structure of a lateral and basic watertight drainage system applied to the controlled industrial waste landfills is presented. The stress forces on landfill and the negative consequences of waste settlement versus time are analysed. The watertight system composed by a natural geological clay barrier and an artificial barrier based on a watertight multi-layer and drainage system is also presented. The seepage calculation through the multi-layer tightening with geo-synthetics, the resistance and stability calculation for watertight and drainage system are elaborated. Key words: geo-synthetic materials, landfill, geo-synthetic permeability 1. INTRODUCTION The protection of soil, underground and terrestrial eco-systems through adequate measures of cultivation, conservation, organisation and territorial management has the goal to prevent the risks of ecological damages, to conserve the bio-diversity and the specific eco-systems in the frame of the natural bio-geography. The waste stuffs are useless elements for the society. But stored and eventually treated with eco-engineering measures, they can be reintroduced as utilizable products in the economic circuit. The spectacular increase of the activities in this domain is caused by the rapidly growing waste quantities, as well as by the restrictions imposed on the placement of these wastes in rapport with the earth massifs, which intend to prevent any possible pollution. The analyse and the choice of the depot s location is made in function of the waste s characteristics, of the local geography and hydro-geology, including the depth of the aquifer, the form of the depot and its harmonic placement in the peisage, the water inflow from the exterior and its position to the sewerage installations, the dominant wind direction and its influence on the residential areas, the distance to the waste sources and to the existing infrastructure. The depot has to assure during a longer time the full security conditions for the environment, as well as for the stored wastes. 2. THE SPECIFICS OF THE CONTROLLED DEPOTS OF INDUSTRIAL WASTES The particularities of the controlled depots for waste stuffs are connected with the presence, the nature and the state of these wastes, and they are manifesting in the structure itself through: - mechanical stress: the deposed stuffs settle steady in time, loss of stability; 1 Olimpia Blagoi, Prof., Eng., Ph.D., D.Sc., Gh. Asachi Technical University of Iassy, Faculty of Hydrotechnics, Mangeron 63, Iassy, Romania, oblagoi@yahoo.com 2 Esmeralda Chiorescu, Eng., Ph.D., D.Sc., Gh. Asachi Technical University of Iassy, Faculty of Hydrotechnics, Mangeron 63, Iassy, Romania, esmeralda_chiorescu@yahoo.com VI - 57

2 - hydro strain: apparition of water infiltrations, specially through precipitations; - chemical demands: production of levigations with polluting character, with different degrees of aggressiveness for the structure; - physical stress: variations of humidity and the actions of plants and organisms (like roots, galleries) favour the inner spreading of polluting substances within the wastes; - production of bio-gas by anaerobe fermentation at the lower part of the depot; - difficulties of intervention on the depot s bottom, after the exploiting phase. The later compression of the wastes appears because they are delivered to the depot in an un-arranged mode (by dumpers or containers) and in time take place considerable rearrangements. The settle-process has unfavourable consequences on the underground and the atmosphere: 1. deformation and rifts in the closing layer of the depot; 2. apparition of depressions on the surface, which favour the stagnation of rainwater; 3. forming of holes in the deposed stuff, which causes the break-down of the covering layer; 4. the apparition of rupture surfaces in the mass of wastes - before or after the closure of the depot - conduces to a loss of stability; 5. seepage of water in the depot, followed by the spilling of soluble noxious substances in the surrounding terrain; 6. if the walls and the bottom of the depot are tight, then the accumulation of the infiltrated water in the depot s mass increases the inner pressure, so that the water can penetrate the tightening layers and contaminate the environment; 7. the start of chemical reactions between water and the waste components, generating new compounds that can be ecologically dangerous; 8. gas emissions can affect the ecologic balance of the zone that surrounds the depot; 9. the actions of organisms and plants (galleries and roots) favour the interior and exterior spreading of substances from the wastes, contributing to the environment pollution. 3. COMPLEX MULTI-LAYER TIGHTENING WITH GEOSYNTHETICS To an ecological construction of depots for industrial wastes, in conformity with the EUnorms regarding the storage of non-reusable wastes and with the H.G. 162/2002 regarding the dumping of residues, it is recommended to replace the usual storage system in concrete tubs with a solution that uses the natural terrain, and at which the sealing is made with natural and artificial means. The main parts of the depot are the body, the surface sealing, the lateral sealing, the bottom sealing and the drainage system. The constructive solution for the depot body consists of a trench combined with side-dams, where the excavations have a depth of 4 5 m and the practicable earth dams around have a height of 3 4 m. The dam crest is 6 m broad. The lateral sealing has the function to prevent the field contamination and it can be executed in two variants: a). through sealing walls, b). through the lengthening of the bottom sealing. The bottom sealing is the lower part of the depot, which protects the underground and has the essential function to prevent the infiltration of noxious stuffs. The minimum admissible distance till to the groundwater level is 1 m. The tightening system consists of a natural barrier and of an artificial sealing. The natural geologic barrier is an existing layer of binding earth (clay, loam) with a thickness of 1 4 m. The permeability of the packet of binding materials has to be: k 10 9 m/s. VI - 58

3 If at the planned depth doesn t exist a natural barrier, then can be filled in a wellcompressed clay layer with a thickness of 1 m. The artificial tightening is a complex system of sealing and draining, which is composed of several different layers. wastes geo-textile stratum of separation 0,4 2,5-4 draining mineral material of ballast geo-textile stratum of protection bentonitic geo-composite: geo-membrane PEHD1,5mm+bentonite 5mm natural geologic barrier Fig. 1. Section through the system of sealing and draining with geo-synthetics The structure of the multi-layer tightening (Fig. 1) is: - bentonitic geo-composite consisting of a 1.5 mm polyethylene geo-membrane with high density (PE-HD) and of a 5 mm bentonite coat pasted on the geo-membrane; the composite is incorporated with the bentonite downside (the estimated thickness is 8 mm); - non-weaved geo-textile stratum for the protection of the geo-membrane, with a unity weight of 1.2 dan/m 2 ; m thick layer of draining mineral material for the drainage of the levigations produced by the infiltration of the rainwater through the deposed wastes; in the draining layer is incorporated the tube system that conducts the accruing levigations to the main sewer; - non-weaved geo-textile stratum for the separation of the wastes from the draining mineral material, with a unity weight of 0.25 dan/m 2. With this structure, the tightening system ensures a triple protection through: a) the natural barrier of existing clay at the lower part, with a minimum thickness of 1 m and with a filtration coefficient of k 10-7 m/s; b) the bentonite layer with k m/s; c) the geo-membrane with k < m/s Infiltration through the multi-layer tightening To use the Darcy-formula, it is necessary to determine firstly the filtration coefficient of the whole system. The equivalent permeability ( k e ) of the bentonitic geo-composite will be expressed in function of the permeability of the component strata: - the synthetic component (geo-membrane) k < m/s, - the bentonitic component k = m/s. Observation The adopted value of the permeability coefficient for the geo-membrane is an equivalent value in the sense of Darcy s Law, because in reality the transition of the substances through the intact geo- membrane isn t made by an advective flux, but by diffusion. The equivalent permeability has the expression: VI - 59

4 The discharge has the value: k e = hi hi (1) k i Q = k i A (2) where: i hydraulic gradient in the hypothesis that the levigated stuff is situated above the draining layer, A surface of calculation, considered 1 m Infiltration through the penetrated or damaged geo-membrane In the case of damage or of a penetration in the geo-membrane of the multi-layer tightening system the discharge (Q) can be calculated with the semi-empiric formula: e 1 = H Q ke H a 1 + (3) 5 10 h where: H hydraulic head on the geo-membrane, considering the liquid at the level of the draining layer (m), h thickness of the mineral component of the bentonitic geo-composite and that of the natural sealing with clay (m), a surface of the damage (m 2 ), k e equivalent permeability of the mineral component of the bentonitic geo-composite and that of the natural sealing with clay (m/s) Time of traversation through the damaged multi-layer tightening system The needed time by the levigated stuffs to traverse the composed tightening system ( t trav. c. s. ) through advective transport, when the geo-membrane is damaged or perforated, can be evaluated with the formula: h n t = e trav. c. s. (4) H ke h where: n e equivalent porosity, h thickness of the composed stratum, H hydraulic head, k equivalent permeability. e 3.4. Resistance and stability calculation of the multi-layer tightening with geosynthetics We consider the lateral sealing on a slope with an angle β, composed of: - bentonitic geo-composite (geo-membrane of PE-HD and bentonite set directly on the slope); - non-weaved geo-textile with 1.2 dan/m 2, for the upper protection of the geo-membrane. VI - 60

5 G N T F 5 F 4 geo-textile stratum of protection F 2 F 3 geo-membrane F 1 bentonitic stratum β arrangement -slope Fig. 2. Calculation scheme of the tightening system with geo-synthetics On the multi-layer system act the following forces (Fig. 2): G weight of the material on the surface; N normal component of the weight: N = G cosβ T tangential component of the weight: T = Gsinβ F 1 acting force of the layers situated on the slope; F 2 friction force between bentonite and the earth surface: F 2 = N tg δ1 F active force of the geo-textile on the geo-membrane; 3 F 4 friction force between geo-textile and geo-membrane: F 4 = N tg δ2 F 5 friction force between the material on the surface and the geo-textile: F 5 = N tgδ3 δ 1 friction angle between completely hydrated bentonite and natural terrain; 1 δ = 5 o 6 o δ 2 friction angle between geo-textile and geo-membrane; for the contact of non-weaved geo-textile with even geo-membrane of PE-HD: δ 2 = 8 o 11 o δ 3 friction angle between the material on the surface and the geo-textile: δ 3 = 26 o. The effective tensile stress in the geo-synthetic ( ) must be smaller than its permissible tension ( R ): gt T g. ef < gt VI - 61 T g. ef The permissible tension of geo-membrane PE-HD is The permissible tension of the protecting geo-textile is R (5) R gt = 40 kn/m. R gt = 50 kn/m. The extension force acting on the geo-membrane ( T gm ) is the difference between the active force of the geo-textile on the geo-membrane ( F 3) and the friction force between bentonite and the earth surface ( F 2 ): The extension force acting on the geo-textile ( T gm ) is: T gm = F 3 F 2 (6) T gt = F 5 F 4 (7)

6 The extension force acting on the geo-membrane or on the geo-textile (T) is: ( tg δ tg δ ) cosβ = γ H T sin β sup inf (8) where: H thickness of the upper strata; β slope angle; δ friction angle of the upper stratum; sup δ inf friction angle of the lower stratum. The stability condition against sliding demands that the friction force between geo-synthetic and the support surface ( F fr. g ) must be greater than the active force ( F act ): F fr. g > act F (9) The loss of stability can happen in two ways: by the sliding of the geo-textile on the geomembrane or by the sliding of the hydrated bentonite on the slope. 4. CONCLUSIONS The exchanges between the stored wastes and the environment (soil, groundwater) can be limited by a separation interface. The quality and the composition of this interface must be well chosen, so that it is able to collect and evacuate the fluids that are coming from the wastes or from infiltrated precipitations. At the same time the interface has to be physically and chemically inert to the agents that originate from the deposed materials or from the surrounding media, and it has to function at the planned parameters over a long time. Geo-membranes and geo-textiles are inert materials, which in combination with adequate mineral materials (clay, gravel), have proved to be well suited for the sealing and draining of the ecologic depots for industrial wastes. REFERENCES 1. BLAGOI, O. MITROI, A.: Construction Hydraulics. Ed. Cermi, Iassy, CHIORESCU, E.: Contributions concerning new technologies and materials suitable for irrigated lands protection. Ph.D. Thesis, Gh. Asachi Technical University of Iassy, EU D. 31/1999 regarding wastes depots 4. H.G. nr. 162/2002 regarding non-reusable wastes storage 5. Law nr. 145/2002 regarding integrated pollution prevention and control VI - 62