The municipal landfill in Rastorf (Schleswig-Holstein) is covered with a temporary till based capping system. Investigations of structural changes (water conductivity, pore function, shrinkage behaviour) were carried out with undisturbed soil cores and two- and three-dimensional measurement techniques.

The usage of different techniques to evaluate the soil volume changes resulted in variations of the available water capacity (ver>ver+hor>3D) and the unsaturated hydraulic conductivity. 

In summary, a critical matric potential between -500 and -1000 hPa in conjunction with a low shrinkage tendency (Vs < 5 %) could be seen as main argument for the utilization of till as sealing substrate

Beck-Broichsitter, Steffen

Fleige, Heiner

Horn, Rainer

English

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Tagungsbeitrag zu: Jahrestagung der DBG, Kommission VI Thema VI/4 (Freie Themen) Veranstalter: DBG Termin und Ort der Tagung: 2. – 7. Sep-tember 2017, Göttingen Berichte der DBG (nicht begutachtete online Publikation); http://www.dbges.de   Approaches to investigate the crack formation of mineral land-fill liner systems  Beck-Broichsitter S.1, Fleige H., Horn R. Abstract The municipal landfill in Rastorf (Schleswig-Holstein) is covered with a temporary till based capping system. Investigations of structural changes (wa-ter conductivity, pore function, shrinkage behaviour) were carried out with undis-turbed soil cores and two- and three-dimensional measurement techniques. The usage of different techniques to evaluate the soil volume changes re-sulted in variations of the available water capacity (ver>ver+hor>3D) and the un-saturated hydraulic conductivity.  In summary, a critical matric potential between -500 and -1000 hPa in con-junction with a low shrinkage tendency (Vs < 5 %) could be seen as main argu-ment for the utilization of till as sealing substrate. Keywords: landfill capping system, shrinkage behaviour, critical matric po-tential  Introduction In its function as pollutant sink, landfills still represent the major option for global waste disposal (Hoornweg and Bhada-Tata, 2012). In Germany, the qualitative criteria of landfills are legally binding and define the technical standards for engi-                                            1 s.beck-broichsitter@soils.uni-kiel.de Institut für Pflanzenernährung und Bodenkunde C.A.U. zu Kiel, Hermann-Rodewald-Str. 2, 24118 Kiel neered barriers according to the Ger-man landfill directive (2009). At the end of the active sedimentation phase, the pollutant potential of the waste makes it necessary to immediate-ly apply a temporary or final surface covering system. The main requirements for a landfill capping system include the minimization of rainwater input (groundwater protec-tion) and the prevention of landfill gas emissions (protection of the atmos-phere) with regard to long-term function-ing (Horn, 2002, Rowe, 2011). There-fore, (a) the recultivation layer in its function as vegetative layer and (b) the sealing layer as water and root barrier are essential components of the capping system to prevent the atmosphere and the groundwater of various pollutants.  A basic prerequisite of functional landfill capping systems is the hydraulic stability of the sealing layer due to weather-induced shrinkage processes that can lead to crack formation resulting in high-er infiltration rates and leachate genera-tion with environmentally degrading ma-terials. Consequently, the sealing layer in its function as last barrier above the waste body must exhibit hydraulic stability dur-ing drier periods by suitable selection of materials and the water content during installation (Horn & Stepniewski, 2004). The functionality of the sealing layer is decreasing due to crack inducing critical matric potentials when the mineral sub-strates dry out with matric potentials more negative than the driest field con-ditions and exceeded the pre-shrinkage stress (Horn and Baumgartl, 2002).  Material and Methods The sealing layer of the Rastorf landfill (Schleswig-Holstein) consists of a com-pacted till (69 % sand, 24 % silt, 7 % clay) with an average dry bulk density between 1.67 and 1.84 g cm-3.  Therefore, the shrinkage induced vol-ume change was estimated with undis-turbed soil cores of 100 cm3 at the dry-ing stages of quasi-saturated, -30, -60, -150, -500 hPa with a suction plate and at -15000 hPa with a pressure pot and the soil cores were subsequently dried with 105°C for 24 hours.  Several non-destructive methods were used to quantify the soil volume change at the different drying stages according to Peng et al. (2006) and Seyfarth et al. (2012). These include:  caliper (only vertical direction = ver),  calliper and 2D image analysis (ver-tical and horizontal direction = ver+hor) in Fig. 1,  3D laser triangulation, considering the geometry of crack formation (3D) in Fig. 2.    Fig.1:  Estimation of soil shrinkage via digital image analyses (Software Image J Version 1.4).    Fig.2:  Estimation of soil shrinkage via laser triangu-lation device Soil LT 100TM (UGT, GmbH).   The volume shrinkage index (Vs) was calculated as follows:  V0 hPa − V105°CV0 hPa (Eq. 1)  where V0hPa is the soil volume at the quasi-saturated stage and V105°C is the soil volume at 105°C (oven dried). The shrinkage tendency (Vs) can be ranked into four classes: low (< 5 %), moderate (5-10 %), high (10-15 %) and very high (> 15 %) according to Vogt et al. (2013).  Results and Discussion The results in Fig. 3 show significant differences between the only caliper method (ver) and the extended 2D im-age analysis (ver+hor) as well as the 3D laser triangulation (3D) with a Vs of 1.68, 3.74 and 4.24 %, respectively. So, the shrinkage tendency could be classified as low. Furthermore, the estimated critical ma-tric potential in a range of -500 to -1000 hPa should not be occurred, especially during dry summer months, to prevent crack formation in the mineral liner.   Fig.3:  Volume shrinkage index (Vs) with significant differences (p<0.05) between the measure-ment methods. v e r v e r+ h o r 3 D0123456Volume shrinkage index Vs [%]* * * *p = 1 .2 e -0 7****p = 1 .3 e -0 6n sp = 0 .2 3Therefore, any desiccation being higher than the previously highest one (mcrit) moreover leads to irreversible shrinkage components and the crack formation will lead to higher infiltration rates and the leachate will increase and thus the tran-sition to the proportional shrinkage phase should be avoided (Horn & Baumgartl, 2002). The till shows a decreasing AWC with 6.78, 6.75 and 6.55 cm3 cm-3 (ver>ver+hor>3D) with increasing soil volume change (ver<ver+hor<3D) (Fig. 3, 4).    Fig.4:  Effect of soil shrinkage on the available water capacity (AWC) of the mineral liner derived from sandy till with measured and fitted data (ver, ver+hor, 3D) by VAN GENUCHTEN formula (1980).  The curve patterns based on the VAN GENUCHTEN parameters (, n, m) also underline the effect of soil volume changes on the hydraulic conductivity according to the applied measuring method (Fig. 5). In that regard, the pa-rameter “” increased and “n” de-creased with increasing soil volume change.    Fig.5:  Effect of soil shrinkage on the hydraulic con-ductivity k(m) of the mineral liner derived from sandy till with measured and fitted data (ver, ver+hor, 3D) by VAN GENUCHTEN formula (1980).  Conclusion The changes of the soil physical proper-ties due to shrinkage processes cannot be excluded and are mostly irreversible. Therefore, deeply drained landfill cap-ping systems quickly lose their hydraulic stability in combination with increasing leachate rates or rather a higher hazard potential for the groundwater.  In our case, the estimated critical matric potential between -500 and -1000 hPa should not be occurred, especially dur-ing dry summer months, to prevent crack formation in the mineral liner (Horn & Stepniewski, 2004). Therefore, the mentioned non-destructive methods were useful to quantify the soil volume change, especially the 3D laser triangu-lation, considering the geometry of crack formation according to Seyfarth et al. (2012). Due to the verified volume change of the till, non-rigidity of soils should not be assumed, even for highly compacted soils with low shrinkage tendency, but the degree of the rigidy depends on the historical hydraulic stresses (Horn & Baumgartl, 2002). An underestimating of the occurrence of soil shrinkage may result in incorrect data for modelling of hydraulic functions or changes of hydraulic properties of mineral liners due to climate change (Alaoui et al., 2011).  1 0 0 1 0 1 1 0 2 1 0 3 1 0 41 41 61 82 02 22 42 6M a tr ic  p o te n tia l  m [-h P a ]Water content [cm3 cm-3]v e rv e r+ h o r3DA W C1 0 -1 1 0 0 1 0 1 1 0 2 1 0 3 1 0 41 0 -1 11 0 -1 01 0 -91 0 -81 0 -71 0 -6M a tr ic  p o te n tia l  m [-h P a ]Hydraulic conductivity k(m) [m s-1]m e a su re d  d a tav e rv e r+ h o r3DAcknowledgement This work was supported by the Innova-tionsstiftung Schleswig-Holstein and the ZMD Rastorf GmbH.  Literature Alaoui, A., Lipiec, J., Gerke, H. H. 2011. A review of the changes in the soil pore system due to soil deformation: A hy-drodynamic perspective. Soil & Tillage Research 115-116: 1-15. Germann landfill directive. 2009. Degree on landfills (ordinance to simplify the landfill law) – Germany, in the form of the resolution of the Federal Cabinet dated April 27th 2009. Horn, R., Baumgartl, T. 2002. Dynamic Properties of Soils. In: A. W. Warrick, Hrsg. Soil Physics Companion. Boca Raton, Florida: CRC Press LLC, pp. 17–48. Horn, R., Stepniewski, W. 2004. Modifi-cation of mineral liner to improve its long-term stability. Int. Agrophysics 18: 317–323. Hoornweg, D., Bhada-Tata, P. 2012. What a Waste: A Global Review of Solid Waste Management. Urban Develop-ment Series, Knowledge Papers (15). The World Bank, Washington DC, 116 pp. Peng, X., Horn, R., Peth, S. & Smucker, A. 2006. Quantification of soil shrinkage in 2D by digital image processing of soil surface. Soil & Tillage Research 91: 173-180. Seyfarth, M, Holdorf, J., Pagenkemper, S. K. 2012. Investigation of shrinkage induced changes in soil volume with la-ser scanning technique and automated soil volume determination ‒ A new ap-proach to analyze pore rigidity limits. Soil and Tillage Research 125: 105‒108.   Vogt, N., Birle, E., Heyer, D., Etz, A. (2013): Entwicklung einer neuen Ver-suchstechnik zur Bestimmung der Gren-zen zwischen halbfestem und festem Zustand. Lehrstuhl und Prüfamt für Grundbau, Bodenmechanik, Felsme-chanik und Tunnelbau. Technische Uni-versität München. BASt-Bericht S 80, 92 S. 

Approaches to investigate the crack formation of mineral landfill liner systems

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Approaches to investigate the crack formation of mineral landfill liner systems

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