^ Wismut GmbH, Chemnitz, Germany *GuD Consult GmbH, 1000 Berlin, Germany

Size: px

Start display at page:

Download "^ Wismut GmbH, Chemnitz, Germany *GuD Consult GmbH, 1000 Berlin, Germany"

Berniece Butler
5 years ago
Views:

1 Seismic stability of tailing dams for the uranium industry H. Klapperich", U. Gross*, R. Hahne', T. Richter^, S. Savidis^ "DMT-Geotechnical Institute, 4300 Essen, Germany *DMT-Geotechnical Institute, 7030 Leipzig, Germany ^ Wismut GmbH, Chemnitz, Germany *GuD Consult GmbH, 1000 Berlin, Germany ABSTRACT Wastes of uranium mining in the former East Germany were disposed in old openpit mines and valleys,which are bordered with rockfill dams. The soil material in these tailings have grain sizes from clay to middle sand. Resulting from partially high water contents the soil material have thixotropical conditions. Long-term protection of these tailings required a dam-stability design due to seismic loading. The ground is formed by metamorphic clay shales or middle-strong sand- and claystones. An analysis of regional seismicity showed value for the seismic site intensity from 7 to 8 degrees at MSK-scale. The computation of the dynamic stresses and strains in the dams were performed in the time-domain by finite-element analyses by the method of Seed. Dynamic soil properties were determined by resonant column and cyclic triaxial tests in connection with in situ borehole tests. For the dam stability analysis the classic theory of failure circle surfaces was used whereby the material softening due to dynamic loading was taken into account. INTRODUCTION In the frame of a concept of preservation, restoration and reactivation the stability of dams retaining large amounts of tailings in slurry form was investigated. Particular attention was given to the behaviour in case of an earthquake. The sites Culmitzsch and Helmsdorf, which are presented

2 508 Soil Dynamics and Earthquake Engineering herein, are located in the region of Sachsen in the southeastern part of Germany. A total of seven dams were considered in this investigation program which separate three tailings ponds with altogether 128 million cubic meters fine uranium tailings. The dams are constructed mainly with dump material and partially using the waste rock from the existing ground (sandstone, phyllite, red sandstone). Only few publications treat the problem of the stability of tailing dams in case of earthquakes, e.g. Klohn et al.\ The stability analysis for the dams under consideration was performed in the time-domain and is based on thefiniteelement method as described by Seed^. In order to assess the possible material degradation in the dams and along the base as well as liquefaction susceptibility in case of earthquake excitation an extensive in-situ and laboratory investigation program was carried out. The steps of the analysis are presented in the following. SITE GEOLOGY The site Helmsdorf belongs to the Erzgebirge basin, which is mainly composed of conglomerates of the upper red sandstone overlain by a thin layer of residual loam. The site Culmitzsch, on the other hand, belongs to the regional geologic unit of the Bergaer Sattel and is part of the foremountain area of Mittelgebirge of Saxony-Thuringhia. The geology is more diversified. Basins of tectonic origin are composed of sandstone sediments underlain by phyllite mudstone bedrock. The bases of the dams in Culmitzsch are made by layers of sandstone, which define sliding surfaces due to the high plasticity clay/silt content, and accordingly low shear strength. DAM STRUCTURE The dams in Culmitzsch are constructed from dump material. With exeption of the ponds-separating dam, all dams were constructed without a core and without compaction of thefillmaterial. On the pond side of the perimeter dams a 25 m high loam zone was placed for protection against mud-water seepage. The dams reach a maximum total height between 27 m and 76 m at inclinations between 22 and 26. The single slope systems reach heights between 10 m and 20 m and inclinations from 30 to 38. The dams in Helmsdorf are different. The Main and the West Dam were self-standing raised on base dams from red sandstone. The Wiistengrund dam was constructed as a zone dam from red sandstone, tailings sand and Crossen dump material and has also a low permeability loam zone on the water side. Underdrains exist in the bases of the Main and the West dam. The dams reach heights up to 61 m and inclinations between 27 and 27. The single slope systems range from 6 m to 11 m at

3 Soil Dynamics and Earthquake Engineering 509 maximum inclinations of The cross section of a representative dam is drawn in Figure 1. Regelquerschnitt Sueddamm Afadeckung FeinscNamione und Sou*'.st Figure 1 : Cross section of the South dam at the Culmitzsch site In order to determine the mechanical properties of the dam materials an extensive investigation program was carried out. For the relevant cross sections a total of 30 borings were conducted. The samples were classified and tested with respect to the mechanical properties according to the German Standards DIN for soil mechanics and foundation engineering. Undrained shear strength, stiffness and density of the hydraulic fill material was determined also in situ by means of penetration tests and vane shear tests. The boreholes were rebuilt to be used for observation of the hydraulic conditions within the dam. Deformation and strength chracteristics under dynamic loading were tested in resonant column and cyclic triaxial apparatus at the Technical University of Berlin. The results and testing procedures for the dynamic tests are given elsewhere, cf. Savidis et ala Seismic up-hole measurements were also conducted to determine in situ densities and dynamic stiffnesses. EARTHQUAKE PARAMETERS For determining the dynamic loading of the dams in case of earthquake a site-specific investigation was carried out by the GeoForschungsZentrum Potsdam and the University of Jena. Both sites are found to be located almost in the center of the seismic region of Saxony-Thuringhia in an area of sub-regional intraplate seismicity. The design earthquake characteristics were determined according to the recommendations for earthquake

4 510 Soil Dynamics and Earthquake Engineering response analysis for nuclear power plants KTA and a probabilistic safety study based on the DIN for water retaining structures. The following characteristics at bedrock level were finally chosen for the seismic response analysis: Intensity / = VII on the MSK scale Focal depth h = 8-20 km Hypocentric distance R = km Magnitude M = maximum mean acceleration amax =210 cm/s~ A representative acceleration response spectrum as well as two acceleration time-histories from the Friaul earthquake of 1976 (sites Tolmezzo and Rocco) were specified for the analysis. Figure 2 shows the acceleration record of the north-south component at the Tolmezzo site. Time in sec com- Figure 2 : Acceleration record of the Friaul earthquake: north-south ponent at Tolmezzo site DYNAMIC STABILITY ANALYSIS In order to assess the nonlinear stress-strain relationship for use in the incremental finite element analysis a hyberbolic equation (a^ 0-3) = f(e) is chosen as suggested by Wong and Duncan^, where a\ and 0*3 are the major and minor principal stresses, respectively. The initial Young's modulus Emax is determined from 2m.z=2oPa(<73/P*r (1) where p<j = 100 kpa is the atmospheric pressure and EQ and a are parameters to be determined from results of the specific drained or undrai-

5 Soil Dynamics and Earthquake Engineering 511 ned static triaxial test. By introducing the failure ratio Rf = (cr\ ^s)//(^i ~^)uitt where (cr\ ovju/t and (a\ erg)/ are the maximum deviator stress and deviator stress at failure, respectively, and applying the Mohr-Coulomb strength relationship, the tangent modulus Et is obtained as follows: " ^ -shv)((7i -0-3)12,, 2c' cos (p' -h 2(73 sin y' 7 '"" l^j where c' and y' are the effective shear strength parameters. For the dynamic analysis the nonlinear soil behaviour is described by means of equivalent elastic parameters which vary as a function of stress and strain level. Resonant column tests have been conducted to determine the dynamic shear modulus Gmax and damping ratio Dmin at very low strains for the particular tailing dam material. The variation of G and D with shearing strain amplitude has been determined according to the procedure outlined by Hardin and Drnevich^ and was checked against experimental curves, i.e. In this equation 7r = Tmax/Gmax is the reference strain, where Tmax is the maximum shear stress at failure as given by the Mohr-Coulomb relationship. For the estimation of the damping ratio we similarly obtain D/Dmax = (7/7r)/(l + 7/7r) (4) where Dmax is the maximum damping ratio, which occurs when G = 0. To assess the increase of pore pressure and accordingly the strength degradation due to earthquake excitation, cyclic triaxial tests were conducted on saturated samples from the tailing dam material. The stability analysis was applied in three steps: i) After discretizing the dam by means of finite elements the system is loaded by its self-weight. The material behaviour is given by the hyperbolic stress-strain law. ii) For the computed stress-state the shear modulus and damping ratio for each element are determined for the actual strength parameters. The system is then subjected to the design earthquake excitation at its base and the developed shear strains are computed. Using an iteration process the stiffness and damping values corresponding to that strain level are determined by means of equations (3)-(4). The resulting irregular stresses-time curves are then converted to an equivalent number of uniform cycles to allow a direct comparison with cyclic laboratory tests, cf. Seed et al.

6 512 Soil Dynamics and Earthquake Engineering An average mean value N<>q is obtained by weighting the induced shear stress Tmaxel over all n elements according to iii) For the equivalent uniform cycles number and the corresponding shearing stress amplitude Tmaxeq = 0.65r^ax (6) where r^ax is the maximum shear stress induced by the earthquake excitation, the pore pressure increase and the cummulative axial strain are determined from the laboratory data. For cohesionless material, the reduced shearing strength y* for slope stability analysis formulated in total stresses is derived from the effective shearing strength y' in terms of the pore pressure u^y and mean total principal stress a* as follows: siny* = siny'(l - %c%/<fm) (7) In order to assess the liquefaction potential at any level of the dam the number of equivalent stress cycles N^i are determined for each element. Then, using the laboratory tests results the magnitude of the cyclic stress required to cause initial liquefaction in the field in N^i cycles is determined for each element and compared to the equivalent cyclic shear stress induced by the earthquake. DISCUSSION AND CONCLUSIONS For the design earthquake chosen the stability analysis of all dams showed a safety factor greater than 1. Thus, an acute threat of the dams does not exist in case of an earthquake loading. Depending on the failure model, the following measures were recommended: Short term: -Observation of the seepage line in the dams by means of a hydraulic measuring system -Installation of seismographs at different levels to measure the dam response in case of an earthquake -Installation of piezometers -Installation of inclinometers for measuring possible movements along the failure planes in order to verify the assumed shear strength variation with strain.

7 Long term: Soil Dynamics and Earthquake Engineering 513 -Reduction of the dam inclination in the South and Main dam -Drainage of the tailings material in order to reduce hydraulic flow and increase the effective shear strength as well to prevent flow of the pond sediments in case of local dam failure -Dowelling of the potential failure surface (e.g. with piles) -Installation of a resistant dam toe at the downstream side. REFERENCES 1. Klohn, E.J., Maartman, C.H., Lo, R.C.Y and Finn, W.D.L. Simplified Seismic Analysis for Tailings Dams. Vol. I, pp , Proc. ASCE Spec. Conf. on Earthq. Eng. Soil Dyn., Pasadena, California, Seed, H.B. Earthquake-Resistant Design of Earth Dams. Vol. Ill, pp , Proc. Int. Conf. Recent Advances in Geotechnical Earthq. Eng. & Soil Dynamics, Rolla, Missouri, Savidis, S.A., Vrettos, C. and Richter, T. Resonant Column and Cyclic Triaxial Testing of Tailing Dam Material. Proc. 6th Int. Conf. Soil Dyn. and Earthq. Eng., Bath, England, Wong, K.S. and Duncan, J.M. Hyperbolic Stress-Strain Parameters for Nonlinear Finite Element Analyses of Stresses and Movements in Soil Masses, Report No. TE-74-3, College of Engineering, University of California, Berkeley, Hardin, B.O. and Drnevich, V.P. Shear Modulus and Damping in Soils. J. Soil Mech. Found. Div. ASCE, Vol. 98, pp , Seed, H.B., Lee, K.L., Idriss, I.M. and Makdisi, F. Analysis of Slides in the San Fernando Dams during the Earthquake of Feb. 9, 1971, Report No. EERC 73-2, College of Engineering, University of California, Berkeley, 1973

Transactions on the Built Environment vol 3, 1993 WIT Press, ISSN

Transactions on the Built Environment vol 3, 1993 WIT Press, ISSN Resonant column and cyclic triaxial testing of tailing dam material S.A. Savidis*, C. Vrettos", T. Richter^ "Technical University of Berlin, Geotechnical Engineering Institute, 1000 Berlin 12, Germany