Prediction of subsoil subsidence caused by opencast mining

Land Subsidence (Proceedings of the Fifth International Symposium on Land Subsidence, The Hague, October 1995). IAHS Publ. no. 234, 1995. 167 Prediction of subsoil subsidence caused by opencast mining HALINA KONDERLA & MACIEJ HAWRYSZ Institute of Geotechnics and Hydrotechnics, Technical University of Wroclaw, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland Abstract Attempts were made to formulate the correlation between the subsidence of the soil mass exposed to opencast mining and the parameters characterizing the development of the depression and the geological structure of the seam. The process of soil surface subsidence was analysed, making use of the results from geodetic and piezometric measurements carried out in one of Poland's brown-coal mines. A simple model was proposed for the prediction of superficial displacements produced by the dewatering of the soil mass. The subsidence and rise of the soil surface, recorded in the course of geodetic measurements, reflect both the effect of seam dewatering and the effect of mining operations. A future verification of the model should include the effect of soil mass decompression concomitant with the stripping of the overburden, the working of the seam, and the construction of the external dumping ground. INTRODUCTION Opencast mining is associated with soil deformations which are primarily due to the dewatering and decompression of the soil mass, and vary with its geological structure. Theoretically, decompression reaches a significant depth on the soil mass, thus accounting for an increase in the rock volume, for the movement of the entire excavation, and for vertical displacements in the adjacent soils. It is difficult to distinguish the displacements produced by decompression alone, because they add to some other movements, such as subsidence due to dewatering, and displacement due to creeping (mostly clays). Thus, the problem of how to predict subsoil subsidence resulting from opencast mining is, in fact, very complex. So far, any attempts to represent the process in terms of conventional soil mechanics have failed to be reliable, because the subsidence predicted via the above route varied from several tens to several hundred percent, as compared to the measured values. Another method of solving the problem in question consists in establishing the correlation between real subsidence and some factors that affect the course of the process. The objective of the present paper was to construct a model describing such relationships, with the assumption that the relations are site-specific. Analysed were the measured displacements of the vertical benchmarks situated in one of Poland's browncoal mines (Fig. 1). The variations of the water table position, produced by the dewatering of the excavated seam, were assumed to be the main factor contributing to subsoil subsidence. Making use of the results from piezometric measurements, water

168 Halina Konderla & Maciej Hawrysz 100/28 Legend S6/36-Borehole 0 29 -Bench mark Fig. 1 Location of measuring points. -fp1 -Piezometer Open pit "Barrier of welis 100 200m Scale level variations in the soil mass were related to the benchmark positions and to the surveying time (Przedsiebiorstwo Geologiczne, 1966). The non-cohesive soil thickness and the dewatered layer thickness were adopted as the parameter characterizing the geological structure of the soil mass. Considering the random distribution of the boreholes and piezometers with respect to the benchmark line, use was made of the SURFER programme packet (Tanski, 1991). GEOLOGICAL STRUCTURE The hydrogeological structure of the opencast mine under study is shown by the cross section of the benchmark line (Fig. 2). The deposit consists of three stratigraphie formations - Upper Cretaceous, Tertiary and Quaternary. The Upper Cretaceous sediments are built of marls displaying fissured zones. Those sediments underlie the Tertiary formation in which the following series can be distinguished: seam-underlying series, brown-coal seam and seam-overlying series. The seam-underlying series is formed of a fine Miocene sand layer with numerous lenticular silty or (less frequently) clayey interbeddings. In the central part of the seam, the thickness of the underlying sand-silt series amounts up to 25 m, decreasing towards the edge part, and these forms are predominantly subject to thinning away before they reach the coal boundary. Coals in the form of a single stratum occur flat at the depth of 30 to 55 m, generally with no disturbances. The seam-overlying series (made of Poznan clays) covers the coal stratum with a thin layer of a thickness ranging approximately between 5 and 15 m. The Quaternary forms overlie the Tertiary ones; they consist of overburden sands (central), boulder clays, and near-surface sands. The central sands (from several to about 19 m thick) show a considerable variability (not only in the vertical profile, but also in the horizontal distribution), and are characterized by the occurrence of gravel agglomerates. The sands are overlain by boulder clays in the form of a continuous layer with a

Prediction of subsoil subsidence caused by opencast mining 169 «teéfemsm^fe»*-*^ 1 4 _t 1 X Clay Olin Brown Coal Test u 0 1 2 3 U Tesr date 1959. 05. 04 1963 05. 13 1963,08.25 196312.06 196V 06.07 196412.10 Symbol Fig. 2 Readings from superficial benchmarks measuring surface displacements and water surface: (a) geological cross section, (b) factor m, (c) displacements of benchmarks, (d) of water surface N II, (e) variations of water surface N III. + i. *, X thickness varying between several and approximately 16 m. The roof part of the Quaternary forms is built of a layer of near-surface sands which differ in thickness. Two major aquifers were distinguished in the seam under study: (a) In coal-overlying sands: the roof and floor being built of boulder clays, respectively (referred to as N II). (b) In coal-underlying sands: separated from the roof by the brown-coal seam (referred to as N III). SUGGESTED FORM OF CORRELATION The following form was postulated: s { = $(A# (.,m (.) (1) where s t is vertical displacement of soil surface; AHj denotes variation of piezometric groundwater table, and m t indicates geological structure of the soil mass which is to be dewatered.

170 Halina Konderla & Made] Hawrysz Soil surface deformation was predicted by making use of the proposed correlation, and taking into account the results of geodetic survey and piezometric measurements carried out in the opencast mine under study. The geodetic line comprised 28 surface benchmarks placed along the west-east direction. The situation of the measuring points is shown in Fig. 1. From 13 May 1963 to 10 December 1964 five geodetic observations were carried out to determine the relative displacements of the benchmarks, without controlling their heights with respect to the benchmark network outside the open pitmined area. In the first version of the equation (1) subsidence was related solely to the soil-mass dewatering which, in that particular case, was expressed in terms of the total depression of the coal-overlying and coal-underlying groundwater tables (AH,-)- To determine the form of the correlation function it is necessary to establish sets of AH',- and m i values at the points which correspond with the location of the benches observed. Not only geological boreholes for the determination of the seam structure, but also hydrogeological wells for the measurement of water horizon positions were located at other points than those of the observation network. Thus it was necessary to make use of approximating procedures so as to find the values of AH t and m,-. The application of the SURFER packet made it possible to draw three-dimensional maps of the values to be found (e.g. Fig. 3(a)) and two-dimensional maps for the inclusion of isolines and measuring points (Fig. 3(b)). ANALYSIS OF MEASURED DATA The geological structure of the soil mass corresponding to the geodetic observation line is shown in Fig. 2(a). Figures 2(d) and 2(e) present the varying positions of the coaloverlying water table (N II) and coal-underlying water table (N III), respectively. The same figures also include undisturbed water table positions, as they were prior to the dewatering of the excavation (symbol u). The dewatering of the excavation (which began in August 1959) was carried out through drilled wells which drew water from overburden sands and coal-underlying sands. The results of geodetic observations are plotted in Fig. 2(c). In addition, Fig. 2 incorporates the plot of parameter m. Its value corresponds with the ratio of non-cohesive soil thickness to total thickness of the active layer. The term "active layer" has been adopted to denote the total thickness of the layers occurring between the actual piezometric level of the groundwater table and the roof of non-deformable strata. The actual piezometric level is defined by the values of NII and N III (which are similar). The depth of active layer occurrence is limited by the roof of the Cretaceous stratum which has a stress-strain characteristics differing considerably from that of the overlying soil series. The relative displacements of the benchmarks (s) were related to measurement 0, carried out in May 1963. Successive measurements were performed in August 1963, December 1963, June 1964 and December 1964 (and are denoted as 1, 2, 3 and 4 respectively). On analysing the variations of the benchmark positions, we noticed two distinct stages of the subsidence process. Thus, stage one consisted of subsidence alone (measurements 1 and 2 of Fig. 2(c)) and stage two was characterized by the commencement of the soil surface rise (measurements 3 and 4). In the course of geodetic observations, both groundwater levels decreased at constant average rates. Thus, NII had a subsidence rate of about 2 m year" 1 and 1 m year" 1 in the time interval between measurement 0 and 2 and in the time interval

Prediction of subsoil subsidence caused by opencast mining 171 (a) HIII m Fig. 3 Plots of factor m: (a) 3D view, (b) contour map. X * - Pie.zorne.ter between measurement 2 and 4, respectively. The subsidence rate of N III amounted to 5 m year 1 for the entire period under study. The rise of the soil surface recorded during measurements 3 and 4 should be attributed to the decompression of the soil mass induced by the stripping of the overburden which began on 1 September 1961. Figure 4 shows the varying positions of the working fronts which correspond with geodetic measurements 2 and 4. The cross section of the open pit is an extension of the observation line in the west-direction. From the data of Fig. 4 it is clearly seen that the overburden volume stripped in the time interval between measurements 2 and 4 was substantially greater than the one removed between measurements 0 and 2. It should be noted that the vertical soil displacements recorded in this study reflect the contribution of both dewatering and decompression.

172 Halina Konderla & Made] Howrysz 111 Brown Coal Overburden stripped between: [ I Sep. 1961 and May 1963 V//A May and Dec. 1963 KR&j Dec. 1963 and Dec. 1964 Fig. 4 Mining operations in the timespan from 1961 to 1964. PRELIMINARY MODEL FOR THE PREDICTION OF LAND SURFACE SUBSIDENCE Analysis of soil displacement measurements, exemplified by the observation of the aforementioned measuring line, allows only preliminary interpretation of the subsidence process by making use of an increment model. A very simple form of the correlation (1) was adopted: As i = Am i AH i (2) where As,- denotes height variation of the ith point on the soil surface (( - ) and (+) being subsidence and rise, respectively); m i indicates the geological structure coefficient (defined in an earlier Section) at the ith point; AH i describes total change of groundwater table at the ith point ((-) standing for depression of water tables, and (+) representing rise of water tables), and A is scale coefficient determined by approximation. The subsidence process was found to run at two distinct stages: Stage I (subsidence) corresponding with the time interval between measurements 0 and 2. Stage II (rise) corresponding with the time interval between measurements 2 and 4. Assuming that the scale coefficient A is constant, its value was assessed by the leastsquare method. Thus, for Stage I: A = 4.1, and correlation coefficient R = 0.79 for Stage II: A = 2.9, and correlation coefficient R = 0.88 The procedure for the assessment of the A-values, and the justification for the low correlation coefficient values are found in Fig. 5. In this way it has been possible to describe the deformation of the soil surface along the analysed measuring line.

Prediction of subsoil subsidence caused by opencast mining 173 mm 30 20 10 0> 0-10 -20-7 -6-5 -4-3 -2-1 0 m-ah v test 0-2 o test 2-4 Fig. 5 Evaluation of scale coefficient. CONCLUSIONS Provided is an example of how to interpret the results from the measurements of vertical soil displacements recorded in the vicinity of a brown-coal mine in Poland. Taking into account the insufficiency of the data base, as well as the complex nature of the process, the correlation obtained has a local meaning and should therefore be regarded as a preliminary approach to the construction of a feasible model which would predict the deformation of the soil surface exposed to opencast mining. Further verification of the model is recommended. A major postulate for the model is that it should incorporate the decompression induced by the removal of the overburden, by the working of the seam and by the construction of the dumping ground. REFERENCES Przedsiebiorstwo Geologiczne (1966) Kompleksowa aktualizacja dokumentacji geologicznej (Follow-up of geological specification). Przedsiebiorstwo Geologiczne, Wroclaw. Tanski, T. (1991) Surfer. PLI, Warszawa.