Surface movement above old coal longwalls after mine closure

Although most subsidence occurs in the months and years after mining by the longwall method, surface movement is still occurring many decades after the mining. The aim of the study is to quantify the long term behavior. Satellite data(radar-interferometry) were analyzed to study an area of about 2 km^2 during the 18 years following the closure of the underground infrastructure and the flooding of the underground workings and rock mass. It was observed that, on average, a residual downward movement took place till 7–12 years after the closure, followed by a clear uplift. However, the first signs of an uplift occurred in certain sub-areas 3–4 years after the closure. Zones within the area studied were identified with either larger or smaller movements. However, the spatial variation of the surface subsidence or uplift could not be directly explained by the characteristics of mining.

Various phases in surface movements linked to deep coal longwall mining:from start-up till the period after closure

This paper covers the entire lifetime of an underground coal mine,including the start-up of the mining operations and the period after the closure and sealing of the underground workings.Measurements of the vertical surface movement are presented for three neighbouring mines in the Belgian Campine coal basin,where the longwall mining method was applied.First,the monitoring of the initial phase showed that the impact of the first panel was smaller than the impacts of the following two neighbouring panels.When the second panel was mined,a change in the induced surface tilt was observed in the immediate proximity of the panels.Second,data for a transect with a length of about 2.5 km were presented,covering a total time of 54 years,of which the first 31 years were during the active life of the mine.Average subsidence rates of up to-0.3 m/year were observed.After the mine was closed and the pumping facilities were stopped,the average uplift rate was situated mostly between 4 and 16 mm/year,but a value of 141 mm/year also was measured.Thirty years after closure,the upward movements are still occurring.Third,the phase of uplift is analyzed in more detail over a full north–south transect.A clear narrow maximum was observed for the uplift,corresponding to a total vertical movement of 0.2 m over approximately an 18-year period.All these phases are relevant,for example when analysing damage to buildings and infrastructure.Damage becomes visible when the cumulative movement and loading exceed critical limits and not necessarily when the largest rate of movement is observed,for example.

Law of surface movement for multi-coal seam strip mining

It is an important part of green mining to control the disasters of coal mining which have caused irreversible damages to buildings and ecological environment. Strip mining is one of the efficient measures to control surface subsidence and mining damage. However, the research on the laws of the surface subsidence are still deficient in multi-coal seam strip mining at present. Based on the Fast Lagrangian Analysis of Continua(short for FLAC3D) numerical simulation software, the laws of the surface subsidence and horizontal movement were systematically studied for different depths, different mining widths, different distances between seams, different mining thickness, different parameters between seams and the special relations of the upper pillar and the lower pillar in the vertical direction in multi-seam strip mining. The function relation between the maximum subsidence and the maximum horizontal movement with the depth, the mining width, the seam distance, mining thickness, different parameters between seams and the partial offset are summarized respectively. Finally the formula integrating the surface maximum subsidence value and the maximum horizontal movement was deduced. The results can be used for reference theory and measure in forecasting the surface displacement in multi-coal seam strip mining.

Shanghai center project excavation induced ground surface movements and deformations

Empirical data on deep urban excavations can provide designers a significant reference basis for assessing potential deformations of the deep excavations and their impact on adjacent structures. The construction of the Shanghai Center involved excavations in excess of 33-m-deep using the top-down method at a site underlain by thick deposits of marine soft clay. A retaining system was achieved by 50-m-deep diaphragm walls with six levels of struts. During construction, a comprehensive instrumentation program lasting 14 months was conducted to monitor the behaviors of this deep circular excavation. The following main items related to ground surface movements and deformations were collected： （1） walls and circumferential soils lateral movements; （2） peripheral soil deflection in layers and ground settlements; and （3） pit basal heave. The results from the field instrumentation showed that deflections of the site were strictly controlled and had no large movements that might lead to damage to the stability of the foundation pit. The field performance of another 21 cylindrical excavations in top-down method were collected to compare with this case through statistical analysis. In addition, numerical analyses were conducted to compare with the observed data. The extensively monitored data are characterized and analyzed in this paper.

Thornthwaite moisture index and depth of suction change under current and future climate‒An Australian study

Climate change is one of the major global challenges and it can have a significant influence on the behaviour and resilience of geotechnical structures.The changes in moisture content in soil lead to effective stress changes and can be accompanied by significant volume changes in reactive/expansive soils.The volume change leads to ground movement and can exert additional stresses on structures founded on or within a shallow depth of such soils.Climate change is likely to amplify the ground movement potential and the associated problems are likely to worsen.The effect of atmospheric boundary interaction on soil behaviour has often been correlated to Thornthwaite moisture index(TMI).In this study,the long-term weather data and anticipated future projections for various emission scenarios were used to generate a series of TMI maps for Australia.The changes in TMI were then correlated to the depth of suction change(H s),an important input in ground movement calculation.Under all climate scenarios considered,reductions in TMI and increases in H s values were observed.A hypothetical design scenario of a footing on expansive soil under current and future climate is discussed.It is observed that a design that might be considered adequate under the current climate scenario,may fail under future scenarios and accommodations should be made in the design for such events.

THEORY OF STOCHASTIC MEDIUM AND ITS APPLICATION IN SURFACE SUBSIDENCE DUE TO EXCAVATION

Research on the stochastic theory and its application have been conducted in China for 40 years.This paper emphasizes on the basic theory of stochastic medium and its practice in predicting the ground movements and deformations induced by underground and open pit mining,near surface excavation of tunnel and so on.