| Large scale mining of coal resources, in parallel with providing support for the national economic and social development, but also brought serious damage to the land and environment, particularly in the plain mining area with high ground-water level in Eastern China, led to a large reduction in farmland, increase in forced relocation of villages, a substantial increase of landless peasants due to mining subsidence. Due to the particular characteristics of mining subsidence of the plain mining area with high ground-water level, the traditional reclamation technique, such as dig deep to fill shallow, filling with coal gangue and fly ash cannot solve the present situation of a sharp reduction in the cultivated land of the region. Taking advantage of some mining area close to the Yellow River, the sediment can be excavated from the Yellow River, and then transported to mining subsidence land by pipeline and filled. This has an associated potential benefit of both disposing of the dredging sediment that is frequently produced by dredging operations while providing reclamation materials, which can reclaim large amounts of arable land and play an important role to ensure the security of the Yellow River flood discharge. Currently there was no report on the research of filling reclamation of mining subsidence land with Yellow River sediment, and the present technology originated from the experiences of improving soil and strengthening dike by warping in flood plains of Yellow River. There were some problems with the present technology, that was the drainage velocity was too fast in the early filling process, resulting in large loss of fine sediment(clay and most of silt), and the lateral drainage velocity and efficiency of the filled saturated sediment was too low in the later drainage process, prolonging the reclamation period. Thus we need to conduct research on new technique of filling, desilting and drainage of water-sediment mixture to reclaim mining subsidence land, in order to increase the lateral drainage velocity of filled saturated sediment in the later drainage process and shorten the reclamation period.Based on the existing achievements, this paper made full use of related knowledge of land use engineering, soil science, agronomy, water conservancy engineering, civil engineering, geotechnical synthetic materials. Firstly, one waterlogged lowland, which was close to Yellow River and with similar topographical features of mining subsidence land, was selected as the experimental site, then it was reclaimed by use of the existing technique of filling and drainage of water-sediment mixture. The landscape features, soil profiles and crop yields of the reclaimed farmland were analyzed and the problems of the existing technique of filling and drainage was discussed in mechanism. Then the new process of filling, desilting and drainage was designed. The key parameters and their experimental acquisition procedures of the new process were proposed. Secondly, the design methods of size and elevation and the calculating methods of water surface profile, sedimentation rate and deposition thickness of the reclamation stripe were discussed, the signification and calculating methods of filling times and interval time of reclamation stripe were proposed. The theoretical simulation calculation of the size, elevation, sedimentation rate, deposition thickness, filling times and interval time of the case site were conducted by use of the basic parameters gained by indoor experiment, and the corresponding design values were provided. Lastly, the best geotextile type and discharge cross section were screened out by the experiments of equivalent aperture, coefficient of vertical permeability, clogging and indoor filling and drainage of water-sediment mixture. The results were as the following.(1) There were three problems with the existing technique of filling and drainage of water-sediment mixture.(1)The drainage velocity of the early filling process was too fast, resulting in large loss of fine sediment(clay and most of silt), not only could deposit the drainage canals nearby, but also reduce the nutrient content of the sediment settled in the filling area.(2)The size of the filling area was not reasonable, leading to the increase in engineering work of texture mixture and leveling of filled sediment.(3)The lateral drainage velocity and efficiency of filled saturated sediment in later drainage process was low, prolonging the reclamation period. Based on the mechanism analysis of the problems, the new process of filling, desilting and drainage of water-sediment mixture was designed, in which the sharp, size, elevation, filling direction, times, interval time, geotextile type and discharge cross section were the key points.(2) The design methods of size of desilting basin for warping and irrigation in Yellow River flood plain were discussed. The result indicated that the quasi hydrostatic settlement method and one-dimensional flow supersaturated sediment method could be used to design the size of the reclamation stripe, in which the average settling velocity of sediment, average depth of water and average flow velocity of water-sediment mixture were the key parameters for design. The design values of size of reclamation stripe were provided by use of the two methods mentioned above. Under three different hypothesizes, the theoretical length of the reclamation stripe calculated by quasi hydrostatic settlement method was between 14.64 and 29.28 meter, revised length was between 36.60 and 73.20 meter, minimum theoretical width was between 0.7 and 1.4 meter, maximum allowed width was between 4.5 and 13.5 meter. While the length of the reclamation stripe was between 31.86 and 65.39 meter, width was between 2.83 and 7.65 meter calculated by one-dimensional flow supersaturated sediment method. Based on the elevation design methods of warping in Yellow River flood plain and filling reclamation of mining subsidence with coal gangue, the theoretical simulation calculation of elevation of mining subsidence land reclaimed with Yellow River sediment was conducted. The results showed that the designed filling elevation should be 1.274 times as high as that of designed using elevation.(3) The calculating methods of water surface profile, sedimentation rate, deposition thickness, filling times and interval time of the reclamation stripe were discussed.(1)The filling times of the reclamation stripe is proportional to the stripe size and filling sediment concentration, and inversely proportional to sediment density. Under the situation of the stripe depth of 2.0 m, the filling number which meets the design requirements is 5 times.(2)As for the case stripe, it can be divided into eight calculation section, the section sediment concentration, sedimentation rate, total sedimentation rate, sedimentation volume and average desilting thickness were simulated and calculated according to the initial flow velocity of water-sediment mixture of 0.4, 0.3, and 0.2 m/s, respectively. The results indicate that the greater the initial flow velocity of filled water-sediment mixture in the stripe, the smaller the sedimentation rate and desilting thickness in the first half stripe. Conversely, the sedimentation rate and desilting thickness is greater.(3)The interval time of multiple filling of the reclamation stripe, includes filling time(T1), hydrostatic settling time(T2), water discharge time(T3), lateral drainage time of filled saturated sediment(T4) and time of backfilling, leveling and compaction of subsoil(T5). Under the traditional situation of drainage in single direction, the maximum consolidation time at the consolidation degree of 75%, which meets the requirements of sediment leveling and subsoil backfilling by construction machinery, is between 61 and 75.5 days, the interval time of multiple filling is 70.5 and 85.2 days. The maximum consolidation time at the consolidation degree of 90% is between 100.4 and 124.3 days, and the interval time of multiple filling is 109.9 and 134 days. However, under the situation of double direction drainage, the maximum consolidation time at the consolidation degree of 75% is between 15.3 and 18.9 days, and the interval time of multiple filling is 17.5 and 21.1 days, while the maximum consolidation time at the consolidation degree of 90% is between 25.1 and 31.1 days, and the interval time of multiple filling is 27.3 and 33.3 days.(4) The screening criteria of geotextiles for drainage and sediment retention was discussed, and a series of experiments of equivalent aperture, coefficient of vertical permeability and clogging of geotextiles were conducted. The results show the five primarily selected geotextiles could meet the soil retention and water discharge principles, but neither of them could meet the prevention requirements of clogging. After discuss with the manufacturer, they adjusted the fiber diameter and increased the porosity of the geotextiles. We finally selected the improved acupuncture non-woven geotextiles with weight of 250 g and 300 g.(5) By use of the selected geotextiles, the indoor simulation experiment of filling and drainage of water-sediment mixture in glass tanks was conducted in 2013 and 2014, respectively. The results of the experiments conducted in 2013 indicated that the treatments using the geotextiles could reduce the sediment concentration in drainage significantly, however, the effectiveness on fine sediment retention was not optimal, and the target on increasing the lateral drainage of filled saturated sediment was not achieved, because the equivalent aperture of the selected geotextiles were too small, and the probe of the equipment used to measure the water content change of the filled saturated sediment was not fixed at the same location. The new experiment was conducted in 2014 by use of the improved acupuncture non-woven geotextiles with weight of 250 g and 300 g. The results were as follows.(1)There was significant difference on the sediment concentration in drainage of the three experimental treatments. In the first stage of drainage(10 min after drainage stabilization), the sediment concentration in drainage of ZT-250 and ZT-300 treatments was higher than that of CK, while at the middle and later stage, the sediment concentration in drainage of the treatments using the geotextiles were significantly lower than that of CK, and ZT-300 treatment had the best effectiveness. The sediment concentration in drainage of the treatments using the geotextiles decrease with time during the experiment, while that of CK treatment had the converse trend.(2)In the first and medium stages, the characteristic grain sizes of sediments in drainage of the treatments using geotextiles were higher than those of CK, while the clay content was lower than that of CK. In the later stage, the characteristic grain sizes of sediments in drainage of the treatments using geotextiles were comparable with those of CK, while the clay content of CK was between that of ZT-250 and ZT-300. The clay content of sediments in drainage of the geotextiles treatments was increasing with time, while CK presented the opposite trend. Besides, the geotextiles had stable effects on retaining sediments in glass tank.(3)Most of the particle of the filled surface sediment of the three treatments was sand. The silt content of surface sediment of ZT-300 using improved acupuncture non-woven geotextiles with weight of 300 g was highest, and the very fine sand content of surface sediment of ZT-250 using improved acupuncture non-woven geotextiles with weight of 250 g was highest.(4)There was difference on the water content of filled surface and bottom sediment. The geotextiles could increase the lateral drainage velocity of the saturated sediments and the treatment of ZT-250 had the best performance.It is an ambitious project to reclaim the mining subsidence land with Yellow River sediment, but the current related technology is not mature. This paper only made some exploration on the new process of filling, desilting and drainage of water-sediment mixture, and gained some understanding. However, a lot of exploration is limited to indoor experiment and theoretical simulation calculation because of the constraints. In the future, the settling rate and deposition thickness of filled sediment under the situation of different stripe size and initial velocity of water-sediment mixture should be measured in the field engineering, and comparing with the indoor theoretical simulation results. Secondly, the consolidation apparatus, which can simultaneously determine the amount of compression and permeability coefficient of sediment sample under different loads, should be used in the future, and the consolidation coefficient of sediment sample under different load should be calculated through the theoretical formula, then the more accurate consolidation time of the sediment can be calculated. Finally, the experiment of filling and drainage of water-sediment mixture should be carried out in the field, in order to further verify the indoor screening results of drainage section form and geotextile type. |
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