1 general principle of soft foundation treatment 1. 1 natural settlement method: that is, preloading soft foundation as soon as possible without deep treatment is the most economical and simple method to gradually achieve subgrade stability through natural settlement. However, at present, the procedures of capital construction can not allocate, requisition and construct calmly as soon as possible, and once the project is put into practice, it is often limited by the construction period, so it will be difficult to realize natural settlement in general. 1.2 engineering technology treatment: that is, under the condition of tight construction period and limited time, different engineering technology methods are adopted to treat soft soil. The design principle should combine the successful experience of local road foundation treatment, adopt applicable, fast and effective treatment methods, ensure the engineering quality and foundation treatment construction progress, minimize environmental pollution and reduce the impact on residents along the route. 3 Comparison of foundation treatment technologies The commonly used foundation treatment technologies in Guangdong Province are: replacement of cushion, surcharge preloading with plastic drainage board (bagged sand drain), vacuum preloading with plastic drainage board (bagged sand drain), cement powder injection pile (mixing pile), gravel pile, compacted sand pile, CFG pile (cement fly ash gravel pile) and dynamic drainage consolidation method. The advantages and disadvantages of various foundation treatment technologies are compared as follows: changing the cushion (reinforcement depth is 3-4m) Advantages: the construction technology is mature, the method is simple and fast, and the cost is low, which can quickly improve the bearing capacity of the foundation. Disadvantages: the greater the replacement thickness, the higher the cost; It is not suitable to treat the foundation with thick soft soil layer and deep buried depth, and the groundwater level in permeable area is high and the cost of pumping and drainage is high. Advantages of plastic drainage board (bagged sand drain) surcharge or overload preloading (reinforcement depth less than 20m): mature construction technology, simple method and low cost, which can effectively eliminate the main consolidation settlement. Disadvantages: the construction period is long, and the high stacking height will obviously increase the cost; It is difficult to effectively solve the secondary consolidation settlement of foundation. Advantages of vacuum preloading (reinforcement depth less than 15m) of plastic drainage board (bagged sand drain): it can be loaded at one time, saving the time of loading and unloading in stages, and the preloading time is shorter than that of surcharge preloading, which can effectively eliminate the main consolidation settlement and is suitable for soft foundation treatment in large areas (roads, yards, docks, airports, etc.). ) There is no problem of abandoning soil. Disadvantages: the process requirements are high, especially the sealing curtain and vacuum sealing film should be sealed, and the cost is slightly higher than that of surcharge preloading; It is difficult to effectively solve the secondary consolidation settlement of foundation. Advantages of cement powder jet grouting pile (reinforcement depth is less than 18m): mature technology, fast progress and short construction period, which can effectively eliminate primary and secondary consolidation settlement. Disadvantages: when the organic matter content and plasticity index of sludge are large, the treatment effect is not good; The treatment depth is usually less than18m; The cost is higher. Advantages of nail-shaped cement-soil bidirectional mixing pile (reinforcement depth of 25m): short construction period, better cement-soil mixing effect than ordinary mixing pile, deeper treatment depth, better effect of improving bearing capacity and reducing post-construction settlement than ordinary cement mixing pile. Disadvantages: the cost is higher than that of ordinary cement mixing pile, which belongs to a new patented technology. At present, it has not been written into the national standard, and there are not many construction units that master this technology, which is not easy to operate. Advantages of dynamic drainage consolidation (reinforcement depth is 8 8- 10/0m): the foundation soil completes most consolidation settlement in a short time and becomes overconsolidated soil, greatly reducing post-construction settlement and rapidly improving bearing capacity; In addition, it can also realize the pre-earthquake effect on the foundation and effectively eliminate the liquefaction of sand; This method is also beneficial to the excavation of underground pipelines. Disadvantages: the construction procedure is complicated; Must ensure that the precipitation and drainage measures; Additional measures should be taken to ensure the treatment effect of reservoir interval; The treatment depth generally does not exceed10m; It has a great impact on the surrounding environment. Advantages of gravel pile (reinforcement depth is 20-25m): short construction period, mature technology and good liquefaction resistance; Conducive to the dissipation of pore water. Disadvantages: high cost; For muddy soil with low shear strength of vane, it is difficult to pile, the pile diameter is difficult to control, and the bearing capacity is slightly improved. Post-construction settlement is difficult to control. Advantages of compaction sand pile (reinforcement depth is about 20 meters): it has the functions of compaction, replacement and drainage consolidation; Can effectively treat the easily liquefied foundation; The cost is lower than that of gravel piles; The construction period is shorter than that of drainage consolidation method. Disadvantages: the treatment of silt foundation needs to be combined with surcharge preloading, and the replacement rate needs to be controlled; Not suitable for foundations with high bearing capacity requirements. CFG pile (cement fly ash gravel pile, reinforcement depth 25-30m) Advantages: The progress is fast, which can reach a larger reinforcement depth (more than 20m), significantly improve the bearing capacity of soft foundation and reduce settlement. Disadvantages: the cost is high, and the pile-forming technology (throwing soil or squeezing soil) used for silt foundation is controversial; Silty foundation should be used with caution. 4 Comparison of foundation treatment process cost and construction period The foundation treatment process is compared according to certain specifications, and the estimated cost and construction period are as follows: replacement cushion: unit price: 150-2 10 yuan/m2, and the construction period is determined according to the construction organization; Vacuum preloading of plastic drainage boards: drainage board spacing 1.2m, unit price 130- 140 yuan /m2, and preloading period is 3 to 4 months; Pre-assembly of plastic drainage boards: the spacing of drainage boards is 1.2m, the unit price is 80- 120 yuan /m2 according to the stacking height, and the pre-assembly period is 6-8 months; DJM pile: pile diameter is 50cm, spacing is 1.5m, unit price is 220 yuan /m2, spacing is 1.8m, and unit price is 190 yuan /m2. Age 30 to 60 days; Dynamic drainage consolidation: spacing of bagged sand wells 1.2m, diameter of 0.07m, unit price 150 yuan /m2, and dynamic compaction period of 60-90 days; Gravel pile: the pile diameter is 0.8m, the spacing is 1.6-2.0m, the unit price is 530 yuan /m2, and the drainage period is 15-20 days; Compacted sand pile: pile diameter is 0.4m, spacing is 1- 1.5m, unit price is 200-280 yuan /m2, and drainage period is 15-30 days; CFG pile: pile diameter is 0.4m, spacing is 1.8-2m, unit price of sand pile is 274 yuan /m2, and age is 30-60 days; No sand pile, unit price: 2 12 yuan/m2. Note: The reinforcement depth is calculated as10m. 5 engineering example Take Student Avenue of Dongshan Education Base in Meizhou City as an example. Zi Xue Avenue connects Xiulan Bridge in the south and ends at Dongshan Avenue. The total length of the road is 2112.2m, and the width of the road is 40m. The building red line on both sides is 5 meters, with a total width of 50 meters. It is not only the main traffic trunk road of Dongshan Education Base, but also a beautiful landscape trunk road in the urban area. 5. 1 Site landform and engineering geological conditions The present landform is platform, hill and plain, the surface is alluvial red clay and fine sand clayey silt, and the foundation bearing capacity is 12- 15kPa/ m2 and 200kPa/ m2 respectively. The groundwater level is about 2-5 meters. The surface layer of the site mainly exposes miscellaneous fill, plain fill and cultivated soil with different thicknesses. There is no soft soil layer on the site foundation. The site soil type is soft soil to hard soil, and the construction site type is Class II. In fish ponds and agricultural areas, the topsoil is silt and cultivated soil with small thickness (usually less than 2 meters), low bearing capacity, high compressibility and poor engineering properties. The underlying layer has high bearing capacity and good engineering properties. Site leveling has little effect on subgrade construction. 5.2 Characteristics of site engineering The present situation of land use along roads is mainly construction sites, villages and farmland, and a large number of houses need to be demolished. Houses along the line are dense, and noise, vibration and dust pollution should be controlled during foundation treatment construction. Dynamic compaction, lime pile, cement powder injection pile and other soft foundation treatment technologies are not applicable. There are many villages along the road, and the village roads are dense. In the process of construction, it can be used as a construction channel to facilitate the transportation of materials and earthwork. There are some fish ponds along the highway, but the scope of land acquisition for fish ponds is limited, so cofferdam and drainage measures should be taken before subgrade construction. Ditches along the line should be reserved or moved, and ditches on both sides of the road should be set through culverts. When burying ditches, the quality of subgrade filling should be controlled. 5.3 Foundation treatment scheme Considering the engineering geological conditions, engineering characteristics and short construction period of the site, this project plans to use the replacement cushion method to treat the road subgrade, and the replacement depth should be controlled within 2.5m The specific measures are as follows: the ground of house demolition site is loose miscellaneous fill and housing infrastructure, which has a great impact on the subgrade filling quality, so it must be excavated, and the excavation depth should be controlled within1.5m. Because most houses to be demolished are located on the platform, Compaction requirements shall be met during rolling and backfilling. For farmland sections and fish ponds, surface soil or floating silt should be removed to a thickness of 0.8- 1.8m, and coarse sand or gravel (the weight ratio of sand to gravel is 7: 3-6: 4) should be replaced to above the groundwater level, and then cohesive soil should be replaced to the elevation of the road bed, and then the pavement structure construction should be carried out. In order to avoid the uneven settlement of the new and old roadbed and pavement, cement concrete slabs, tie rods and dowel bars with the same thickness are added in the extended pavement range, and the differential settlement can be effectively reduced by combining the method of laying geogrids in vertical and horizontal directions. In order to avoid uneven settlement of subgrade caused by different foundation treatment methods, a layer of biaxial stretching geogrid is laid longitudinally in the range of 15m before and after the joint of different treatment methods. In order to avoid the uneven settlement of embankment caused by the difference of foundation bearing capacity at the filling-excavation interface, a biaxial stretching geogrid with the length of 10-20m is laid along the longitudinal and transverse directions.
Only by carrying out detailed geological investigation on the coastal soft soil foundation site, finding out the topography, geomorphology and hydrogeology of the site, carefully designing, repeatedly studying, comparing schemes according to different engineering properties and geological characteristics, and adopting the best treatment method, can we design safe, reasonable and economical buildings and structures.
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