Commonly used foundation treatment methods include: cushion replacement, dynamic compaction, gravel pile method, vibroflotation method, cement-soil mixing method, high-pressure jet grouting method, preloading method, rammed cement-soil pile method, cement-fly ash gravel pile method, lime pile method, lime soil compaction pile method, soil compaction pile method, column hammer compaction pile method, single liquid silicification method and alkali solution method.
1, the replacement cushion method is suitable for the treatment of shallow soft foundation and uneven foundation. Its main function is to improve the bearing capacity of foundation, reduce settlement, accelerate the drainage consolidation of soft soil layer, prevent frost heave and eliminate the expansion and contraction of expansive soil.
2. Dynamic compaction method is suitable for treating gravel soil, sandy soil, silt, cohesive soil, collapsible loess, miscellaneous fill and plain fill with low saturation. Dynamic compaction replacement method is suitable for projects with poor deformation control on high saturated silt, soft plastic cohesive soil and other foundations, and its applicability and treatment effect must be determined through field tests before design. Dynamic compaction method and dynamic compaction replacement method are mainly used to improve soil strength, reduce compressibility, improve soil anti-vibration liquefaction ability and eliminate soil collapsibility. Saturated cohesive soil should be combined with surcharge preloading and vertical drainage.
3. Sand-gravel pile method is suitable for compaction of loose sand, silt, cohesive soil, plain fill, miscellaneous fill and other foundations, which can improve the bearing capacity of foundation and reduce compressibility, and can also be used to treat liquefiable foundation. For projects with lax deformation control on saturated clay foundation, sand-gravel piles can also be used for replacement treatment, so that sand-gravel piles and soft clay can form a composite foundation, accelerate drainage consolidation of soft soil and improve foundation bearing capacity.
4. The vibroflotation method can be divided into two types: with filler and without filler. Filling soil is usually called vibro-replacement gravel pile method. The vibroflotation method is suitable for treating foundations such as sandy soil, silt, silty clay, plain fill and miscellaneous fill. For the treatment of cohesive soil and saturated loess foundation with undrained shear strength not less than 20kPa, its applicability should be determined by field test before construction. Vibration compaction without filler is suitable for treating medium-coarse sand foundation with clay content not exceeding 10%. Vibro-replacement gravel pile is mainly used to improve the bearing capacity of foundation and reduce the settlement of foundation, and can also be used to improve the sliding stability of soil slope or the shear strength of soil.
5. Cement-soil mixing method is divided into slurry deep mixing method (wet method for short) and powder jet mixing method (dry method for short). Cement-soil mixing method is suitable for treating normal consolidated silt and silt, cohesive soil, silt, saturated loess, plain fill and saturated loose sand without flowing groundwater. It is not suitable for treating peat soil, clay with plasticity index greater than 25, foundation with corrosive groundwater and high organic matter content. If it needs to be adopted, its applicability must be determined through experiments. This method is not suitable when the natural water content of foundation is less than 30% (loess water content is less than 25%), more than 70% or the pH value of groundwater is less than 4. Continuous overlapping cement mixing piles can be used as waterproof curtain for foundation pit. Because of its mixing ability, it is difficult to apply this method in cohesive soil and silt foundation with foundation bearing capacity greater than 140kPa.
6. The high-pressure jet grouting method is suitable for treating silt, muddy soil, cohesive soil, silt, sandy soil, artificial fill and gravel foundation. When the soil foundation contains a large number of large-size stones, a large number of plant roots or high organic matter, its applicability should be determined according to the field test results. It is not suitable for the situation that the groundwater velocity is too high and the shotcrete cannot solidify around the grouting casing. High-pressure jet grouting pile has a deep treatment depth, which can be used as a waterproof curtain for deep foundation pit or dam in addition to foundation reinforcement. At present, the maximum treatment depth has exceeded 30m.
7, preloading method is suitable for dealing with silt, muddy soil, fill and other saturated cohesive soil foundation. According to the preloading method, it is divided into surcharge preloading method and vacuum preloading method. The surcharge preloading is divided into plastic drainage belt or sand drain foundation surcharge preloading and natural foundation surcharge preloading. When the thickness of soft soil layer is less than 4m, the natural foundation surcharge preloading method can be used; When the thickness of soft soil exceeds 4m, vertical drainage preloading methods such as plastic drainage belt and sand drain should be adopted. For vacuum preloading engineering, drainage wells must be set in the foundation. Preloading method is mainly used to solve the settlement and stability of foundation.
8. Compacted cement-soil pile method is suitable for treating silt, plain fill, miscellaneous fill, cohesive soil and other foundations above groundwater level. This method has the advantages of short construction period, low cost, civilized construction and easy cost control. At present, it has been successfully applied to the old city reconstruction projects in Beijing, Hebei and other places.
9. Cement fly ash gravel pile (CFG pile) method is suitable for treating cohesive soil, silt, sandy soil, self-compacting plain fill, etc. The applicability of muddy soil should be determined according to regional experience or field test. A cushion with a certain thickness should be set between the foundation and the top of the pile to ensure that the pile and soil share the load and form a composite foundation. This method is suitable for strip foundation, independent foundation, box foundation and raft foundation, and can be used to improve the bearing capacity of foundation and reduce deformation. For the liquefiable foundation, the multi-pile composite foundation of gravel pile and cement fly ash gravel pile can be used to eliminate the liquefaction of foundation soil and improve the bearing capacity.
10, lime pile method is suitable for treating saturated cohesive soil, silt, muddy soil, miscellaneous fill and plain fill. When it is used in the soil layer above the groundwater level, the strength of the pile can be improved by reducing the amount of quicklime and increasing the water content of the admixture. This method is not suitable for underwater sand.
1 1, lime-soil compaction pile method and soil compaction pile method are suitable for treating collapsible loess, plain fill and miscellaneous fill above groundwater level, and the depth can be 5 ~ 15m. When it is used to eliminate the collapsibility of foundation soil, the soil squeezing pile method should be adopted. When it is used to improve the bearing capacity of foundation soil or enhance its water stability, lime-soil compaction pile method should be adopted; This method is not applicable when the water content of local foundation soil is more than 24% and the saturation is more than 65%. The lime-soil compaction pile method and soil compaction pile method have basically the same effect in eliminating soil collapsibility and reducing permeability. The bearing capacity of soil compaction pile method and high pressure jet grouting method is suitable for treating silt, muddy soil, cohesive soil, silt, sandy soil, artificial fill and crushed stone foundation. When the soil foundation contains a large number of large-size stones, a large number of plant roots or high organic matter, its applicability should be determined according to the field test results. It is not suitable for the situation that the groundwater velocity is too high and the shotcrete cannot solidify around the grouting casing. High-pressure jet grouting pile has a deep treatment depth, which can be used as a waterproof curtain for deep foundation pit or dam in addition to foundation reinforcement. At present, the maximum treatment depth has exceeded 30m.
12, column hammer punching pile method is suitable for treating miscellaneous fill, silt, cohesive soil, plain fill, loess, etc. For saturated soft soil below the groundwater level, its applicability should be determined through field tests. The depth of foundation treatment should not exceed 6m. Collectors will retreat.
13, single liquid silicification method and alkali dissolution method are suitable for treating collapsible loess and other foundations with permeability coefficient of 0. 1 ~ 2m/d above groundwater level. In the site of self-weight collapsible loess, the applicability of alkali liquor method should be determined through experiments for grade II collapsible loess foundation.
14. When determining the foundation treatment scheme, different methods should be selected for comparison. For composite foundation, the scheme selection is to improve the amplitude and select the appropriate pile-forming technology and reinforcement materials according to different soil quality and design requirements.