Treatment and reinforcement methods of bad foundation
Abstract: This paper discusses the treatment method and reinforcement scheme of bad foundation soil and abnormal foundation soil before building.
[Keywords:] poor foundation; Basic anomaly; Foundation treatment; Construction technology; Foundation stiffness
Abstract: This paper introduces the treatment scheme and reinforcement method of bad and abnormal foundation before project implementation.
Building construction.
Keywords: bad foundation; Abnormal basis; Foundation treatment; Construction technology; stiff
In practical engineering, bad foundation and abnormal foundation often appear, which will have adverse effects on buildings if not handled properly. This paper will briefly introduce the treatment of bad foundation and abnormal foundation in order to better solve the foundation problems in engineering practice.
1 Treatment of bad foundation
1. 1 substitution method
1. 1 replacement filling method: the poor foundation soil on the surface is dug up, and then the soil with good compaction characteristics is backfilled for tamping or compaction to form a good bearing layer, thus changing the bearing characteristics of the foundation and improving the deformation resistance and stability.
Key points of construction: dig out the soil layer to be converted, and pay attention to the stability of the pit edge; Ensure the quality of the filler; Packing should be compacted in layers.
1. 1.2 vibroflotation replacement method: use a special vibroflotation machine to vibroflotation under high-pressure water jet, make holes in the foundation, and then fill the holes with coarse particles such as gravel or pebbles in batches to form piles. Pile and in-situ foundation soil form composite foundation, which can improve foundation bearing capacity and reduce compressibility.
Precautions in construction: the bearing capacity and settlement of gravel pile depend largely on the lateral constraint of in-situ foundation soil. The weaker the constraint, the worse the effect of gravel pile. Therefore, this method must be used cautiously in soft clay foundation with low strength.
1. 1.3 tamping (squeezing) replacement method: put the pipe (hammer) into the soil by sinking the pipe or tamping hammer, so that the soil is squeezed aside, and put fillers such as gravel or sand into the pipe (or tamping pit). Pile and in-situ foundation soil form a composite foundation. Due to soil compaction, the soil is squeezed laterally, the ground rises, and the excess pore water pressure of the soil increases. When the excess pore water pressure dissipates, the strength of soil increases accordingly.
Precautions in construction: When the filler is sand and gravel with good water permeability, it is a good vertical drainage channel.
1.2 preloading method
1.2. 1 surcharge preloading method: before construction, temporarily surcharge the foundation (sand and gravel, soil, other building materials, goods, etc.). ) and given a certain preloading period. After the foundation preloading completes most of the settlement and the bearing capacity of the foundation is improved, the load is unloaded and the building is built.
Construction technology and key points: ① the preloading load should be equal to or greater than the design load; ② Dump trucks and bulldozers can be used for large-scale stacking, and light machinery or manual operation can be used for the first-stage stacking of ultra-soft soil foundation; ③ The width of the top surface of the crib should be smaller than that of the bottom surface of the building, and the bottom surface should be enlarged appropriately;
⑤ The load acting on the foundation should not exceed the ultimate load of the foundation.
1.2.2 dewatering method: lowering the groundwater level can reduce the pore water pressure of the foundation, increase the self-weight stress of the overlying soil and increase the effective stress, thus preloading the foundation. This is actually to achieve the purpose of preloading by lowering the groundwater level and relying on the self-weight of the foundation soil.
Key points of construction: light well point, jet well point or deep well point are generally adopted; When the soil layer is saturated clay, silt, silt and muddy cohesive soil, it is appropriate to combine electrodes at this time.
1.3 The compaction method combined with surcharge preloading is adopted to accelerate the consolidation of saturated cohesive soil foundation.
1.3. 1 surface compaction method: loose surface soil shall be compacted by manual tamping, low-energy tamping machinery, rolling or vibration rolling machinery. Layered fill can also be compacted. When the water content of topsoil or fill soil is high, lime and cement can be spread in layers for compaction, so that the soil can be reinforced.
1.3.2 Heavy hammer tamping method: Heavy hammer tamping is to tamp the shallow foundation by using the greater tamping energy generated by the free fall of the heavy hammer, so as to form a relatively uniform hard shell layer on its surface and obtain a bearing layer with a certain thickness.
Key points of construction: before construction, conduct tamping test to determine relevant technical parameters, such as the weight of tamping hammer, diameter of bottom surface and falling distance, final settlement, corresponding tamping times and total settlement; Before compaction, the elevation of tank bottom and pit bottom should be higher than the design elevation; When tamping, the water content of foundation soil should be controlled within the optimal water content range; Large-scale compaction should be carried out in sequence; When the basement elevation is different, it should be deep first and then shallow; In winter construction, when the soil has been frozen, the frozen soil layer should be dug out or heated to melt; After completion, the compacted topsoil shall be removed in time or the floating soil shall be compacted to the design elevation at the drop close to1m.
1.3.3 Dynamic compaction: Dynamic compaction is short for dynamic compaction. When the heavy hammer falls freely from a height, it exerts high impact energy on the foundation and tamps the ground repeatedly, so that the particle structure in the foundation soil is adjusted and the soil becomes dense, thus improving the foundation strength and reducing the compressibility to a great extent. Its construction process: ① leveling the site; (2) laying graded macadam cushion; (3) setting gravel piers for dynamic replacement; (4) leveling and filling graded macadam cushion; ⑤ Full
Hit it again; ⑥ Level and lay geotextile; ⑦ Backfill the cushion of aeolian slag, and roll it with vibratory roller for eight times. Generally, before large-scale dynamic compaction, typical tests should be carried out in a site not exceeding 400m2 to obtain data and guide design and construction.
1.4 compaction method
1.4. 1 vibro-compaction method: Repeated horizontal vibration and lateral extrusion produced by special vibro-compaction equipment gradually destroy soil structure and rapidly increase pore water pressure. Due to structural damage, soil particles may move to the position with low potential energy, making the soil from loose to dense.
1.4.2 construction technology: ① level the construction site and arrange the pile positions. (2) The construction vehicle is in place, and the vibrator is aimed at the pile position. (3) Start the vibrator and slowly sink into the soil until it exceeds the reinforcement depth of 30~50cm, record the current value and time of the vibrator at each depth, and lift the vibrator to the orifice. Repeat the above steps 1~2 times to dilute the mud in the hole. (4) Pouring a batch of fillers into the hole, and sinking the vibroflot into the fillers for vibration to enlarge the pile diameter. Repeat this step until the depth current reaches the specified dense current, and record the amount of filler. ⑤ Lift the vibrator out of the orifice, continue the construction of the upper pile section until the whole pile vibrates, and then move the vibrator and the machine to another pile position. ⑥ In the process of pile formation, each pile should meet the requirements of dense current, grouting quantity and vibration time, and the basic parameters should be determined by field pile formation test. ⑦ A mud drainage system should be set in advance at the construction site to introduce the mud generated in the process of pile making into the sedimentation tank. The thick mud at the bottom of the tank can be dug out regularly and sent to the pre-arranged storage place, and the clear water at the upper part of the sedimentation tank can be reused. ⑧ Finally, the pile body with the thickness of 1m at the top of the pile is excavated or rolled, and compacted by dynamic compaction, and the cushion is laid and compacted.
1.4.3 grit pile (gravel pile, lime-soil pile, OG pile, low-grade pile, etc. ): After hammering, vibrating or static pressure sinking into a hole in the foundation by using a pipe sinking machine, it feeds into the pipe, and lifts (vibrates) the sinking pipe while feeding, forming a dense pile body and forming a composite foundation with the original foundation.
1.4.4 Tamping gravel pile (block stone pier): Tamp gravel (block stone) into the foundation with a heavy hammer or dynamic compaction, and gradually fill the rammed pit with gravel (block stone) and tamp it repeatedly to form gravel pile or block stone pier.
2 Treatment of abnormal ground conditions
2. 1 Disposal of loose soil pits (fill, grave pit, silt, etc.). )
2. 1. 1 excavate the soft and virtual soil in the pit, so that the natural old soil can be found at the bottom of the pit and the trench wall, and then backfill it with materials with compressibility similar to the natural soil near the pit. Backfill materials and methods: ① When the local foundation is sandy soil, backfill with sand or gravel, and the thickness of each layer of backfill is not more than 20cm, and it should be watered by layers or compacted with a flat vibrator. (2) When the local foundation is dense hard clay, 3∶7 lime soil can be used for layered compaction. (3) When the local foundation is medium-dense plastic clay, it shall be compacted and backfilled by layers with 1∶9 lime soil. (4) When excavating virtual soil, in case of groundwater, the underwater part will be backfilled with graded sand, and the water part can still be compacted with lime soil.
2. 1.2 When there is a virtual soil pit under the single-column foundation, it can be treated as follows: ① When the pit depth is greater than the groove width, or the pit area is greater than 1/3 of the groove bottom area, the groove bottom should be completely lowered to the pit bottom. (2) In cohesive soil, the height difference between the trough bottoms of two adjacent single-column foundations should not be greater than the clear distance between the adjacent column foundations, otherwise, the bottom of the shallow column foundation should be lowered accordingly, so as to make the bottom elevation of the two column foundations flush. (3) In sandy soil, the height difference between the bottoms of two adjacent single-column foundations should not be greater than 1/2 of the clear distance, otherwise the bottoms of two column foundations should be leveled. (4) If the pit bottom is too deep, consider increasing the area of the bottom of the foundation, or connecting it with the adjacent column foundation to form a combined foundation.
2.2 When there are hard soil or old structures locally (such as old foundation, old lime soil, septic tank, old brick kiln, compacted pavement, big tree roots, big stones, etc.), there are hard soil or old structures locally. ) below the foundation, dig it out completely, and then backfill or deepen the foundation according to the above method (it should be pointed out that only soft foundation is treated when foundation treatment is carried out. If the too solid foundation is not treated, it will also cause the building to stop production.
Large differential settlement).
2.3 Treatment of equipment and pipelines
When the pipes of water supply and drainage equipment pass above the tank bottom, a gap larger than the expected settlement of the building should be left above the pipes at the foundation wall to avoid the damage of the pipes when the building settles. At the same time, measures should be taken to prevent pipeline leakage and avoid uneven settlement caused by water soaking foundation. When the pipeline foundation passes through the foundation, the foundation can be buried deeply, so that the pipeline can pass through the foundation wall, and at the same time, enough space can be left above the pipeline according to the above principles.
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