With the popularization and application of manual digging pile technology in foundation engineering construction, its unique large bearing capacity has been recognized by many construction units, and it is considered that manual digging pile is indeed an economical and practical foundation form suitable for construction in soft soil foundation. Because the construction of soft foundation is often influenced by groundwater, sediment and silt layer, the construction technology of manual digging pile will encounter some difficulties. The following are solutions to several common problems.
I. Groundwater
Groundwater is the most common problem in deep foundation construction, which brings many difficulties to manual digging pile construction. Due to the excavation of soil layer, the dynamic equilibrium state of water in soil layer is destroyed, and the surrounding still water penetrates into the pile hole, which affects the normal construction of manual digging pile. Especially in the soil layer with dynamic water pressure, it is not only difficult to excavate, but also the retaining wall concrete is easily washed away by water pressure, resulting in pile quality problems. For example, when the pile encounters fine sand and silty sand layer during excavation, sand flow and lost circulation are easy to occur under the action of water pressure. According to different situations, the following methods should be adopted to treat groundwater.
1. When the amount of underground water is not large, pump water with submersible pump and dig while pumping. After the hole is formed, the concrete retaining wall of the corresponding section shall be poured in time, and then the construction of the next section shall be continued.
2. The amount of underground water is large. When the water pump in the hole of digging pile can't be excavated, the construction sequence should be considered. Firstly, the surrounding pile holes are pumped at the same time to reduce the water inflow in the excavation holes, and the construction method is alternately circulated and arranged reasonably, which can achieve good results.
3. When the excavated pile is shallow, a unified light tube well can be reasonably arranged around the site for dewatering and diversion. When the foundation surface area is large, it can be solved by increasing the number of rows of dewatering tube wells. In a word, we must pay attention to the surrounding environment when dealing with groundwater problems in the construction of manual digging piles. Sometimes the surrounding environment, infrastructure and so on. It has influence, so it is impossible to pump water indefinitely; If there are rivers, lakes and swamps around, the purpose of pumping water may not be achieved. Therefore, reliable measures should be taken before pumping water, and the most effective way to deal with this kind of problem is to cut off the water source and close the waterway. When the pile hole is shallow, sheet pile can be used for sealing; When the pile hole is deep, borehole pressure grouting can be used to form a curtain to stop water, so as to ensure that the normal pumping can meet the requirements of excavation.
Second, quicksand.
When digging by hand, quicksand is easy to form when it meets flowing fine sand and silty sand, and in serious cases, well leakage will occur, resulting in quality accidents. Therefore, effective and reliable measures should be taken to ensure construction safety.
1, when the quicksand situation is light, the effective method is to shorten the excavation depth of this cycle and shorten the normal section of 1m to 0.5m, so as to reduce the exposure time of the excavated sand hole wall and timely pour the retaining wall concrete. When the hole wall collapses and the pile hole cannot be formed due to the inflow of sediment, the outer wall of the pile hole can be formed by compiling bag soil piles to control the inner wall of the pile hole to meet the design requirements.
2. When the quicksand situation is serious, the common method is to lower the steel sleeve. The steel sleeve is similar to the steel formwork for wall protection. It can be divided into 4-6 circular arcs with the outer diameter of the pile hole as the diameter, and connected with appropriate ribs by bolts or steel fasteners. When the pile hole is excavated to a depth of about 0.5m, it can be inserted into the sleeve in sections, extending into the bottom of the hole not less than 0.2m, and inserted into the outer side of the upper concrete retaining wall not less than 0.5m. After installation, the retaining wall concrete is poured. If the quicksand is still upwelling after being inserted into the casing, concrete can be used to seal the bottom of the hole. After the concrete is solidified, the concrete in the center of the hole will be chiseled to form a pile hole. This method can also be applied to the construction of the bottom protection section of the lowest section of the concrete retaining wall, so that the hole wall inclines beyond the lower retaining wall, grouting pipe is driven, and cement slurry is poured, so that the lower soil is hardened, the water permeability of the surrounding and bottom sand is reduced, and the occurrence of quicksand is prevented.
Three. Silty soil layer
When encountering muddy soft soil layer, timbers, planks, templates and other supports should be adopted, and the excavation depth of this section should be shortened, and concrete retaining wall should be poured in time. The wood square and formwork used for support should be driven into the bottom with a depth of not less than 0.2m along the periphery. After the upper part is embedded in the poured concrete retaining wall in the upper section, it can be placed obliquely and arranged in two rows, which can achieve good support effect.
Fourth, the pouring method of pile concrete
In the process of pile concrete construction, to ensure the quality of pile concrete, we should not only deal with the problems of water accumulation at the bottom of the hole and water seepage at the hole wall, but also ensure the compactness of pile and concrete.
1, the method of eliminating water at the bottom of the hole. When pouring concrete into piles, ensure that the design strength requirements are met. In order to ensure the uniformity and compactness of concrete, the key point should be to prevent the water in the hole from affecting the mixture ratio and compactness of concrete. Before pouring, the accumulated water in the pile hole should be drained, and the submersible pump for pumping water should be equipped with a check valve to avoid the residual water in the pumping pipe from flowing into the pile hole again during pumping. If the water in the hole can't be pumped out, dry-mixed concrete or pure cement can be laid at the bottom of the hole after the pump is lifted, and then concrete can be poured. If there is a large amount of accumulated water at the bottom of the hole, which really cannot be solved by pumping, pile concrete construction should be carried out according to the requirements of underwater concrete construction technology.
2. The problem of water seepage in the hole wall can't be ignored, because it takes a long time to pour pile concrete. If there is too much water seepage, it will affect the quality of concrete and reduce the strength of pile concrete. Therefore, waterproof materials should be used to block the leakage parts before the concrete pouring of the pile body. For the hole with large water output, use a wooden wedge to drive it, and seal it with waterproof material around it, or embed a drainage pipe in the concentrated water leakage part, and install a valve. When the pile hole is constructed, open the valve to let the water flow out, and close it when the cast-in-place pile body is concreted, which can also solve the problems affecting the concrete quality of the pile body.
3. Measures to ensure the compactness of pile concrete In order to ensure the compactness of pile concrete, the method of string tube or conduit blanking and layered vibrating pouring is generally adopted. Pouring speed is the key to ensure the quality of concrete, so a pile of concrete should be poured in the shortest time, especially when there is groundwater pressure water, enough concrete should be poured in a short time, so that the concrete itself can inhibit the infiltration of water and ensure the quality of pile concrete. The reasonable arrangement of manual digging pile construction sequence plays an important role in reducing the construction difficulty, and should be arranged reasonably according to the actual situation in the construction scheme. If possible, shallow pile holes should be constructed first, then deep pile holes should be constructed. Generally speaking, the deeper the pile hole, the more difficult the construction is. After shallow pile hole construction, the stability of the upper soil layer will be strengthened and the pressure during deep hole construction will be reduced. When constructing in an aquifer or soil layer with hydrodynamic pressure, the pile hole in the periphery (or the water-facing part) should be constructed first. After the concrete wall protection of this pile hole is completed, a small number of pile holes can be reserved, which can be used as a drainage well to facilitate the construction of other pile holes without pouring pile body concrete. This ensures the construction speed and hole-forming quality of pile holes.
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