Manual bored pile construction is convenient, does not require large machinery and equipment, and is cost-effective. Therefore, it is widely used in roads and civil buildings. However, the underground working conditions of bored piles are poor, the environment is harsh, and the labor intensity is high, so safety and quality are particularly important.
Groundwater issues
Groundwater is the most common problem in deep foundation construction, which brings many difficulties to the construction of artificial bored piles. The water in the aquifer destroys its equilibrium state during excavation, causing the surrounding static water to fill into the pile hole, thus affecting the normal construction of artificial bored piles. If dynamic water pressure soil layer construction is encountered, not only is excavation difficult, but even the retaining wall concrete is easily penetrated by water pressure, resulting in pile body quality problems. If fine sand and silt sand soil layers are encountered, under the action of pressurized water, it is also very easy to cause sand flow and well leakage.
1 When the amount of groundwater is not large, a submersible pump can be used to pump water, and excavation can be carried out while pumping water. After the hole is formed, the concrete retaining wall of the corresponding section is poured in time, and then the construction of the next section can be continued.
2 When the amount of water is large , when it is not easy to excavate with the water pump of the construction hole itself, the construction sequence should be considered, and the surrounding pile holes should be pumped at the same time to reduce the amount of water in the excavation hole, and the method of alternating cycle construction should be adopted. Reasonable organization and arrangement can achieve good results.
3. For bored piles that are not too deep, uniform light pipe wells can be reasonably arranged around the site to divert water. When the foundation plane occupies a large area, the number of rows of drainage pipe wells can also be increased, which can generally solve the problem.
4. Environmental impact during pumping . Sometimes the surrounding environment of the construction is special. First, the surrounding environment and infrastructure have a lot of influence when pumping out groundwater, and unlimited pumping is not allowed; second, there are river sand, lakes, swamps, etc. around, and it is impossible to achieve the purpose of pumping water without restrictions. Therefore, reliable measures must be taken before pumping. The most effective way to deal with such problems is to cut off the water source and close the waterway. When the pile hole is shallow, it can be closed with sheet piles; when the pile hole is deep, use drilling pressure grouting to form a curtain to block water to ensure normal excavation during normal pumping.
Quicksand Problem
When excavating artificial holes, if there are fine sand and silt layers, coupled with the effect of groundwater, quicksand is very likely to form. In severe cases, well leakage may occur, causing quality accidents. Therefore, effective and reliable measures must be taken.
1 When the quicksand situation is relatively light, the effective method is to shorten the excavation depth of this cycle, shortening the normal 1m section to 0.5m, so as to reduce the exposure time of the excavated hole wall and timely pour the wall concrete. When the hole wall collapses and mud and sand flow in and the pile hole cannot be formed, textile bags can be used to gradually pile up soil to form the outer wall of the pile hole, and control to ensure that the inner wall meets the design requirements.
2 When the quicksand situation is more serious , the commonly used method is to install a steel sleeve. The steel sleeve is similar to the steel membrane plate used for wall protection. It can be divided into 4-6 arc sections with the outer diameter of the hole as the diameter, and then added with appropriate ribs. It is connected to each other with bolts or steel ring buckles. When the excavation is about 0.5m, the sleeve can be installed in pieces, penetrating into the bottom of the hole by not less than 0.2m, and inserted into the outside of the upper concrete retaining wall by not less than 0.5m. After installation, the retaining wall concrete is poured with formwork. If the quicksand still surges up after the sleeve is placed, the method of sealing the bottom of the hole with concrete after the protruding sand is dug out can be adopted. After the concrete solidifies, the concrete in the center of the hole is cleared to form a pile hole.
Muddy soil problem
When encountering soft soil layers such as silty soil layers, wooden planks, wooden formwork, etc. can generally be used for support, and the excavation depth of this section should be shortened, and concrete retaining walls should be poured in time. The wooden planks for support should be driven into the bottom along the perimeter to a depth of not less than 0.2m, and the upper part should be embedded behind the concrete retaining wall that has been poured in the upper section. They can be placed obliquely, with double rows arranged in opposite directions and crossed from each other, which can achieve a good support effect.
Problems in pouring concrete of pile body
When pouring pile body concrete, it is mainly necessary to ensure that it meets the design strength, and to ensure the uniformity and density of the concrete, so as to prevent water accumulation in the hole from affecting the mix ratio and density of the concrete.
1 Eliminate the influence of water Before pouring, the water in the hole should be drained, and the submersible pump for pumping water should be equipped with a reverse flow valve to ensure that when the water pump is raised, the residual water in the pumping pipe will not flow into the pile hole again. If the water in the hole cannot be pumped out, after the water pump is raised, part of the dry mixed concrete mixture or dry cement can be spread into the bottom of the hole, and then the concrete can be poured.
2 Water accumulation at the bottom of the hole If the amount of water at the bottom of the hole is large and it is really impossible to solve it by pumping water, the construction of the pile body concrete should adopt underwater pouring construction technology.
3 Water seepage at the hole wall Water seepage at the hole wall cannot be ignored. Because the pouring time of the pile body concrete is relatively long, if there is too much water seepage, it will affect the quality of the concrete and reduce the strength of the pile body concrete. Waterproof materials can be used to seal the leakage parts before pouring the pile body concrete. For holes with large water output, wooden wedges can be driven in, and then the surrounding area can be sealed with waterproof materials, or drain pipes can be embedded in the concentrated leaking parts, and valves can be installed. When constructing the pile hole, the valves can be opened to let the water flow out, and then closed when pouring the pile body concrete. This can also solve the problem of affecting the quality of the pile body concrete.
4 Ensure the compactness of the pile body concrete The compactness of the pile body concrete is a necessary condition to ensure that the concrete reaches the designed strength. In order to ensure the compactness of the pile body concrete pouring, the method of serial flow tube feeding and layered vibration pouring is generally adopted. The pouring speed is the key, that is, strive to complete the pouring of a pile body concrete in the shortest time, especially when there is underground pressure water, it is required to concentrate enough concrete in a short time to suppress the infiltration of water flow ahead of the concrete’s own weight.
For the pile body line with a depth greater than 10m, the impact force formed by the concrete’s own drop and the pressure surface of the concrete’s own weight can be relied on to make it dense, and this part of the concrete does not need to be vibrated. Experience has shown that the pile body concrete can meet the uniformity and compactness. The speed is better than that of string flow tube construction. Normal construction methods should be adopted for pouring concrete on the upper part of the pile body. Because there is generally little influence of groundwater on the upper part, the pouring speed does not need to be very fast, and the special construction method of free fall cannot be used.
Construction sequence issues
Reasonable arrangement of the construction sequence of manual bored piles plays an important role in reducing the difficulty of construction. Careful coordination should be made in the construction plan and reasonable arrangements should be made according to the actual situation.
Under possible conditions, shallower pile holes should be constructed first, and deeper pile holes should be constructed later. Because the deeper the pile hole, the greater the difficulty. After the shallower pile hole is constructed, it will strengthen the stability of the upper soil layer and reduce the pressure during deep hole construction. When constructing in an aquifer or a soil layer with dynamic water pressure, the pile holes on the periphery (or the water-facing part) should be constructed first. After the concrete wall of this part of the pile hole is completed, a small number of pile holes can be retained without pouring the pile body concrete, but as drainage wells, to facilitate the construction of other holes. The construction speed and hole quality of the pile hole are guaranteed.
Post time: Jan-15-2025