A common problem with the quality of manually dug piles? The following Zhongda Consulting brings common quality problems and prevention and treatment of common quality problems of manually dug piles. The relevant content is for reference.
Common quality problems of artificially dug piles mainly introduces the quality problems and prevention measures that occur during the construction of artificially dug cast-in-place piles.
1. Advantages of manually dug piles
Because manually dug cast-in-place piles have simple equipment, cheap labor, and low construction costs; manual digging can avoid the problems caused by mechanical hole formation. It eliminates the disadvantages of vibration and requires jumping construction, so multiple holes can be operated at the same time, which can greatly increase the hole forming speed; piles in the same row can be constructed at the same time, providing convenience for the construction of the next process; no vibration, no noise, environmentally friendly; suitable for field sites It is a project that is narrow and difficult to place a drill rig; the hole shape is easy to check, and the empty soil at the bottom of the hole is easy to clean. The depth of the hole can be changed at will depending on the soil quality, and the bottom of the hole can be expanded at will, which can ensure the quality of the hole and improve the bearing capacity of the single pile; artificial Vibration can be used to vibrate the concrete in the upper part of the cast-in-hole pile, which can improve the compactness and strength of the concrete and effectively avoid shortcomings such as shrinkage of the upper part of the pile and low concrete strength due to the small drop of concrete at the top of the cast pile. Due to the above advantages of manual digging, manual digging is more common at construction sites that have conditions for manual digging. Manually digging cast-in-place piles should be used when there is no underground water layer, or when there is water, but it can be drained with a water pump, or when it is inconvenient to use mechanical drilling. In recent years, some construction units have used manual digging in better strata; or manual digging in shallow layers and mechanical drilling in deep layers; manual digging is also a good method when dealing with broken piles. Sometimes piles are broken due to construction reasons, and it is an engineering accident not to use the original pile position to form the pile. After comparing the technical solutions and economics, it is an effective way to use artificial hole forming method to connect the piles.
2. Problems that are prone to occur when manually digging holes
Due to various reasons, there are many common problems with manually dug cast-in-place piles, which should be paid attention to by all parties. Now let’s talk about our superficial views on the following aspects.
⑴The strength of the retaining wall concrete directly affects the bearing capacity of the pile foundation. Generally, construction units believe that the role of the retaining wall is only to protect the hole wall from collapse, and do not make strict proportioning designs for the retaining wall concrete. Moreover, during construction, Failure to carry out strict vibration results in extremely low strength of retaining concrete. From the perspective of force, for friction piles, the upper load is transmitted to the foundation through the retaining concrete. Although the retaining concrete is outside the calculated pile diameter range, all loads on the pile are transmitted through the retaining concrete. . In this way, if a stronger soil layer is encountered, such as a gravel layer, a rock layer, etc., it will cause two hard and one soft situations. When the rock-embedded pile or friction pile foundation is subjected to lateral thrust and produces a bending moment, it is easy to break the low-strength concrete retaining wall. When the concrete is crushed and the friction pile foundation is subjected to vertical force, the retaining concrete may also be sheared into pieces. This will reduce the bearing capacity of the pile due to the low strength of the retaining concrete soil.
⑵ The degree of close contact between the retaining concrete and the surrounding soil directly affects the bearing capacity of the cast-in-place pile. Because the load of the pile is transmitted to the soil layer through the retaining concrete, the relationship between the retaining concrete and the surrounding soil foundation Adhesion - The load transfer capacity of the retaining concrete must be considered. When manual digging of holes encounters a soil layer with serious groundwater seepage, it is easy to cause the protective wall to collapse, or the soil to become unstable and collapse; when digging a hole to encounter a quicksand layer or the groundwater pressure is high, the soil layer at the bottom of the protective wall is likely to suddenly lose stability, and the soil will suddenly lose its stability. The water gushes out rapidly, causing a well kick, causing the retaining wall and soil to become hollow, or forming irregular hole shapes. When a cavity is created between the retaining concrete and the surrounding soil foundation, the friction pile has greatly reduced the friction coefficient of the pile, which will inevitably reduce the bearing capacity of the bored pile.
⑶ The damage to the concrete of the protective wall affects the quality of the pile body and the performance of lateral resistance. If a manually dug hole encounters groundwater, it is necessary to pump water while digging, causing the groundwater to drop, and the protective wall is susceptible to the negative friction generated by the sinking soil layer. The action makes the protective wall produce tensile force, which can cause circumferential cracks in the protective wall. If the retaining wall is subjected to uneven soil pressure around it, bending moments and shear forces may occur, and vertical cracks may occur in the retaining wall. After the concrete pouring of the pile body is completed, the retaining wall and the concrete of the pile body form an integral body. The retaining wall is a part of the pile body. Cracks and dislocations in the retaining wall will definitely affect the quality of the pile body and the performance of lateral resistance.
⑷The quality of the concrete cast-in-place pile itself is another important factor affecting the load-bearing capacity of the pile. The mechanically formed concrete cast-in-place pile adopts the conduit pouring method. The concrete achieves the effect of non-segregation and uniform density through the pressure difference between the inside and outside of the tube. .
In actual construction, some construction units often do not use conduits to pour concrete when pouring manually dug piles, which can easily cause the poured concrete to segregate. In addition, the vibration is insufficient. For example, when the slump of the concrete used is small, the density of the concrete will be affected. Extremely poor performance will inevitably reduce the concrete strength of the pile body and affect the bearing capacity of the pile foundation.
⑸ There are many hidden dangers in production safety. Because manual digging construction workers generally have low quality and weak awareness of production safety, they are afraid of trouble at work, do not construct the wall seriously, and sometimes do not do the concrete for the wall protection, or only do it. If the upper protective wall is not used as the lower protective wall, if there is a sand layer or other easily collapsed soil layer, the hole wall will easily collapse and cause sand gushing, and the construction workers will be buried in the sand and difficult to rescue and die. Others do not pay attention to the gas supply into the hole before entering the hole and during construction. Due to the lack of oxygen in the hole, it is easy to cause casualties.
3. Preventive measures In order to solve the construction quality problems and safety production problems of artificial controlled hole concrete cast-in-place piles, the following preventive measures should be paid attention to.
⑴ Before starting the manual digging of holes, a detailed construction plan must be formulated and submitted for approval before construction can start. After the plan is approved, first lay out the center of the pile foundation, and determine four protective piles around each pile foundation along the longitudinal and transverse directions of the bridge. The position of the protective piles is determined in order to facilitate the review of the position of the pile foundation during construction. The pile protection shall be fixed and protected with concrete, and shall be reviewed and signed by the supervision engineer to ensure that the pile position is correct.
⑵It is very important to protect the orifice well. For land-based piles, the site must first be leveled, and dangerous rocks and stones on the ground or slope must be removed; when the pile foundation is on a mountain slope, if there are signs of cracks or collapse, necessary protection must be added, the soft soil layer must be eradicated and compacted, and the holes must be removed. Dig a drainage ditch around the hole to remove surface water in a timely manner, and set up a canopy at the hole to prevent rainwater from flowing into the hole; install lifting equipment and lay out a slag discharge road; reasonably stack materials and machinery to ensure that the pressure on the hole wall is not increased and the impact is not affected. Construction is the principle. For shallow water piles, islands must be built first. Before building an island, debris from the river bed must be cleared first, and when building the island, filling is done from upstream to downstream; the size of the island depends on the water level and site terrain, and the height must be higher than the construction water level. A circle full of river pebbles must be piled around the island. Stone bags for protection. Use concrete to make a frame around the hole. The height should be 20cm-30cm above the ground to prevent soil, stones and debris from rolling into the hole and hurting people. If the ground at the hole is soft, a concrete wall must be used at the hole, with a height of not less than 2m. , to prevent the hole entrance from collapsing; if there is a large amount of water seepage in the hole entrance stratum, the well point method should be used to lower the groundwater level; the pile foundation located next to the construction access road should be protected by a wooden or steel barrier next to the hole entrance. , and install warning red lights.
⑶ It is very important to have good support and wall protection. Especially when the pile hole is deep, the geological conditions are complex, or the groundwater is large, concrete wall protection should be used in the hole. The protective wall concrete must start from the hole position, and the height of the protective wall is determined according to the degree of geological stability. If the geological conditions are good, for every 1m of excavation depth, make a layer of protective wall and excavate the next layer. For sand layers, silt soil layers, and gravel layers with poor stratigraphic conditions, it is advisable to dig every 0.3-0.5m as a tunneling wall protection section, continuously pump water, quickly tunnel, and quickly protect the wall, and observe the settlement of groundwater and soil layers at any time. If subsidence is detected, pumping must be stopped and emergency measures must be taken. The protective wall with good geological conditions should be 10-15cm thick, and the reinforced concrete protective wall with a thickness of 20cm should be used in poorer strata. Generally, the longitudinal reinforcement is 42Ф14 and the ring reinforcement is 4Ф8. After each excavation section, the steel bars are rolled first, and then the steel formwork is installed. The formwork is corrected according to the center of the pile to ensure that the pile diameter size and pile position are correct. When pouring concrete, pour it evenly from all sides and compact it with steel bars. Each section of the protective wall must be completed on the same day. If any leaking or irregular parts are found after the formwork is removed, they must be reinforced and corrected in time. If there is a weak fossil layer, the wall protection may not be used. If it is necessary to dig holes manually to deal with broken piles, and the holes need to be dug very deep, the thickness of the retaining concrete can be appropriately increased. There are two types of guard wall shapes: external tooth type and internal tooth type (see the picture below). The overlap between the upper and lower sections of the guard wall is 50-75mm. There is a small step between the sections of the external tooth type, which can increase the lateral resistance of the pile. The retaining wall concrete should use the same strength ratio as the pile body concrete. When digging holes in unstable strata, steel cylinders can be used to protect the walls, and overweight, powerful, and high-frequency vibration impactors can be used to quickly drive into the steel cylinders for surprise drilling. The spray grouting method can be used for individual hole sections to consolidate the unstable layer before excavation.
⑷ Digging holes is the main process of artificial hole formation, and it should depend on the properties of the soil layer and the layout of pile holes. If the soil layer is tight and the groundwater is not large, all pile holes under one pier foundation can be excavated at the same time to shorten the construction period; holes with large water seepage should be excavated in advance and pumped intensively to reduce the water level of the surrounding holes.
Holes with soft soil and large groundwater should be excavated diagonally. When using plum blossom-shaped excavation, the middle hole should be dug first to avoid collapse caused by too thin spacing between holes. If sand surges or collapses in the quicksand layer during hole excavation, C15-C20 mortar or sand bags can be used to backfill the empty section in time, and the protective wall can be reinforced to ensure safety. During the digging process, the size and plane position of the pile hole must be checked frequently. The hole position and hole depth must meet the design requirements. If water seeps in during hole control, it should be drained out and the hole wall should be supported in time to prevent water from soaking and flowing on the hole wall, causing hole collapse. After the bottom of the hole reaches the designed elevation, the loose soil, sediment and debris at the bottom of the hole must be removed; if the geology is complicated, use a steel drill to detect the following geological conditions, and only after obtaining the approval of the supervisor can concrete be poured. When digging piles, manual digging records must be filled in in a timely manner, and samples must be taken where the soil layer changes to determine the soil quality so that it can be checked with the geological profile map. Strengthen supervision and strict control during the construction process to avoid incidents of cutting corners and cutting corners, and nip various quality accidents and safety casualties in their bud.
⑸If manual digging of holes encounters soft or hard rocks or boulders, blasting must be carried out in the holes. When blasting inside a hole, holes must be drilled and blasts fired. It is strictly forbidden to expose the explosive package. The depth of soft rock blastholes should not exceed 0.8m, and the depth of hard rock blastholes should not exceed 0.5m. The number, location and oblique insertion direction of the blastholes should be determined according to the direction of the rock formation section. The center group should be focused on drilling the core, and the surrounding blastholes should be inserted into the edges. The dosage of medicine is mainly controlled by loose workers and strictly controlled. The support near the hole of the head should be reinforced or protective measures should be installed to prevent the hole wall support from being blown out and causing hole collapse. If there are water holes, use waterproof explosives to avoid blind shots. After blasting in the hole, high-pressure air ducts or electric blowers must be used to blow air and smoke from the bottom quickly to promote air convection. When the hole depth is greater than 12m, the concentration of toxic gases in the hole should be measured immediately after each blast. When there is no instrument for inspection, a small animal with strong political sensitivity can be hoisted into the bottom of the hole for verification. Only after the safety is confirmed can personnel proceed to the hole for construction. The storage and processing of detonating bodies, detonating tubes and explosive packets must be kept away from living areas, and processing operations must comply with safety conditions. The detonator detection workbench should be laid with anti-static soft mats, and the number of detonators on each workbench should not be too many. Detonators that are flattened, damaged, or corroded cannot be used. Explosives need to be detonated with an electric detonator. After each blasting, it is necessary to carefully check whether there are any blind guns in the blasting area according to regulations. If blind gunfire occurs or is suspected, report it immediately and handle it according to safety procedures. For blind guns that cannot be handled, obvious signs should be set up nearby and corresponding safety measures should be taken. If the blind blast is confirmed to be caused by blasting the network, it can be reconnected and detonated; naked explosives can be dropped near the blind blast hole to induce the explosion.
⑹ After digging the hole to the designed depth, carefully check the hole position, hole shape, hole diameter, hole depth and geological conditions. If the geological conditions are found to be inconsistent with the design, it should be reported in time to all parties. The final hole will be considered only when it is confirmed that the bearing capacity of the pile foundation meets the design requirements.
⑺ Before pouring concrete, the bottom of the hole must be treated and the soft base layer soaked in water must be chiseled away. The base must be smooth, free of loose residue, sludge and sedimentation and other soft layers. When the amount of water in the hole is large, the underwater concrete pouring method is used. When there is no groundwater in the hole, the dry concrete pouring method is used; in order to ensure the quality of the concrete on the pile body, a conduit must be used for pouring. The bottom of the conduit should not be greater than 2m from the bottom of the hole. Use a vibrator to vibrate every 0.5-0.6m of pouring. Dense; concrete must be poured continuously.
⑻ Safety production must be paid attention to during construction, and safety production implementation of hole digging and wall protection construction must be checked frequently. Gas must be supplied before personnel go down the well to ensure that there is enough fresh air in the hole. To ensure sufficient brightness on the underground construction working surface, 36V low-voltage waterproof lamps should be used for lighting. Machinery such as buckets, hooks, wire ropes, and winches for extracting soil ballast must be inspected frequently and maintained in good condition. Construction workers are required to pay attention to soil layer changes at all times, report any unstable soil layers in a timely manner, and propose safety measures before continuing construction.
⑼ Under special geological conditions (such as broken pile treatment), manual digging construction methods are used. The construction progress must be controlled. First, the groundwater cannot drop too fast, and the construction cannot be affected by water; second, there is no Make the protective wall well, do not continue to dig down, and do not be sloppy at all. The inner diameter of the protective wall must be equal to or slightly larger than the original designed pile diameter; third, the broken pile must be cut in place to ensure that the concrete at the broken pile head can see new growth; fourth, the broken pile must The head should be cleaned to ensure that the old and new concrete are well connected.
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