With the development of social economy, there are more and more high-rise buildings. In the construction process, the punching pile equipment is relatively simple and suitable for construction under various geological conditions. Therefore, it is widely used in various high-rise buildings and foundation pit retaining schemes. According to years of work experience, the author expounds several problems that punching pile has a great impact on the quality in the construction process, For reference only.
Factors affecting the construction quality of punching pile
1 Analysis of geological report
During construction in some areas, it is found that most areas are sandy gravel, conglomerate, argillaceous sandstone and red sandstone. At the same time, in the construction process, many sections can occur. Even if the rock surface is relatively flat, special situations will occur in a section here: first, the height difference of rock surface is large. There are multiple weak intercalations under the second slightly weathered rock. For example, a high-rise is a dug pile foundation, and two layers of weak interlayer are found under the slightly weathered Bedrock (argillaceous sandstone) by core pulling; As mentioned above, I believe that for sections with complex geology, it is necessary to carefully analyze the geological report, and appropriately adopt advance drilling to clarify the final hole depth, so it can avoid adding piles after core pulling and problems are found.
Establishment and improvement of 2 mud circulation system
The key to the quality of punching pile lies in the perfection of mud circulation system. In recent years, due to various reasons, some pile foundation subcontractors have adopted a makeshift attitude towards the mud circulating system (although the budget quota has considered the cost of the mud circulating system), so they do not build a mud tank before construction, let alone set up a mud sedimentation tank and mud circulating tank. Usually, they dig a pit near the pile, In the past, the phenomenon that special personnel were often sent to fish slag in the mud pit and mud ditch during punching construction no longer exists, let alone using the slag extractor (this item often appears only on the paper of the construction scheme). Therefore, when the punching reaches the design elevation, the mud has no sedimentation at all, and the repeated circulation is the mud with slag, resulting in the failure to clean the hole bottom after 7 ~ 8h circulation. Due to the poor quality of mud (high sand content), sediment will appear when the conduit is connected. At this time, the construction unit adopts a so-called "solution", that is, mixing a bucket of mud and flushing the sediment with the plug opening method in the storage hopper. However, the washed sediment is still in the hole after all, so the sediment will sink after the concrete is plugged, It will lead to the phenomenon of mud inclusion at the pile edge.
3 punching positioning and measurement of orifice elevation and ground elevation
At present, the construction units pay more attention to the hole location of punching pile, and the measurement is more serious. However, some construction units do not pay attention to the measurement of ground elevation and pile casing opening elevation, but this will also directly affect the construction quality. Because some sites are flat when entering the site, but after the construction of punching pile, with the deposition of mud, especially the construction in deep foundation pit, sometimes the site backfilling and reinforcement treatment are required, so the actual ground elevation difference is much different, and these differences will directly affect the judgment of design elevation.
4 Determination of rock sample of bearing stratum of punching pile
The rock sample of punching pile is usually obtained through slurry circulation, and it is also sampled by welding a sampling cylinder on the rib at the bottom of the punch. To accurately determine the rock sample, first analyze it according to the geological report. If there is construction data of adjacent piles, it can be analyzed together with the geological report. The sampling shall be conducted at the pile casing mouth, and whether to enter the rock stratum required by the design shall be judged according to the proportion of rock sample of hard rock in rock slag. In the past, some operators would pour some rock samples taken from other piles into the air and falsely report that they are the rock samples of the hole. Some people on the construction site also take the rock pillars during drilling to break them and pretend to be rock samples. Therefore, when judging the rock samples, pay special attention to the rock samples with fresh edges or rounded edges. If necessary, take more samples for judgment. For sandy conglomerate area, when finishing the hole, measure several more points along the periphery of the hole with a measuring hammer to prevent one side of the hole bottom from being hard and the other side from being soft. In addition, the rock sample can be carefully analyzed in combination with the footage of punching. In moderately weathered rock stratum, the footage is generally about 15 ~ 20cm per hour. However, it is worth noting that the footage cannot be used as the only standard for judging rock samples, because sometimes there are a lot of rock slag or sand at the bottom of the hole and there is no timely circulation during construction, so the footage will be very slow.
5. Measurement and re measurement of pile depth
In the hole inspection, the measurement of pile depth is a problem worthy of attention. Due to the slow footage in the rock stratum, some operators often cut off a section of the measuring rope and reconnect it in order to finish the hole earlier. If the footage is not repeated, the depth into the rock will not meet the design requirements and even lead to quality accidents.
6. Control of sediment at hole bottom
The key to the control of bottom sediment lies in the control of mud quality. At present, due to various reasons, the mud circulation system does not receive serious attention, so the bottom sediment exceeds the standard. At present, most construction units dig a pit near the pile flushing hole as a mud pit, and the sedimentation cycle is this pit. The rock slag in the mud has not been precipitated, and the mud circulating from the hole bottom has not been precipitated, so the footage is also affected. In the past, there was a section of mud ditch from the pile hole to the mud pit to precipitate rock slag, and some workers specially fished the slag, but now some construction units have also omitted this process, and the process of cleaning the slag in the hole with a slag extractor in the construction scheme has also been omitted. For this reason, some pile sediments in some punching pile construction sites reach 48CM. The subsequent core pulling test shows that the actual sediment thickness is only slightly smaller than the value during acceptance inspection. Practice has proved that the mud circulation system is essential, but it can be flexibly arranged according to the actual situation of the construction site. For example, some construction sites use the site conditions, use the excavation pit to set up mud sedimentation tank and circulation tank respectively, and make full use of mud ditch to settle and remove slag, which also has good results; Due to the narrow site and the deep elevation of some pile tops, the mud pit is excavated after the completion of the pile, and the effect is also good.
7 fabrication and quality control of reinforcement cage
The reinforcement cage of punching cast-in-place pile is generally made of hot-rolled reinforcement. The steel shall have material qualification certificates and pass the on-site re inspection. The reinforcement binding and welding quality shall meet the relevant requirements of reinforcement sub project in code for acceptance of construction quality of concrete structures (GB50204-2002). Technical requirements for fabrication of main reinforcement cage: 1) the diameter of reinforcement cage shall meet the design size. 2) The length of each section should not be more than 12m or less than 5m, because if it is too long, it is easy to bend and deform during hoisting, and if it is too short, it will increase the welding time, which is unfavorable to the quality of pile. 3) When using flange joint conduit, the stiffening hoop should be set outside the main reinforcement. Generally, the main reinforcement is not provided with a hook. According to the process requirements, the hook shall not be exposed to the inner circle, so as to prevent the conduit hook from causing the reinforcement cage to float up. 4) The fabricated reinforcement cage shall be stacked horizontally on a flat and clean site, and the stacking height shall not exceed two layers. 5) The reinforcement cage shall be hoisted section by section and welded at the orifice. The welding method of the main reinforcement of the reinforcement cage is vertical welding. The welding personnel are required to have a high level of welding process to ensure the welding quality. 6) The reinforcement cage shall be lowered quickly, and generally the pile hole shall be completed within 2 ~ 4H.
8. Plug opening and buried pipe depth control of underwater concrete pouring
1) The plug opening during underwater concrete pouring also has a great impact on the pile quality. After the initial stock of concrete is determined, different plug opening methods have different results. Generally, the plug used for opening the plug is made of concrete, but it is less used now because it takes a long time to make. Now commonly used plugs are made of nylon cloth wrapped cement mortar. This kind of plug is easy to make and easy to pass through the conduit. Some construction units set a valve at the connection between the storage hopper and the conduit. When the initial stock is enough, open the valve. However, the disadvantage of this method is that the impulse force is not enough after opening the plug. If the mud at the hole bottom is thick or the hole cleaning is not clean enough, the quality of the hole bottom will be affected. From the core pulling results of a project, it can be seen that the combination of pile bottom and rock surface with sliding door opening is worse than that with shearing plug technology.
2) The control of buried pipe depth in the pouring process of underwater concrete is an important problem to ensure the pile quality. As long as the constructors have a strong sense of responsibility, this problem is not difficult to solve.
