At present, rotary drilling rigs have become one of the mainstream construction machinery for bored piles due to their advanced technology, fast and stable hole formation, and other advantages. Due to the great differences in engineering geological conditions in various places, it is unlikely that different engineering designs will be completely consistent with the actual strata, and sometimes there are even large differences. Therefore, when geotechnical construction units undertake bored pile foundation projects, pile foundation drilling faces considerable uncertainty. If some technical problems are not handled properly during the pile foundation drilling process, it will not only cause great economic losses, but may also pose a threat to engineering equipment and even personnel safety. Combined with the practical experience of using rotary drilling rigs to drill in various strata in recent years, the technical problems and solutions often encountered by rotary drilling rigs in drilling construction in some more special strata are summarized. I hope to provide a reference for colleagues who encounter similar problems.
In soft soil, newly accumulated backfill soil and newly deposited loose fine sand soil layers, if the groundwater level is high, after the rotary drilling rig digs the soil, even if the drilling hole is protected by mud, it is difficult to keep the hole wall stable, which is very likely to cause hole collapse or necking. Since this type of stratum is mostly distributed in the shallow surface layer, as the amount of collapse gradually increases, the ground around the borehole will collapse, often causing the casing to sink. In severe cases, it will also pose a threat to drilling equipment, operators, etc. When encountering such a situation, especially when it is found that the hole casing has been suspended or sunk during construction, you should not continue drilling with luck, but backfill the borehole as soon as possible, re-fix the hole casing, and then drill.
In soft soil, recently backfilled soil, and other water-bearing strata that cannot be effectively protected by mud, the most efficient and safe rotary drilling method is to take long casing wall measures. Each rotary drilling rig is equipped with about 3 steel casings, 1 vibrating hammer, and a crane with sufficient pulling force depending on the difficulty of drilling. The length of the steel casing should be able to be lowered to the bottom of the soft layer, and the diameter of the steel casing should be about 100mm larger than the pile diameter (too large will cause concrete waste; too small will easily get stuck in the casing. It is difficult to ensure smooth construction).
(1) It should be basically confirmed that there are no large-diameter stones larger than 100mm within the casing lowering depth range; select a vibrating hammer with appropriate power according to the long casing diameter and lowering length.
(2) When constructing with small diameters, the outer diameter of the drill rod and the diameter of the casing should be comprehensively considered. The outer diameter of the drill rod should be at least 200mm smaller than the diameter of the casing. When the length of the casing is large, the difference should be larger to facilitate construction.
(3) The verticality of the steel casing should be ensured, especially for bored piles with smaller diameters.
(4) The casing should be pulled out in time after pouring concrete.
(5) The sinking of steel casing and rotary drilling of piles should be closely coordinated. After the steel casing is pulled out, it should be able to be directly sunk into the next borehole to be constructed. This will form a flow operation and greatly improve construction efficiency.
When drilling holes in fine sand or silty soil layers, if the groundwater level is high, mud is needed to protect the wall of the hole. Since there is no adhesion between the particles in this type of stratum, it is very easy to be washed away by water flow. Because the rotary drilling rig directly takes soil to form a hole, the soil in the hole is raised to the ground by the drill bucket in a reciprocating cycle. The drill bucket moves in the mud, and the water flow speed outside the drill bucket is relatively high, which can easily cause scouring of the hole wall. The sand particles washed off the hole wall further reduce the wall protection effect of the wall protection mud. It is more likely to cause problems such as neck protection or even hole collapse.
(1) Appropriately reduce the lowering and pulling speed of the drill bit, reduce the flow rate of mud between the drill bucket and the hole wall, and reduce scouring.
(2) Appropriately increase the outer angle of the drill teeth. Increase the distance between the hole wall and the side wall of the drill bucket.
(3) Appropriately increase the area of the water flow hole in the drill bucket to reduce the negative pressure at the top and bottom during the pull-out process of the drill bucket, thereby reducing the flow rate of the mud in the hole.
(4) Configure high-quality mud wall protection and measure the sand content of the mud in the hole in time. Take effective measures in time when it exceeds the standard.
(5) Check the sealing of the bottom cover of the drill bucket after closing. If it is found that the gap is large due to distortion, it should be repaired in time to avoid sand leakage.
During the construction of the pile foundation of the Tiaohe Bridge in the mountainous area, boulder formations are often encountered during the drilling process. Because the diameter of the boulder particles is large and they are embedded in each other, conventional sand drill bits cannot drill in; if forced drilling is carried out, it is very easy to break the drill teeth and even damage the drill rod. In addition, large boulders with a diameter similar to the hole are often encountered, or probe stones that are partially located in the hole and mostly buried in the formation around the hole. If the boulders are not handled properly, the hole will collapse due to long-term vibration of the hole wall. If the probe stones are not handled properly, it is easy to cause inclined holes, which will pose practical difficulties for the subsequent lowering of the steel cage; if they collapse during the pile pouring process after the steel cage is lowered, it will also seriously affect the quality of the pile.
When the rotary drilling encounters a large boulder formation, if the boulder formation is located at the shallow surface, it is advisable to use an excavator to remove the boulder backfill it with clay, and lower the hole casing to the bottom of the boulder layer before drilling; if the boulder is located in a deeper position when the boulder particle size is larger than the opening diameter of the sand bucket and normal drilling is impossible, the large boulder can be removed with a cylindrical drill bit, and then the sand bucket can be used to gradually drill.
When the bored pile encounters a solitary boulder, if the solitary boulder happens to be completely located in the hole, the solitary boulder can be directly removed with a cylindrical drill bucket; if most of the solitary boulder is buried in the formation around the hole to form a probe stone, drilling should not be forced. Instead, the probe stone should be slowly cut off and removed with a cylindrical outer head before normal drilling can be carried out.
The biggest problem that hard clay brings to the drilling process is the difficulty of drilling and unloading soil. Compared with the Quaternary sedimentary formations, the typical characteristics of mudstone are high strength, softening after contact with water, high viscosity, and easy slipping. Because the general circulation drilling rig has very little pressure when drilling in mudstone, the drill teeth find it difficult to cut the rock formation, the drill bit will always slip on the mud surface, and the drilling efficiency is extremely low. When using a rotary drilling rig to drill mudstone, especially when the hole is deep and the machine-locked drill rod that can be directly pressurized cannot be used, if the drill bit is not designed properly, it is easy to have low drilling efficiency or even failure to drill.
When drilling in hard clay, you should consider using a body drill bucket or opening a notch on the side of the bottom of the drill bucket to improve the efficiency of unloading soil. When drilling and boring piles in mudstone formations, the following points can be taken into consideration to improve construction efficiency.
(1) Prefer to use machine-locked drill rods for dry drilling. When friction drill rods are used for drilling, the weight of the drill rod and drill bucket can be increased so that the drill teeth can cut into the mudstone and smoothly build up pressure. (2) For strongly weathered and more weathered formations, V19 drill teeth should be used as the priority when drilling. If necessary, the layout design can be made by cutting notches, making the height difference of the teeth on the left and right sides of the drill bucket, and appropriately increasing the rock penetration angle. Alloy pick teeth must be used when arranging the teeth of the drill bucket to improve the wear resistance and anti-fracture ability.
(3) When the diameter of the foundation pile is large, graded drilling should be considered. For the first-stage drilling, a cylindrical drill bucket with a relatively large length-to-diameter ratio can be used to directly core; for the second-stage drilling, a cutting tooth with the same size as the hole diameter should be used for external drilling.
(4) Avoid sparse teeth. Avoid having the same cutting trajectory on both sides of the drill bucket. Repair the severely worn side teeth in time to effectively avoid drilling difficulties caused by excessive wear of the drill teeth and bottoming.
Limestone caves are often encountered during construction in mountainous areas in the south. The bedrock surface of this type of formation is undulating due to uneven erosion, and the size and shape of the caves vary. When drilling and pouring in this type of formation, it is common to encounter situations such as inclined holes, sudden leakage of slurry resulting in hole collapse, stuck drill teeth, and even damage to drill rods and drilling tools, and excessive concrete overfilling.
When drilling and pouring piles in limestone cave formations, the following measures can be taken to reduce the risk of hole formation. When encountering bedrock surfaces, slow down the drilling speed to avoid inclined holes.
(1) Carefully compare drilling data. When drilling near the top of the cave, slow down the drilling speed and prepare sufficient wall slurry and backfill clay. When drilling through the cave top causes the wall slurry in the hole drops rapidly, the hole should be backfilled in time to avoid hole collapse.
(2) It is preferred to use a straight and extended drill bucket with better guidance. If an inclined hole has been formed and the drill is stuck near the cave top, the drill should be slowly rotated and lifted. The top of the drill bucket should be welded in time to facilitate the inclined beam structure of guidance.
(3) When encountering a large cave with little or no filling, clay should be backfilled in time-reversed reversed, and compacted with a rotary drill bucket. Avoid excessive over-squareness during the pouring of concrete after the hole is formed.
Many large bridges require the foundation piles to penetrate the medium-weathered hard rock to a certain depth when designing pile foundations, and the pile diameters of these pile foundations are mostly above 1.5mm, or even up to 2m. Drilling in such large-diameter hard rock formations places high demands on equipment power and pressure, and generally requires equipment with a torque of more than 280kN.m. When drilling in such formations, the wear of the drill teeth is very large, and high requirements are placed on the vibration resistance of the equipment.
When constructing in hard rock formations such as granite and sandstone, the following measures should be taken to improve the hole formation efficiency and reduce the risk.
(1) Drilling construction with a power of 280kN.m or above should be selected. Prepare drill teeth with higher hardness and better abrasiveness in advance. Water should be added to waterless formations to reduce the wear of the drill teeth.
(2) Reasonable configuration of drilling tools. When drilling large-diameter piles in this type of formation, a graded drilling method should be selected. The first stage should be an extended barrel drill with a diameter of 600 mm~800 mm, directly coring to create a free surface, or a small-diameter spiral drill should be selected to create a free surface.
(3) When an inclined hole occurs in a hard rock formation, it is extremely difficult to sweep the hole. Therefore, when encountering an inclined rock surface, it must be corrected before normal drilling.
From the practice of using rotary drilling rigs for drilling and grouting in recent years, it can be applied to the construction of most strata; however, for a specific project, rotary drilling can be smoothly and effectively implemented. It is necessary to reasonably select the machine model and configure the drill rod and drilling tools according to the actual strata and the designed hole diameter and hole depth, and formulate and implement practical and reliable specific measures. In summary, there are the following points:
(1) Take measures to protect the whole mouth. For boreholes that need wall protection, mud wall protection should be given priority. For soft soil and newly backfilled soil, mud wall protection cannot guarantee the wall protection effect, and full casing wall protection should be decisively adopted.
(2) Choose the appropriate machine model. When the drilling rig is flexible and the speed of lifting and lowering the drill is fast, it is more suitable for pile foundation construction with good strata, small depth, and pile diameter. Large drilling rigs have large power and can drill into harder strata and construct large diameter holes. Using high-power drilling rigs to drill simple strata is inefficient; using low-power drilling rigs cannot drill into hard rock strata.
(3) Rationally configure drilling tools and optimize the layout of drill teeth according to actual conditions. Before the drilling rig enters the site, the drilling tools should be configured according to the geological survey data; drill several holes for trial exploration first, and after understanding the actual geological conditions, the layout of drill teeth should be optimized in time according to the actual conditions.
(4) Drill carefully at key points. At key locations such as inclined rock surfaces, probe rocks, and cave tops, the drilling speed should be slowed down, and emergency measures should be prepared in advance.
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