Common Problems And Methods of Rotary Drilling Rig Drilling in Special Strata

  At present, rotary drilling rig has become one of the mainstream construction machinery of bored pile because of its advanced technology, rapid and stable hole formation. Due to the various engineering geological conditions in different places, different engineering designs are unlikely to be completely consistent with the actual strata, and sometimes there are even large differences.

  Therefore, when the geotechnical construction unit contracts the pile foundation drilling project, the pile foundation drilling faces no small uncertainty. Some technical problems in the drilling process of pile foundation will not only cause great economic losses, but also pose a threat to the safety of engineering equipment and even personnel.

  Based on the practical experience of drilling in different strata with rotary drilling rig in recent years, this paper summarizes the technical problems and solutions encountered by rotary drilling rig in drilling in some special strata.

01

Rotary drilling method in soft soil and other collapsible strata

• Characteristics and risk of pore-forming in soft soil and other easily collapsing strata

  In soft soil, newly accumulated backfill soil and newly deposited loose fine sand soil layer, if the water table is high, the hole wall is difficult to maintain stability even if the drilling is in the case of mud wall protection, and it is easy to cause hole collapse or shrinkage. Because these strata are mostly distributed in the shallow layer of the surface, with the gradual increase of the collapse amount, the ground around the drilling hole will collapse, and often cause the sinking of the protection cylinder. In serious cases, it will also pose a threat to drilling equipment and operators. When encountering such situations, especially when the orifice guard is found to be supended or sunk during construction, it should not be lucky to continue drilling, but backfill the drilling hole as soon as possible, and re-fix the orifice guard before drilling.

• Construction method of rotary drilling

In the water-bearing strata where mud wall protection is not effective, such as soft soil and newly backfilled soil, the most efficient and safe method of rotary drilling is to adopt long wall protection measures. Each rotary drilling rig is equipped with about 3 steel protection barrels, 1 vibration hammer and 1 crane with enough lifting force according to the difficulty of drilling. The length of the steel casing can be lowered to the bottom of the soft layer, and the diameter of the steel casing should be greater than about 100mm of the pile diameter (too large to cause concrete waste; Too small drill bit easily stuck barrel. It is difficult to guarantee smooth construction. When using the long drum wall protection to dig holes, the following points should be noted:

(1) It should be basically confirmed that there are no large diameter stones above 100mm within the lower depth of the protective cylinder; Select the vibration hammer with the right power according to the diameter of the long protection cylinder and each lowering length.

(2) During small diameter construction, the outer diameter of the drill pipe and the diameter of the casing should be considered comprehensively. The outer diameter of the drill pipe should be more than 200mm smaller than the diameter of the casing. When the length of the casing is larger, the difference should be larger to facilitate construction.

(3) The perpendicality of the steel casing should be ensured, especially for borehole piles with smaller diameters.

(4) After pouring concrete, the protective cylinder should be pulled out in time.

(5) Sunk steel casing and rotary dug pile closely cooperate. After the steel casing is pulled out, it should be able to be sunk directly into the next borehole to be constructed. This formation of flow operation, will greatly improve the construction efficiency.

02

Sand and silt layer rotary drilling method

• The characteristics of sandy soil and silty soil layer become risks

  When drilling holes in fine sand or silty soil, if the groundwater level is high, mud should be used to form holes for wall protection. This kind of stratum is easy to be washed under the action of water flow because there is no adhesion between particles. Because the rotary drilling rig directly takes the soil into the hole, the drilled soil is recycled by the drill bucket to the ground. The drilling bucket moves in the mud, and the water flow speed outside the drilling bucket is large, which is easy to cause erosion of the hole wall. Sand washed by the hole wall further reduces the wall protection effect of the wall protection mud. It is more likely to cause problems such as neck protection and even hole collapse.

• Construction method of rotary drilling

When using mud wall protection in the construction of better sand or silt soil layer, the following measures should be considered:

(1) Properly 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 erosion.

(2) Appropriately increase the Angle of the drill teeth. Increase the spacing between the hole wall and the side wall of the drill bucket. 

(3) Appropriately increase the area of the water hole in the drilling bucket, reduce the negative pressure at the top and bottom of the drilling bucket during the extraction process, and then reduce the flow rate of the mud in the small hole.

(4) Configure high-quality mud wall protection, timely measure the sand content of the mud in the hole. Take effective measures in time when exceeding the standard.

(5) Check the tightness of the bottom cover of the drill bucket after closing. If it is found that the gap caused by distortion is large, it should be repaired in time to avoid sand leakage.

03

Rotary drilling method in hard clay and mudstone strata

• Characteristics and risk of pore formation of hard clay and mudstone strata

  The biggest problem that hard clay poses to the drilling process is the difficulty of drilling and unloading. Compared with quaternary sedimentary strata, mudstone is typically characterized by higher strength, greater viscosity and easier to slip after water softening. General circulation drilling machine because the pressure is very small, when drilling in mudstone, drilling teeth are difficult to cut the rock, the drill bit will always slip on the mud surface, drilling efficiency is very low. When using rotary drilling rig to drill mudstone, especially in the case of large hole depth can not use directly pressurized machine lock pipe, if the drill bit is not properly designed, it is easy to have low drilling efficiency or even no drilling.

• Construction method of rotary drilling

When drilling in hard clay, it should be considered to adopt the bulk drilling bucket or make a gap on the bottom side of the drilling bucket to improve the unloading efficiency. In order to improve the construction efficiency, the following points can be used to make holes in the mudstone stratum.

(1) The machine lock drill pipe is preferred for dry drilling. When you have to use friction drill pipe drilling, you can consider increasing the weight of the drill pipe and drill bucket so that the drill teeth can cut into the mudstone and successfully build up pressure.

(2) For formations with strong weathering and higher weathering degree, priority should be given to drilling with V19 drill teeth when the drill bucket is deployed. If necessary, the layout design can be properly increased by cutting the gap, the height difference between the left and right sides of the drill bucket teeth, etc. Alloy teeth must be used when the drill bucket teeth are deployed to improve the anti-wear and break ability.

(3) When the diameter of the foundation pile is large, the use of stage drilling into holes should be considered. The first stage of drilling can be directly coring with a barrel drill bucket with a relatively large length and diameter. The secondary hole is drilled with an external cutting tooth of the same size as the bore hole.

(4) Avoid spreading teeth too sparsely. Avoid the same cutting path of the teeth on both sides of the drill bucket, and repair the side teeth with serious wear in time to effectively avoid the drilling difficulties caused by excessive wear of the drill teeth and support.

04

Rotary drilling method of granite and other hard rock strata

• Characteristics and risk of pore formation of hard rock such as granite silicon

In the design of pile foundation for many large Bridges, the pile is required to enter the weathered hard rock to a certain depth, and the pile diameter designed for these pile foundations is more than 1.5mm, or even up to 2m. Such a large diameter hard rock formation drilling, the equipment power and pressure put forward higher requirements, generally require torque above 280kN.m equipment. In this kind of formation, the loss of drill teeth is great, and the anti-vibration ability of equipment is put forward higher requirements.

• Construction method of rotary drilling

In the construction of hard rock strata such as granite and sandstone, measures should be taken from the following points to improve the efficiency of pore formation and reduce risks.

(1) Drilling construction with power of 280kN.m and above should be selected. Prepare drill teeth with higher hardness and better grinding performance in advance. Water should be added to the anhydrous formation to reduce the wear of drill teeth.

(2) Reasonable configuration of drilling tools. When the large-diameter piles are forming holes in this kind of formation, the fractional drilling method should be selected. The first stage should choose a diameter of 600mm~800mm long barrel drill, direct core, create a free surface; Or choose a small diameter auger to drill to create a free surface.

(3) When inclined holes occur in hard rock strata, it is extremely difficult to sweep holes. Therefore, when the inclined rock face is encountered, it must be corrected before normal drilling.

05

conclusion

Based on the practice of using rotary drilling rig in the construction of drilling holes in recent years, it can be applied to the construction of most strata. However, for a specific project, in order to successfully and effectively implement rotary drilling, it is necessary to rationally select models, rationally configure drill pipe and drill tools according to the actual formation, design aperture, hole depth, etc., and formulate and implement practical and reliable specific measures, which are summarized as follows:

(1) Do a good job of hole wall protection measures. For boreholes requiring wall protection, priority should be given to mud wall protection drilling. For soft soil and newly backfilled soil and other mud wall protection can not guarantee the wall protection effect, should decisively adopt the full protective tube wall protection.

(2) Select the appropriate model. When the rig is flexible and the speed of lifting and releasing drilling is fast, it is more suitable for pile foundation construction with good formation, small depth and pile diameter. Large drilling RIGS are more powerful and can drill hard formations and perform large diameter holes. Using high-power drill to drill simple strata is inefficient. Hard rock formations cannot be drilled with low power drilling.

(3) Reasonable allocation of drilling tools, according to the actual situation to optimize the layout of drill teeth. Before the drilling rig enters the field, the drilling tool should be configured according to the geological prospecting data. Drill several holes first for testing, after understanding the actual geological conditions, should be timely optimized according to the actual situation.

(4) Drilling carefully at key points. In key parts such as inclined rock face, probe rock and top of karst cave, the drilling speed should be slowed down and emergency measures should be prepared in advance.

END