Rotary Drilling Rigs Drilling Piles in Hard Limestone Formations

1. Preamble

In the construction of bridges in hard geological areas, the commonly used pile foundation construction methods are hand-dug pile method and percussion drilling method. Manual drilling method is gradually eliminated due to the long pile construction period, backward technology, and the need for blasting operations, the greater the potential danger, and the use of percussion drilling for construction there are certain problems, mainly reflected in the geology of the hard rock layer percussion drilling into the hole speed is very slow, and even appeared throughout the day does not enter the foot phenomenon, if the geology of the dissolved rock is more developed, there are often jammed drilling, but the jammed drilling situation, a The construction of a bored pile often takes 1-3 months or even longer.

2, the characteristics of the method

2.1 Fast drilling speed

Rotary drilling rig rock coring bit tooth arrangement and tooth structure is combined with the theory of rock crushing design, can be directly drilled into the rock layer, so the hole formation speed, construction efficiency is greatly improved.

2.2 Outstanding quality control advantages

Rotary drilling rigs are generally equipped with 2m or so hole mouth protection cylinder (such as thicker backfill soil can be extended), and the drilling rig itself can be buried in the protection cylinder, so as to avoid the impact of backfill soil from the hole mouth on the drilled piles as much as possible; rotary drilling rigs adopt the more mature underwater catheter filling concrete filling process, which can avoid the adverse effects of mud dropping from the mouth of the holes, and the slag produced in the process of filling; rotary drilling rigs are a collection of modern advanced science and technology, and can be utilized in a single machine. The rotary drilling rig is a pile foundation construction machine integrating modern advanced science and technology, which has high accuracy in verticality, rock layer inspection at the bottom of the hole and pile length control during the process of hole formation, and the quality of pile foundation construction is fully guaranteed due to the low slagging at the bottom of the hole, which is easy to clear the hole.

2.3 Strong ability to adapt to the stratum

Equipped with different drilling bits, the rotary drilling rig can be used in different geological strata such as sand layer, soil layer, pebble gravel, rock layer, etc., and is not restricted by geography.

2.4 Convenient movement and strong mobility

The chassis of rotary drilling rig adopts crawler excavator chassis, which can walk by itself. In addition, the rotary drilling rig can work independently, has strong mobility, adapts to complex terrain, does not need auxiliary facilities to complete the installation and dismantling of the drilling rig, occupies little space, and can be operated against the wall.

2.5 Environmentally friendly and clean construction site

Rotary drilling rigs can work without mud in rocky ground, which not only reduces the waste of water resources, but also avoids the pollution of mud in the surrounding environment. Therefore, the construction site of rotary drilling rig is neat and tidy, and the pollution to the environment is small. 3、Scope of application

This method is mainly applicable to the construction of rotary drilling rig bored piles in the middle weathered and weakly weathered rock stratum with hard rock quality.

4、Principle of process

4.1 Design principle

According to the working principle of rotary drilling rig, combined with the mechanical properties of rock and the basic theory of rock crushing by rotary drilling rig, test piles are made for the drilling piles in hard rocky medium-weathered limestone stratum, and the related technical parameters and economic indexes of rotary drilling rig adopting different drilling processes are counted, and the drilling piles constructed in hard rocky medium-weathered limestone by rotary drilling rig are determined through systematic technical and economic comparison and analysis.

4.2 Principle of rotary drilling rig drilling process in rock formation

The rotary drilling rig is equipped with different types of drilling bits to carry out graded reaming of the hard rock strata, constructing the free surface of the bottom of the hole for the rotary drilling rig, improving the ability of the rotary drilling rig to enter into the rock, and ultimately realizing the high efficiency of the rotary drilling rig to enter into the rock, and at the same time, saving the cost of the project construction.

5、Construction process and operation points

5.2 Construction operation points

1. Pile position control

In order to ensure the quality of pile position, adopt precise measurement method, i.e. use total station to locate the position, after the barrel is buried, repeat the measurement again, and use the welded coordinate frame to correct the center of the barrel and the center of the pile position to keep the same.

2.Pile slope control

The buried barrel adopts the cross-crossing statutory centering at the upper and lower ends of the inner diameter of the barrel, which can ensure the verticality of the barrel through the two center points. Determine the hole slope in time during drilling to ensure that the hole slope rate is less than 1%. If the hole slant is too big, immediately take measures to correct the slant.

3.Pile diameter control

Reasonable selection of drill diameter according to the stratum, the pile diameter control has an important role, the hole diameter can be 10-20cm larger than the diameter of the drill.

4. Pile length control

During the construction, the elevation of the mouth of the barrel and the design elevation should be correctly converted. The length of the machine should be measured accurately, and the length of the steel rope should be measured correctly, and the elongation value after considering the load should be taken into account, and the error should be corrected in time.

5.Pile top control

When the concrete is filled to the last bucket, accurately detect the thickness of floating slag and calculate and adjust the capacity of the last bucket of concrete. Review the height of pile top after filling, remove the conduit when it reaches the design requirement, or else replenish the material.

6.Pile structure control

The production of reinforcing cage should not exceed the tolerance allowed by the specification, including the lap method and length of main reinforcement. All data of reinforcing cage are accepted and recorded according to concealed works. Ensure that the conduit is well sealed, and the conduit should not be raised too much when moving it during filling, so as to prevent the occurrence of quality accidents such as mud entrapment and pile breakage.

7.Raw material control

For each batch of incoming reinforcement, cement, additives and other raw materials, strictly check the standard, factory date and factory test report and other material documents and sampling, all performance indicators are in line with the design requirements before use, strictly prohibit the use of unqualified or expired hardened cement.

5.3 Main construction process

1.Pile measurement and sampling

Adopt total station to carry out pile center position sampling, after sampling, set up guard piles around and retest, the error is controlled within 5mm. According to the control points provided by the design, use total station to set up the control network on site and review. According to the coordinate value of the center axis of the pile foundation, use the coordinate method to lay out the center line of the pile foundation, the center point of the pile foundation, etc., and drive in the marking piles, the error of the center line is controlled to be within the range of 5cm, and set up the cross-shaped control piles for easy checking, and the piles are marked with the pile number.

2.Site leveling

The chassis of the drilling rig should not be placed directly on the unsolidified fill to avoid uneven subsidence. For the construction site is dry land and the groundwater level is below the original ground during the construction period, the site will be leveled and tamped to remove debris; when the site is located in the shallow water, the drilling rig will be placed on the top surface after building an island, and the top surface of the island will be about 1.0m higher than the construction water level.

3. Drilling Rig Positioning

Drive the drilling rig to the construction pile position, the maximum inclination angle between the drilling rig and the plane is not more than 4° when the rig is in position, adjust the angle of the mast, operate the winch, align the center of the drill bit with the center of the drilling hole, adjust the verticality parameter of the drilling rig to make the drilling rod vertical.

4. Casing burial

Shield burial adopts digging and burying method, i.e. use special drilling bucket to dig out the soil layer where the shield is to be buried, and then put the shield into it. Buried accurately, horizontally, vertically and stably, backfill clay around the casing and tamp it. The centerline of the guide bar of the drilling rig, the centerline of the rotary disk and the centerline of the casing should be kept in the same straight line. The deviation between the center of the barrel and the center of the designed pile shall not be more than 20mm, and the deviation of the verticality of the steel barrel shall not be more than 0.5%, so as to ensure that the drilling rig works smoothly along the vertical direction of the pile.

5.Rotary drilling rig hole formation

During the drilling process of rotary drilling rig, the operator observes whether the drill pipe is vertical at any time, and controls the drilling depth through the depth counter. The drilled piles are drilled using a graded reaming construction method: firstly, a small-diameter cylinder core drill bit is used to drill to the design hole depth with a sand bucket, and then a larger-diameter drill bit is used to carry out the next level of reaming operation with a sand bucket, until the hole diameter and depth of the drilled piles satisfy the design requirements, and finally, the hole is completed by the rotary drilling rig.

6.Inspection of hole formation

After the hole reaches the design elevation, check the depth, diameter, wall and verticality of the hole, and take measures to deal with unqualified holes. Hole checking method according to the hole diameter, when drilling a dry hole, the hole can be tamped with a heavy hammer to compact the virtual soil, using a direct measurement of the rope and measuring device, if there is groundwater in the hole, can be used to pump suction anti-circulation slurry pumping method of clearing the hole, the method of underwater pouring of concrete construction of the drilling of the hole measurement work.

7. Hole clearing

Clear the sediment out of the hole. The thickness of sediment at the bottom of the hole before pouring underwater concrete is not more than 5cm for column pile and 20cm for friction pile.

END