Safety Measures for Bored Pile Construction in Tunnel Civil Works
Safety Measures for Bored Pile Construction in Tunnel Civil Works
Prevention of Borehole Collapse
In line with the construction process for bored piles and the characteristics of the drilling equipment, the following safety measures are adopted to prevent borehole collapse:
Before rock drilling for pile construction, a drilling plan must be prepared based on the engineering geological conditions of the ground. Proper selection and control of slurry parameters is the key to preventing borehole collapse.
The temporary casing is fabricated from 10 mm thick steel plate, with an inner diameter 20–40 cm larger than the design pile diameter, and two overflow openings are set at the upper part. When installed, the top of the casing must be 0.5 m above ground level. In deep riverbeds with soft soil, silt or sandy layers, the top of the casing must be at least 3 m above the water surface. Where the soft soil or silt layer is relatively thick, the casing must be embedded at least 1.5 m into the underlying impervious clay stratum.
Clay slurry is used for borehole wall support during the formation of the borehole. Slurry properties must be strictly controlled within the following ranges: relative density 1.2–1.6; sand content ≤ 4%; viscosity 16–28 s; colloidal rate ≥ 95%; fluid loss ≤ 20 ml/30 min; filter cake thickness ≤ 3 mm/30 min; pH value between 8 and 11. During construction, the slurry level must always be maintained not lower than 0.5 m below the casing top. In sections with poor ground conditions, the internal borehole pressure is balanced by increasing slurry density and appropriately reducing drilling speed, so as to effectively avoid collapse.
Prevention of Borehole Deviation
Before rock drilling starts, the ground beneath the drilling rig base must be leveled and well compacted, and sleepers laid under the rig to prevent rig displacement caused by uneven settlement. Rig positioning must be accurate, ensuring that the drill bit centering error is less than 2 cm, and that the centers of the rotary table, crown block, and casing lie on the same vertical line, so as to avoid base movement and resulting deviation.
During construction, the horizontal level of the rig must be checked periodically. Borehole verticality must be measured when the depth reaches half of the design depth and again after completion of drilling, in order to eliminate borehole deviation.
Treatment Measures for Collapse, Deviation, and Slurry Loss Around the Casing
If borehole inclination is detected during rock drilling, the drill bit should first be lifted to the deviated section, and the deviation corrected by repeatedly sweeping the borehole at low speed. If the deviation is severe and cannot be corrected, the borehole must be backfilled to about 0.5 m above the deviated section. After the backfill has settled and become dense, drilling can be restarted.
If, during drilling, the slurry level inside the casing drops sharply and is accompanied by bubbling, a borehole collapse is likely. For minor collapse, construction can proceed through the affected zone by reducing drilling speed and increasing slurry density. In the event of severe collapse, drilling must be stopped immediately, and the borehole backfilled with clay. Drilling may resume only after the borehole wall has stabilized.
Bored pile construction must strictly follow the principle of “one borehole formed, one pile concreted,” to avoid excessive idle time between hole completion and concrete placement that could lead to collapse. Construction must be organized in accordance with the sequence defined in the construction technical brief. The minimum time interval between the construction of two adjacent piles must not be less than 24 hours, in order to avoid interconnection between bores (pile communication).
After completion of concrete pouring, the open area at the pile head and around the borehole must be covered and protected using rebar mesh or planks, to prevent personnel from accidentally falling into the hole and causing safety accidents.
