Borehole Instability in Decomposed Granite Seabed for Rock-Socketed Monopiles during “Drive-Drill-Drive” Construction Process: A Case Study
Abstract
:1. Introduction
2. Background
2.1. Project Information
2.2. Construction Technology
2.3. Borehole Instability
2.4. Typical Borehole Collapse Cases
2.4.1. Case 1
2.4.2. Case 2
2.4.3. Case 3
3. Methodology
3.1. Sampling Method
3.2. In Situ Tests
3.3. Laboratory Tests
4. Results and Discussion
4.1. Physical and Mechanical Properties of Decomposed Granite
4.2. Disintergration Properties of Decomposed Granite
4.3. Causes for Borehole Instability
4.4. Judgement of the Possibility of Borehole Instability
4.5. Improvement Measures to Prevent Borehole Instability
- (1)
- Using a smaller diameter drilling rig during drilling. In this case, the range of exposed soil is reduced, and considering the soil arching effect, the possibility of borehole instability is significantly reduced. Even if borehole instability occurs, the buffer effect provided by the distance between the borehole wall and the monopile can prevent pile running. Additionally, constructing a hollow cylindrical structure of soil within the monopile can also significantly reduce pile driving resistance.
- (2)
- Using a casing that can be lifted and lowered freely. After drilling is completed, the casing can be lowered to protect the borehole wall and then lifted back up after the next step of pile driving is completed. This method can provide support to the borehole wall to prevent borehole instability.
- (3)
- Using biopolymer drilling fluid during drilling to form a filter cake on the borehole wall. This can prevent decomposed granite from directly contacting seawater, avoiding disintegration. It can also exert static water pressure on the borehole wall to counteract the stress release caused by excavation and maintain borehole stability. Compared with traditional slurry, biopolymer drilling fluid causes minimal pollution to the marine environment, but its disadvantage is its high cost.
5. Conclusions
- (1)
- The results of the sampling operation demonstrate that employing a double-tube rotary sampler with biopolymer drilling fluid significantly minimizes disturbance to decomposed granite, enabling the acquisition of complete samples. A series of in situ and laboratory tests revealed an increase in the weathering degree of granite from SDG to MDG, HDG, and CDG, while mechanical strength and integrity declined considerably. These four types of decomposed granite can be effectively classified based on apparent characteristics, in situ tests, and laboratory tests.
- (2)
- The long window period between drilling and monopile driving operations, the insufficient soil shear strength, and the lack of hole wall protection are the main causes of borehole instability. Due to HDG’s insufficient strength and the highly fragmented structure of MDG, hole collapse predominantly occurs within CDG and MDG strata. The water-immersion disintegration properties of HDG exacerbate the collapse rate in HDG strata. Additional factors, such as large pile foundation dimensions, geological fractures or faults, and adverse construction environments, may worsen this issue.
- (3)
- The occurrence of hole collapse can be comprehensively determined by considering factors such as stratum type, soil exposure range, and exposure duration. The key to preventing hole wall instability lies in reducing exposed soil range and duration or implementing suitable retaining wall measures. Drilling smaller holes has proven effective in engineering practice. Employing casing or plant-based drilling fluid for hole wall protection may also be considered a potential improvement measure.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
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Type | Grade | General Characteristics | Typical Characteristics |
---|---|---|---|
CDG | Ⅴ | Gray-yellow with brown yellow | High content of clay minerals |
Destroyed completely, soil like | Can be crumbled by hand | ||
HDG | Ⅳ | Gray-white with black | Mainly quartz and feldspar |
Original rock texture preserved, sandy soil like | Can be broken by hand | ||
MDG | Ⅲ | Brown yellow | Part of feldspar is weathered |
Clear structure of parent rocks, clastic blocks | Not easily broken by hand, easily broken by hammer | ||
SDG | Ⅱ | Gray-white with black | Mainly quartz, feldspar, and mica |
Distinct structure of parent rocks, column-like | Cannot usually be broken by hand, not easily broken by hammer |
Parameter | CDG | HDG | MDG | SDG |
---|---|---|---|---|
ρn/(g/cm3) | 1.83~2.04 | 1.87~2.21 | / | / |
ρd/(g/cm3) | 1.50~1.73 | 1.46~1.92 | 2.68~2.72 | 2.62~2.81 |
e | 0.552~0.814 | 0.397~0.850 | / | / |
ω/% | 17.7~29.9 | 14.3~30.6 | / | / |
Gs | 2.69~2.72 | 2.68~2.72 | / | / |
av0.1~0.2/MPa−1 | 0.30~0.54 | 0.23~0.28 | / | / |
Es1~2/MPa | 3.35~5.49 | 5.53~11.61 | / | / |
Ee/GPa | / | / | / | 16.8~51.9 |
c/kPa | 21.3~25.3 | 18.3~23.4 | 205.9~6900 | / |
φ/° | 27.4~28.8 | 28.4~35.2 | 34.9~52.4 | / |
Is/MPa | / | / | 0.33~0.68 | / |
R/MPa | / | / | 5.38~18.20 | 30.63~107.20 |
k/(10−6 cm/s) | 58.2~87.5 | 61.2~426.0 | / | / |
Vs/(m/s) | 311~346 | 316~495 | 447~681 | 610~936 |
ρ/(Ω·m) | 1.46~1.91 | 2.18~11.36 | 10.73~44.26 | 23.66~152.55 |
N | 33~47 | 55~67 | / | / |
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Sun, B.; Zhang, Q.; Zhu, W.; Leng, J.; Ye, G. Borehole Instability in Decomposed Granite Seabed for Rock-Socketed Monopiles during “Drive-Drill-Drive” Construction Process: A Case Study. J. Mar. Sci. Eng. 2023, 11, 990. https://doi.org/10.3390/jmse11050990
Sun B, Zhang Q, Zhu W, Leng J, Ye G. Borehole Instability in Decomposed Granite Seabed for Rock-Socketed Monopiles during “Drive-Drill-Drive” Construction Process: A Case Study. Journal of Marine Science and Engineering. 2023; 11(5):990. https://doi.org/10.3390/jmse11050990
Chicago/Turabian StyleSun, Bo, Qi Zhang, Wenxuan Zhu, Jian Leng, and Guanlin Ye. 2023. "Borehole Instability in Decomposed Granite Seabed for Rock-Socketed Monopiles during “Drive-Drill-Drive” Construction Process: A Case Study" Journal of Marine Science and Engineering 11, no. 5: 990. https://doi.org/10.3390/jmse11050990
APA StyleSun, B., Zhang, Q., Zhu, W., Leng, J., & Ye, G. (2023). Borehole Instability in Decomposed Granite Seabed for Rock-Socketed Monopiles during “Drive-Drill-Drive” Construction Process: A Case Study. Journal of Marine Science and Engineering, 11(5), 990. https://doi.org/10.3390/jmse11050990