--- title: "Overbreak and tunnel geological survey" subject: "Engineering Geology II" chapter: 4 key: "semester_4/engineering_geology_ii:overbreak-tunnel-survey" source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/overbreak-tunnel-survey site: BCT Engineering --- # Overbreak and tunnel geological survey Engineering Geology II · Chapter 4: Engineering geology in site selection, investigation & construction/excavation - Asked in: 2073 Magh, 4 marks - Video: [Tunnel geological overbreak prognosis and optimization system](https://www.youtube.com/watch?v=6ifOogUIznU) — Geonovelty - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/overbreak-tunnel-survey - Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-4 ## Question What is overbreak? Give a brief outline of the geological survey of a tunnel. ## Short answer Overbreak is rock excavated beyond the designed tunnel profile, expressed as (actual area − design area) / design area ×100. Geological survey steps: desk study, surface mapping, seismic/ resistivity profiling, core boreholes with RQD and Lugeon tests, rock mass classification, geological longitudinal section, and face mapping during construction. ## Answer **Overbreak** is the excavation of rock beyond the designed tunnel profile (the volume or thickness outside the neat line), caused by geological conditions and blasting. It is expressed as a percentage of the design area: $$ \text{Overbreak}(\%) = \frac{A_{actual}-A_{design}}{A_{design}}\times 100 $$ It increases excavation, filling and support cost and decreases safety. #### Outline of the geological survey of a tunnel 1. **Desk study:** topographic and geological maps, air photos and satellite images, earlier reports, to fix the corridor. 2. **Surface mapping:** along the alignment at 1:5,000-1:1,000; map rock types, joints, bedding, foliation, faults, folds, shear zones, springs, and slope stability at portals. 3. **Geophysical survey:** seismic refraction and resistivity profiles along the axis to find weak zones, water, and depth of weathering and overburden. 4. **Drilling:** core boreholes at portals, below valleys, and at suspected faults; RQD, packer tests, groundwater level. 5. **Rock testing and classification:** RMR, Q; lab strength and swelling tests. 6. **Geological longitudinal section** along the axis with predicted rock classes, water inflow and support types, and hazards such as squeezing, rock burst, gas. 7. **During construction:** face mapping, probe drilling ahead, monitoring, and updating the geological model. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is overbreak in tunnelling? A. Excavation of rock beyond the designed tunnel profile B. Rock left inside the design profile after blasting C. The collapse of the portal slope only D. Water inflow beyond the design discharge Answer: A. Overbreak is the volume or thickness excavated outside the neat line of the design profile. (Key point: Overbreak definition) 2. How is overbreak percentage calculated? A. (A actual + A design) / 2 B. (A actual - A design) / A design x 100 C. (A design - A actual) / A actual x 100 D. A actual / A design Answer: B. It is the excess area over the design area expressed as a percentage of the design area. (Key point: Overbreak formula) 3. What is the first stage of a tunnel geological survey? A. Face mapping during construction B. Final concrete lining C. Desk study of maps, air photos and earlier reports D. Core drilling at suspected faults Answer: C. The desk study fixes the corridor before surface mapping, geophysics and drilling. (Key point: Survey sequence) 4. What does the geological longitudinal section along the tunnel axis show? A. Only the surface topography B. The cost of lining C. Only the portal location D. Predicted rock classes, water inflow, support types and hazards Answer: D. It gives predicted rock classes (RMR, Q), inflow, support types and hazards such as squeezing and rock burst. (Key point: Geological longitudinal section) ## Related in Engineering Geology II - [Geological problems in tunnel construction](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Atunnel-construction-problems) — Engineering Geology II, chapter 4, asked 3× - [Geological investigation of tunnel and road](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Atunnel-road-investigation) — Engineering Geology II, chapter 4 - [Overbreak and its importance](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Aoverbreak-factor) — Engineering Geology II, chapter 4, asked 8× - [Geological documentation in tunnelling](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Atunnel-geological-documentation) — Engineering Geology II, chapter 4, asked 6× - [Geology in tunnel site selection](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Atunnel-site-selection) — Engineering Geology II, chapter 4, asked 4× ## Related in other subjects - [Tunnel alignment survey and ideal bridge location](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Atunnel-alignment-survey-ideal-bridge-location) — Transportation Engineering II, chapter 5 - [Ideal bridge site; tunnel ventilation need and methods](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aideal-bridge-site-tunnel-ventilation) — Transportation Engineering II, chapter 5