Engineering geology in site selection, investigation & construction/excavation

Overbreak is the excavation of rock beyond the designed (neat) tunnel profile. It is the volume or thickness of rock removed outside the theoretical excavation line, usually expressed as a percentage of the design area or as extra thickness in cm.

$$\text{Overbreak} (\%) = \frac{A_{actual}-A_{design}}{A_{design}}\times 100$$

Importance

• Increases excavation, mucking and disposal volume.
• Needs extra shotcrete/concrete lining to fill the void, so cost increases.
• Reduces stability: loosened rock above the crown can fall.
• Delays the construction schedule and endangers workers.
• Shows the quality of the rock mass and of blasting practice, so it is a performance indicator in the contract (payment is normally only for the neat profile plus a permitted tolerance).

Factors of overbreak

Geological:

1. Joint spacing and orientation (joints dipping to the crown or parallel to the axis).
2. Bedding, foliation, faults and shear zones.
3. Weak or weathered rock, low rock strength.
4. Groundwater flow and weathering.
5. High in-situ stress (spalling).

Construction:

1. Excavation method (drilling and blasting vs. TBM).
2. Drilling accuracy (look-out angle) and charge per hole.
3. Delay and time lag before support.
4. Tunnel size and shape.

Good practice: controlled blasting (smooth blasting, presplitting), systematic mapping and prompt support.

Site investigation is the systematic collection, analysis and interpretation of geological, geotechnical and hydrological data of a site, to decide its suitability and to provide the information required for the design and construction of a civil engineering project.

Why geological site investigation is required

1. To select a suitable site or alignment among alternatives.
2. To find the soil and rock types and their sequence, thickness and extent.
3. To determine engineering properties: bearing capacity, shear strength, compressibility, permeability.
4. To locate geological defects: faults, shear zones, joints, cavities, landslides and weak layers.
5. To determine groundwater conditions: water table, seepage and quality.
6. To assess hazards: earthquake, flood, landslide, erosion and subsidence.
7. To design foundations, slopes and support systems economically and safely.
8. To estimate quantity and quality of construction materials.
9. To choose the construction method and equipment, and to estimate cost and time.
10. To prevent failures, unexpected costs, claims and delays in the contract.
11. To assess the environmental impact of the project on the ground.

Investigation is done in stages: reconnaissance, preliminary and detailed.

Site investigation is carried out in stages (reconnaissance, preliminary, detailed) using surface and sub-surface methods.

A. Surface investigation

1. Direct methods: field geological mapping, traversing, measurement of strike/dip and joints, outcrop study, geomorphological mapping, study of existing cuts and landslides.
2. Indirect methods: study of topographic maps, aerial photographs and satellite images (remote sensing), GIS, and review of earlier reports.

B. Sub-surface investigation

1. Direct methods:
   • Test pits and trenches: to 3-5 m depth; for soil and shallow rock.
   • Shafts, adits and drifts: to inspect rock in tunnels and dam abutments.
   • Boring (auger, wash, percussion) and core drilling: to get samples and rock quality (RQD); depth as required.
   • Sampling: disturbed and undisturbed samples.
2. Indirect methods: geophysical exploration:
   • Seismic refraction (depth of bed rock, velocity),
   • Electrical resistivity (soil/rock layers, groundwater),
   • Gravity, magnetic and ground penetrating radar.

C. In-situ and laboratory tests

Standard penetration test, plate load test, permeability (Lugeon/packer) test, shear test, rock mechanics tests and soil tests.

D. Reporting

Logs, geological sections, maps and recommendations.

A bridge site must have a safe foundation for piers and abutments, stable banks and a stable river channel.

Geological considerations

1. Foundation rock: sound, massive, strong bed rock at shallow depth is the best; avoid deep soft alluvium or weak rock.
2. River behaviour: a straight, stable reach with narrow channel; avoid meanders, braided reaches and confluences; assess scour depth and flood level.
3. Geological structure: avoid faults, shear zones and highly jointed rock; bedding should not dip downstream steeply toward the river on abutment slopes.
4. Abutment slopes: stable, no landslides, rock fall or debris; low erosion.
5. Weathering and overburden thickness.
6. Groundwater: water table and seepage affect excavation of foundations.
7. Seismicity: active faults and liquefiable sand near the site.
8. Construction material for concrete and approach roads nearby.
9. Approach alignment should be stable.

Investigation of the bridge foundation

• Geological mapping of both banks and river bed.
• Boreholes at each pier and abutment location, with core drilling in rock to at least 3 m into sound rock.
• Test pits; SPT and sampling in soil.
• Geophysical profiles (seismic refraction/resistivity) across the river.
• Laboratory tests for bearing capacity, scour and shear strength.
• Hydrological data for flood and scour assessment.

Geological documentation in tunnelling is the systematic recording of the geology encountered in the excavated face, walls and roof, during construction, in the form of maps, logs, photos and reports.

What is documented (face and tunnel mapping)

1. Rock type, colour, grain size and mineral content.
2. Rock strength and weathering grade.
3. Discontinuities: bedding, joints, foliation, faults and shear zones, with strike, dip, spacing, persistence, roughness, aperture and filling.
4. Groundwater: inflow rate, location, seepage, water quality.
5. Rock mass classification (RMR, Q) of each round.
6. Overbreak and instability: falls, squeezing, rock burst, gas.
7. Support installed (bolts, shotcrete, steel ribs) and any problems.
8. Photographs, sketches and plan/section of the tunnel (usually at 1:100-1:200), drawn after each blasting round, with chainage.

Procedure

• Mapping is done at the face right after excavation and before shotcrete covers it.
• Data are plotted on the tunnel log (development map of roof and walls) and longitudinal geological section.
• Prediction of the geology ahead using probe drilling and seismic methods.

Importance

1. Checks design assumptions and the pre-construction geology.
2. Choice and modification of support based on the actual rock class.
3. Provides basis for payment, claims and contract disputes.
4. Safety: warns of unstable zones, water and gas.
5. Provides a permanent record for maintenance, future repair and for similar projects.
6. Helps to predict conditions ahead and plan construction.

Geology controls the cost, stability and maintenance of a road, especially in the hills. The alignment is chosen after a geological study.

Engineering geological parameters

1. Rock and soil type: strong, massive rocks (quartzite, granite, gneiss) give stable cut slopes; phyllite, schist, shale, Siwalik mudstone and loose colluvium are weak and landslide-prone.
2. Geological structure: relation of bedding, foliation and joints to the road cut.
   • Dip into the hill (against the slope) is favourable.
   • Dip out of the slope (daylighting) gives planar and wedge failures; avoid it.
   • Avoid alignments parallel to faults and shear zones.
3. Slope and topography: gentle, stable slopes; avoid steep slopes, gullies and scarps; prefer ridges and stable benches.
4. Landslides and unstable ground: avoid old landslides, debris slopes and eroding areas.
5. Weathering and overburden thickness: deep soil and highly weathered rock means large cut volume and failure.
6. Groundwater and springs: avoid wet areas, seepage zones and swampy ground.
7. Streams and floods: minimise crossings; stay above flood level; choose stable stream banks.
8. Seismic hazard: avoid active faults.
9. Construction materials: quarry, aggregate and borrow areas along the route.
10. Foundation of structures: bridge and culvert sites.

Investigation

Reconnaissance with air photos, geological mapping, test pits, trenches and geophysics at critical sections; slope stability assessment with the kinematic analysis. Select the alignment that has the least geological risk, and then design cuts, drainage and protection works.

A tunnel passes through the ground, so the rock mass is both the load and the structure. Site and alignment are selected on the basis of the following.

Geological factors

1. Rock type and strength: hard, massive, homogeneous rocks (granite, gneiss, quartzite, limestone) are best; avoid soft, weak or swelling rocks (shale, phyllite, clay).
2. Geological structure: the tunnel axis should cross the strike of beds at a large angle; avoid alignment parallel to the strike, folds and fault planes. Avoid faults, shear zones and thrusts (MBT, MCT).
3. Rock mass quality: RQD, RMR, Q values; joint spacing and orientation.
4. Groundwater: inflow, high pressure and water-bearing fractures or karst increase risk; select alignment above the water table when possible.
5. Overburden (rock cover): enough cover for stability but not too deep (squeezing, rock burst); shallow cover below a river or valley is weak.
6. Portal sites: stable slopes with good rock, free from landslide, rock fall and flood.
7. Weathering and alteration at the portals and along the route.
8. Special hazards: gas, hot water, high temperature, rock burst, squeezing, karst cavities.
9. Seismicity and active faults.
10. Construction facilities: access, muck disposal and water supply.

Selection is based on mapping, geophysics (seismic refraction, resistivity), exploratory drill holes and geological sections along the alignment.

Geophysical exploration

It is the study of the sub-surface by measuring physical properties of the ground (velocity, resistivity, density, magnetism) from the surface, without excavation. It is an indirect method.

Methods:

• Seismic refraction/reflection: wave velocity gives layer depth and rock quality.
• Electrical resistivity: detects bed rock, groundwater, clay and cavities.
• Gravity and magnetic methods: large structures and faults.
• Ground penetrating radar and electromagnetic methods for shallow features.

Advantages: quick, cheap, covers large area, non-destructive. Limits: results are interpretation-based and need calibration.

Geotechnical exploration

It is the direct investigation of the ground by boring, test pits, trenches, sampling, and in-situ and laboratory tests, to obtain the engineering properties needed for design (bearing capacity, strength, compressibility, permeability).

Methods: boreholes with SPT, undisturbed sampling, core drilling with RQD, plate load test, vane shear, Lugeon test and laboratory tests.

Difference

The two are used together: geophysics between boreholes and geotechnical data to calibrate it.

Geophysical exploration is the investigation of sub-surface geology by measuring physical properties of the earth (seismic velocity, electrical resistivity, density, magnetic susceptibility, radar reflection) at the surface or in boreholes, and interpreting the readings to find the thickness, depth and nature of strata, bed rock, water and structures. It is an indirect and non-destructive method.

Main methods

1. Seismic refraction: a hammer or explosive creates waves; geophones record first arrival times; time-distance graph gives velocities and layer depths. Velocity shows rock quality (soil 0.2-0.8 km/s; hard rock above 3-5 km/s).
2. Electrical resistivity: electrodes in a Wenner or Schlumberger array send current; apparent resistivity vs. spacing identifies water table, clay and bed rock.
3. Gravity and magnetic methods: variations detect cavities, faults, dykes.
4. Ground penetrating radar (GPR): shallow detail, cavities, buried objects.
5. Borehole logging: gamma, resistivity and sonic logs.

Uses in civil engineering

• Depth to bed rock for foundation, dam and bridge sites.
• Thickness of overburden and landslide slip surface.
• Groundwater exploration.
• Locating faults, cavities, weak zones along tunnel alignments.
• Rippability and quality of rock.

Advantages and limits

Fast, cheap, covers a large area; but results are not unique, and need calibration with boreholes.

Tunnel construction in the Himalaya meets many geological problems, which depend on the rock mass, structure, water and stress.

1. Rock fall and roof collapse (overbreak): in highly jointed, blocky or weak rock, wedges fall from the crown; worst when joints dip steeply and the tunnel axis is parallel to strike.
2. Squeezing ground: weak rocks (phyllite, schist, shale, clay-rich shear zones) under high overburden deform slowly into the opening, closing it; needs yielding support.
3. Swelling ground: clay minerals (montmorillonite) and anhydrite swell with water and damage lining.
4. Rock burst and spalling: in brittle, hard rock (granite, gneiss, quartzite) under high stress, the rock breaks violently; common under high overburden in the Higher Himalaya.
5. Water inflow and flooding: fault zones, karst cavities and fractured rock give heavy inflow and high pressure; wash out infill, causing flow of sand and collapse.
6. Faults, shear zones and thrusts: crushed rock with low strength; collapses and big water flow (MBT, MCT).
7. Running and flowing ground: loose sand or saturated silt flows into the face.
8. Karst cavities in limestone and dolomite.
9. Gas and high temperature: methane, hydrogen sulphide, hot water.
10. Portal instability: landslides and rock fall at the entry.
11. Abrasive rock (quartzite) wears cutters in TBM.

Remedies: pre-investigation, probe drilling, forepoling, pipe-roof, grouting, systematic rock bolts and shotcrete, steel ribs, drainage, yielding support and a controlled advance rate.

Direct sub-surface methods expose or sample the ground directly so that rock and soil can be seen, tested and logged.

1. Test pits (trial pits): open excavations, 1-3 m wide and up to 3-5 m deep; give a clear view of the soil profile, bed rock, water table; allow undisturbed block samples and in-situ tests. Used for shallow foundation and borrow areas.
2. Trenches: long narrow excavations across slopes, faults or landslides to expose structure and thickness of overburden.
3. Shafts: vertical excavations (up to 100 m) to examine rock at depth, used at dam sites and tunnels.
4. Adits, drifts and exploratory tunnels: horizontal tunnels into abutments of dams; allow geological mapping and in-situ tests such as shear, plate load and flat jack test on rock.
5. Boring and drilling:
   • Auger boring in soft soil;
   • Wash boring and percussion boring in soil;
   • Rotary core drilling in rock, giving cores for logging, RQD and lab tests; diamond bits used in hard rock. Samples taken: disturbed (SPT split spoon) and undisturbed (thin-walled Shelby tube).
6. In-situ tests in boreholes: standard penetration test, packer (Lugeon) permeability test, pressuremeter, borehole camera.
7. Water-level observation in holes.

Limits: costly, time-consuming and gives information only at points, so it is supplemented with geophysics.

Direct surface methods are field studies by the geologist at the ground surface. For a road in rock slopes they are used to find stable alignment and safe cut slopes.

1. Geological (field) mapping at 1:5,000-1:2,000: traverses along the proposed centre line and on both sides; record rock type, weathering grade, soil cover and its thickness.
2. Structural measurements: strike and dip of bedding, foliation and joints by Brunton compass; spacing, persistence, roughness, aperture and infilling are noted. Joint sets are plotted on stereonet to check planar, wedge and toppling failure with respect to the proposed slope direction.
3. Study of natural slopes and existing cuts: performance of old road cuts and natural scarps shows safe angles for each rock type.
4. Mapping of landslides, rock fall, debris flow, gullies and erosion zones; old slide scars and tension cracks on the road line.
5. Hydrogeological observation: springs, seepage zones and wet areas, stream crossings and flood marks.
6. Rock strength estimates: Schmidt hammer, hammer-blow tests and RMR classification of outcrops.
7. Locating construction materials, quarries and spoil areas.

The result is an engineering geological map and a slope-stability zoning of the route, used for choosing the alignment and designing cut slopes (e.g. 1:4 in hard rock and 1:1 or flatter in weak rock).

Indirect surface methods get information without direct contact with the ground, by use of maps, images and instruments.

1. Study of existing maps and records: topographic, geological, soil and hydrological maps, previous reports, rainfall and flood data.
2. Aerial photo interpretation: stereo pairs show lithology, lineaments, faults, landslides, drainage patterns, and land forms.
3. Remote sensing and satellite imagery (Landsat, Sentinel, Google Earth), DEM, LiDAR, used for large areas, landslide inventory and structure mapping.
4. GIS analysis: overlay of slope, geology, land use and rainfall for hazard zoning.
5. Surface geophysical methods:
   • Seismic refraction: depth of bed rock and layer velocity.
   • Electrical resistivity: groundwater, clay and bed rock.
   • Magnetic and gravity methods.
   • Ground penetrating radar.
6. Local knowledge and interviews about floods, past slides and springs.

Advantages: cheap, quick, cover wide areas, guide the planning of direct investigation. Limit: interpretations must be checked in the field.

Indirect sub-surface methods estimate underground conditions from measured physical properties, mainly by geophysics, often in boreholes.

Tunnel alignment

• Seismic refraction survey along the alignment: gives rock velocity and so rock quality, depth of weathered layer and bed rock, and fault or shear zone (low velocity zones).
• Electrical resistivity profiling/tomography: detects water-bearing fractures, clay-filled shear zones, karst cavities (low resistivity for water and clay, high for dry cavity).
• Ground penetrating radar for shallow cover at portals.
• Borehole geophysical logging (gamma, resistivity, sonic, caliper) and cross-hole/down-hole seismic for stiffness and fractured zones.
• Seismic reflection/tomography to predict ahead of the face (TSP) during construction.

Dam site

• Seismic refraction for depth to sound bed rock, thickness of overburden and rippability on abutments and river bed.
• Resistivity for sand-gravel thickness, groundwater and seepage paths; cavities in limestone.
• Gravity/magnetic methods for buried faults and dykes.
• Borehole logging and packer tests for permeability, with cross-hole seismic for the rock mass between holes.
• Spontaneous potential and tracer methods for seepage.

These are checked with a few cored boreholes.

The detail stage (detailed design stage) is the final stage of investigation after the site or alignment has been selected, and gives the complete and accurate geological data for the final design and construction.

Activities

1. Large-scale engineering geological mapping (1:1,000-1:500) of the site, with detailed structural study.
2. Dense sub-surface exploration: closely spaced core drilling (including supplementary holes), test pits, trenches, shafts and adits.
3. Geophysical surveys to fill the gaps between holes.
4. In-situ tests: permeability (Lugeon), plate load, shear, flat-jack, SPT and in-situ stress tests.
5. Laboratory tests on rock and soil for strength, deformability, durability and index properties.
6. Rock mass classification (RMR, Q, GSI) and design parameters.
7. Groundwater study: observation wells, piezometers.
8. Detailed investigation of construction materials and their reserve.
9. Geological sections and 3D model; final report with recommendations for foundation treatment, slope support, grouting, excavation and monitoring.

Scale and spacing are chosen from the importance of the structure and the geological complexity.

The detail stage gives the exact ground data needed for design after the bridge site or reservoir has been selected.

Bridge

• Large-scale mapping of both banks and the river bed.
• Boreholes at every pier and abutment position, and core drilling at least 3 m into sound rock; SPT in soil; undisturbed samples.
• Geophysical profiles across the river for bed rock depth.
• In-situ and laboratory tests for bearing capacity, shear strength, and permeability.
• Study of scour depth, flood levels, bank stability, and river training needs.
• Seismic and liquefaction assessment; materials for aggregate and fill.

Reservoir

• Detailed mapping of the reservoir rim, dam and abutments, with special attention to leakage paths: faults, solution cavities in limestone, permeable layers, and low saddles in the rim.
• Core drilling and water-pressure (Lugeon) tests to get permeability of foundation and rim.
• Groundwater table, springs and water divide study.
• Slope stability of reservoir banks and landslides which may enter the reservoir.
• Sedimentation estimates and silt load.
• Seismic study (reservoir-induced seismicity) and construction material survey.

Sub-surface investigation with supplementary core drilling is carried out in the detailed (final design) phase, after site selection.

• In the reconnaissance and preliminary phases, only a few exploratory holes are used.
• In the detailed phase, additional (supplementary) core-drill holes are placed at closely spaced positions, to define foundation conditions, weak zones, rock quality (RQD), depth of weathering and permeability, and to fill the gaps between earlier holes and geophysical profiles.
• The results are used in the final design and for foundation treatment (grouting, excavation depth).

Tunnel

1. Reconnaissance: desk study, air photos and satellite images, geological map; identify possible alignments and portal sites.
2. Preliminary: field mapping along the alignment, measurement of joints, faults and folds; seismic refraction and resistivity; a few boreholes at portals and at deep cover; rock tests.
3. Detailed: large-scale mapping, core drilling along the axis (spacing based on geology), packer tests, groundwater observation, geological longitudinal section with rock classes (RMR/Q), prediction of support, water inflow, squeezing and rock burst. Pilot tunnel or adits for a long tunnel.
4. During construction: face mapping, probe drilling, monitoring.

Road

1. Reconnaissance: study of maps, aerial photos; identify landslides, unstable zones, river crossings.
2. Preliminary: geological traverse along alternatives, mapping of rock type, structure, slope stability and hydrology; select alignment.
3. Detailed: large-scale mapping, test pits and trenches, boreholes at bridges, high cuts, embankments; geophysics at critical sections; slope-stability analysis with joint orientation (stereonet); drainage design; materials survey for aggregate and borrow.
4. Construction: inspect cuts, treat failures, monitor slopes.

A canal carries water over long distances, so geology decides its stability, seepage loss and cost.

Geological parameters

1. Soil and rock type: firm, compact, impervious soil (clay, silty clay) gives low seepage loss and stable banks. Avoid loose sand and gravel (high seepage), very soft clay (settlement), black cotton or swelling soil, and rock needing costly excavation.
2. Permeability and seepage: avoid cavernous limestone, fractured rock and coarse gravel unless lined. Canal loss reduces supply and raises the water table.
3. Slope stability: in the hills, use stable slopes; avoid landslide areas, steep cuts in weak rock, and talus. Dip of beds toward the canal on a hillside causes slides.
4. Geological structure: avoid faults, shear zones, and highly jointed rocks along the line; cross faults at right angles.
5. Topography: a gentle gradient is needed; side-hill cut-and-fill canals need a stable ground.
6. Groundwater: high water table causes waterlogging and bank failure; salinity affects soil.
7. Drainage crossings: number and type of cross-drainage works; geology at their foundations; flood-prone areas.
8. Seismic and flood hazards.
9. Construction materials: earth for embankment, stone, gravel and sand nearby.
10. Excavation conditions: depth of cutting, rippability, rock excavation cost.

Investigation: geological mapping, test pits at intervals, borings at structures, permeability tests, and soil tests (compaction, shear strength, swelling).

A dam must be founded on ground that is strong, stable and watertight.

Geological factors

1. Foundation rock: strong, massive, sound, with low weathering. Granite, gneiss, quartzite and massive sandstone are good; weak shale, phyllite, schist and soft clay are poor. The type of dam (concrete gravity, arch, earth fill) is chosen from the foundation.
2. Geological structure: avoid faults, shear zones, folds with weak planes, closely jointed rock; bedding should be favourable (dipping upstream is best, steep dip downstream is dangerous for sliding).
3. Watertightness: avoid highly permeable rock, solution cavities in limestone, and open joints; check with Lugeon tests; plan grouting and cut-off.
4. Abutments: stable, no landslides or rock fall; adequate rock cover.
5. Valley shape: narrow, with sound rock at shallow depth, so smaller dam volume.
6. Seismicity: avoid active faults near the site; design for earthquakes.
7. Reservoir: rim and basin should be watertight and stable (see reservoir site).
8. Construction materials: aggregate, sand, clay and rock fill near the site.
9. Groundwater and seepage conditions around the dam.
10. Sedimentation rate of the catchment.

A reservoir must hold water, remain stable and stay silt-free as long as possible.

Geological considerations

1. Watertightness of the basin and rim: impermeable rocks (shale, clay, massive crystalline rock) are suitable. Avoid cavernous limestone, highly jointed or fractured rock, coarse gravel and permeable fault zones, which cause leakage. Check there is no low saddle or permeable gap along the rim and divide.
2. Geological structure: beds dipping upstream or toward the reservoir are favourable; downstream dip may lead to leakage. Faults and folds may be leakage channels.
3. Stability of reservoir banks: avoid landslide-prone slopes; filling can reactivate old slides; slide into the reservoir causes waves.
4. Water table: the groundwater level on both sides should be higher than the reservoir level, to avoid loss.
5. Sedimentation: catchment with weak, erodible rocks and landslides leads to quick siltation.
6. Seismicity: reservoir-induced seismicity; avoid active faults.
7. Water quality: soluble salts (gypsum, rock salt) spoil the water.
8. Valuable resources (minerals, forests, settlements) submerged.
9. Construction materials nearby.

Geological considerations for road site

• Stable slopes with strong rock; avoid landslides, steep weak slopes, and old debris.
• Favourable structure: beds dipping into the hill; avoid faults, shears and daylighting joints.
• Avoid wet, swampy ground and springs; limit stream crossings; stay above flood level.
• Materials (aggregate, borrow) along route.

Geological considerations for dam site

• Sound, strong, massive foundation rock with low weathering; narrow valley.
• No active faults, shear zones or highly jointed zones.
• Watertight foundation and rim; no solution cavities.
• Stable abutments and favourable dip (upstream); materials nearby.

Investigation of foundation site for a building

• Desk study and surface mapping of the site.
• Test pits and boreholes (depth 1.5 times the footing width or at least up to 1.5 B below foundation level) to find soil/rock type, thickness, water table.
• SPT, plate load test, soil samples for laboratory tests (bearing capacity, settlement).
• Look for fill, soft clay, expansive soil, liquefiable sand, slope stability.

Investigation of foundation site for a bridge

• Mapping of both banks and the river; scour and flood study.
• Boreholes at each pier and abutment, with core drilling 3 m into sound rock; SPT in soil.
• Geophysics (seismic refraction/resistivity) across the river.
• Lab tests for bearing capacity and foundation depth below scour level.

Dam site

1. Foundation rock: strong, sound, massive, little weathered (granite, gneiss, quartzite); avoid weak shale, phyllite and soft soil.
2. Structure: avoid faults, shear zones, closely jointed rock and unfavourable bedding dip; upstream dip is preferable.
3. Watertightness: no cavernous limestone, open joints or permeable gravel; plan for grouting.
4. Abutments: stable and free from landslides; adequate rock cover.
5. Valley shape: narrow with sound rock at shallow depth.
6. Seismicity: no active faults near the site.
7. Reservoir: watertight, stable rim, low sedimentation.
8. Materials: aggregates, clay and sand nearby.

Bridge site

1. Foundation: sound rock or dense gravel at shallow depth for piers and abutments; avoid thick soft clay and loose sand.
2. River channel: straight, narrow, stable reach; avoid bends, braided reaches, confluences and fast-shifting channels; estimate scour depth and maximum flood level.
3. Structure: avoid faults and weak shear zones; stable bedding.
4. Banks and abutment slopes: no landslides, rock fall or active erosion.
5. Groundwater: affects excavation of foundations.
6. Seismic hazard, liquefaction of sandy soil.
7. Approach roads on stable ground; construction materials and access.

Investigation by mapping, core drilling at piers and abutments, geophysics and in-situ tests supports both selections.

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.

Documentation (geological mapping and recording) is done in tunnelling in order to:

• Compare the actual ground with the predicted geology and verify or revise the design.
• Select and adjust support according to the real rock class.
• Warn of unstable zones, water inflow, gas and squeezing.
• Provide a legal record for payment, claims and disputes.
• Keep a permanent record for maintenance, repairs and later projects.

Tunnelling is the excavation of an underground passage through rock or soil, without removing the overlying ground, by drilling and blasting, mechanised excavators or tunnel boring machines, followed by support and lining. It is used for roads, railways, hydropower waterways, water supply and metro.

Geological site investigation process

Common stages for all structures: (1) desk study, (2) reconnaissance, (3) preliminary investigation, (4) detailed investigation, (5) reporting.

Roads

• Study maps, air photos; mark landslides, steep and weak slopes.
• Map geology along alternatives: rock type, joints, bedding dip relative to the cut, weathering, springs.
• Test pits and trenches, boreholes at high cuts, bridges, and fills; geophysics at doubtful places.
• Stereographic analysis of slope failure, materials survey.

Bridges

• Map both banks and river channel; study flood and scour.
• Boreholes at piers and abutments, core drilling in rock; SPT in soil; geophysics across the river.
• Lab tests for bearing capacity and tests of aggregate.

Dams

• Regional geology, structure, seismicity, and reservoir watertightness.
• Mapping of the dam axis, abutments, and borrow areas.
• Core drilling with Lugeon tests; adits and trenches; geophysics.
• In-situ tests for strength and deformation of foundation rock; grouting design; materials investigation.

Outputs: engineering geological maps, sections, logs and recommendations for design.

Underground excavation is the construction of tunnels, caverns and shafts below ground.

Process:

1. Investigation and design of alignment, shape and support from the rock mass classification.
2. Portal construction: slope cutting, support and drainage at the entrance.
3. Excavation:
   • Drill and blast: drilling holes, charging with explosives, blasting, ventilating.
   • Mechanised: roadheader, TBM in soft/medium rock.
   • Full-face or top-heading and bench method according to rock quality.
4. Mucking: removing broken rock.
5. Support: rock bolts, shotcrete, wire mesh, steel ribs; installed immediately in poor rock.
6. Geological mapping, monitoring of convergence.
7. Drainage and ventilation, then final concrete lining.

Geological problems: squeezing, rock burst, water inflow, overbreak and faults.