Geology and Civil Engineering

Geology is the science of the Earth: its materials, structure, processes and history. Engineering geology applies geological knowledge to the planning, design, construction and maintenance of civil engineering works.

Scope

• Study of rocks, minerals, soils and their engineering properties.
• Study of structures (folds, faults, joints) and their effect on stability.
• Study of surface processes: weathering, erosion, landslides, floods.
• Study of groundwater, earthquakes and other hazards.
• Selection and investigation of sites for dams, tunnels, roads, bridges and buildings.

Objectives

• To select a safe and economical site.
• To find the nature and strength of foundation rock.
• To identify geological hazards early and plan remedial measures.
• To locate suitable construction materials (aggregate, sand, stone).

Importance

• Prevents failures such as dam leakage, tunnel collapse and slope failure.
• Reduces cost by avoiding redesign and unexpected ground conditions during construction.
• Helps in the design of safe slopes, cuttings and foundations.
• Essential in Nepal, where young and fractured Himalayan rocks, landslides and earthquakes are common (for example, hill roads and hydropower tunnels).

Engineering geology is the application of geological knowledge, methods and data to civil engineering so that structures are planned, designed, built and maintained safely, economically and with the least harm to the environment.

Scope

1. Site selection and investigation for dams, tunnels, bridges, highways, buildings, canals and airports.
2. Materials: identification of rocks and soils, and of sources of aggregate, sand, gravel and building stone.
3. Geological structures: attitude of beds, folds, faults and joints and their effect on foundations, slopes and tunnels.
4. Ground and surface water: groundwater occurrence, seepage, reservoir leakage and drainage problems.
5. Geological hazards: landslides, rockfall, erosion, floods, subsidence and earthquakes.
6. Geophysical and geotechnical methods: seismic and resistivity surveys, drilling, sampling and in-situ tests.
7. Mapping and reporting: preparation of engineering geological maps, sections and reports.

Objectives

• Choose the best site and alignment.
• Determine the bearing capacity, strength and permeability of the ground.
• Predict hazards and recommend treatment.
• Estimate the quantity and quality of construction materials.

Importance in civil engineering

• Gives the engineer a reliable picture of the ground before design, so cost and time overruns are reduced.
• Prevents disasters; a dam on a weak, leaky foundation or a road on an unstable slope will fail.
• Helps to choose the right type of foundation, slope angle, tunnel support and excavation method.
• Supports environmentally sound design.

Examples from Nepal

• Hill roads (for example, Mugling-Narayanghat, Sindhuli road, BP Highway): frequent landslides in weak, sheared Lesser Himalayan rocks need geological mapping and slope treatment.
• Hydropower projects (Kulekhani, Marsyangdi, Upper Tamakoshi): dam sites, headrace tunnels and powerhouses depend on rock quality, joints and shear zones.
• Kathmandu Valley: soft lacustrine clays and sands increase earthquake damage, as seen in 1934 and 2015.
• Chure (Siwalik) region: weak, easily eroded rocks cause flash floods and bank cutting.

Geology is a broad science and is divided into branches, each studying one aspect of the Earth. They are closely linked.

Main branches

• Mineralogy and crystallography: minerals and their crystal forms.
• Petrology: origin, composition and classification of rocks.
• Structural geology: folds, faults, joints and other deformation features.
• Physical geology: surface and internal processes (weathering, erosion, volcanism).
• Historical geology and stratigraphy: Earth history and the order of rock layers.
• Palaeontology: fossils and ancient life.
• Geomorphology: landforms and their development.
• Hydrogeology: groundwater.
• Economic geology: ore and fuel deposits.
• Engineering geology: application to civil works.
• Geophysics and geochemistry: physical and chemical study of the Earth.

Interrelationship

• Mineralogy gives the base for petrology, because rocks are aggregates of minerals.
• Petrology and structural geology together explain how rock bodies are formed and deformed.
• Stratigraphy uses palaeontology (fossils) to date and correlate layers.
• Physical geology and geomorphology explain how structures are shaped into landforms.
• Engineering geology uses nearly all of them to solve practical civil engineering problems.

Site investigation is the study of the geological and ground conditions at a proposed site so that a safe and economical structure can be built. It is done in stages (phases), and the methods are of two groups: surface and subsurface.

Phases of investigation

1. Desk study / reconnaissance: collect existing maps, aerial photographs, satellite images, reports and records; walk over the area to see main features.
2. Preliminary investigation: geological mapping, a few boreholes and pits, and geophysical profiles to compare alternative sites or alignments.
3. Detailed investigation: closely spaced drilling, trenches, in-situ tests, laboratory tests and geophysics on the selected site to supply design parameters.
4. Construction stage: inspection of excavations and foundations, and checking of assumptions.

A. Surface methods

• Geological mapping: rock types, bedding, folds, faults, joints, springs and slope stability recorded on a topographic base, giving an engineering geological map.
• Aerial photographs and remote sensing: show lineaments, landslides, drainage and rock boundaries.
• Geophysical methods (done from the surface):
  • Seismic refraction gives depth of bedrock and rock quality.
  • Electrical resistivity gives depth of water table, bedrock and soil layers.
  • Others are gravity, magnetic and ground-penetrating radar.

B. Subsurface methods

• Test pits and trenches: shallow, direct view of soil and weathered rock; give undisturbed samples; trenches across faults and shear zones.
• Drifts and adits: tunnels driven into a hillside to inspect rock at depth (dam abutments, tunnels).
• Boring and drilling:
  • Auger boring for soft soils.
  • Wash boring and percussion boring for soils.
  • Rotary core drilling for rocks, giving continuous cores.
  • Borehole logs record strata, core recovery and RQD.
• Sampling: disturbed and undisturbed samples for laboratory tests.
• In-situ tests: standard penetration test, plate load test, permeability (Lugeon) test, and shear tests.
• Borehole geophysics and logging for fractures and water.

 Desk study -> Reconnaissance -> Preliminary
        -> Detailed -> Construction stage

The results are combined into a report with maps, sections and recommendations.

Topographic map

A topographic map shows the shape and relief of the ground surface using contour lines, and also natural and man-made features such as rivers, roads, settlements and vegetation, drawn to scale. It shows the surface only, not the rocks beneath.

Aerial photograph

An aerial photograph is a photograph of the ground taken from an aircraft (or drone) with the camera pointing vertically or obliquely downward. Overlapping pairs viewed under a stereoscope give a 3-D view. They are used to identify rock units, lineaments, faults, landslides, drainage patterns and land use, and to prepare base maps.

Engineering geological map

An engineering geological map is a map, made on a topographic base, that shows rock and soil types, their engineering properties, geological structures (dip, strike, folds, faults, joints), weathering, groundwater, slope stability, hazards and construction material sources, for use in civil engineering projects.

Importance for site investigation

1. Shows the distribution and nature of foundation rocks and soils at one glance.
2. Helps to compare alternative sites or alignments quickly and cheaply.
3. Identifies hazards such as landslides, active faults, unstable slopes, swelling soil and flood zones.
4. Guides the planning of boreholes, trenches and geophysical lines, so that investigation is placed where it is needed.
5. Shows groundwater conditions, springs and seepage areas.
6. Locates sources of construction material.
7. Helps design slope cuts, foundations, tunnel support and drainage.
8. Serves as a permanent record for later stages and maintenance.

Engineering geology versus general geology

Disciplines (branches) of geology

• Mineralogy and crystallography
• Petrology
• Structural geology
• Physical geology
• Historical geology and stratigraphy
• Palaeontology
• Geomorphology
• Hydrogeology
• Economic geology
• Geophysics and geochemistry
• Engineering geology

A dam is a huge structure that must stand on rock able to carry the load, resist sliding and stay watertight, and its reservoir must hold water without leakage or slope failure. Investigation is therefore done at the dam site and the reservoir area.

What is to be found out

• Rock type, strength and weathering of foundation and abutments.
• Structures: dip and strike of beds, joints, faults, shear zones, folds.
• Permeability and groundwater conditions.
• Depth to sound bedrock and thickness of overburden.
• Slope stability of reservoir rim; sedimentation; seismicity.
• Availability of construction materials.

A. Direct methods

1. Geological mapping of the dam site (scale 1:500 to 1:2000) and reservoir (1:5000 to 1:25000).
2. Test pits, trenches across suspected faults and in overburden.
3. Core drilling at the axis, abutments, spillway and stilling basin; core recovery, RQD and logging; permanent record of the rock.
4. Drifts/adits in abutments to see rock at depth and carry out in-situ tests.
5. In-situ tests: Lugeon (packer) water pressure tests for permeability, plate load test, shear tests and borehole dilatometer tests.
6. Laboratory tests on cores: compressive strength, density, porosity, durability.

B. Indirect methods

1. Seismic refraction: depth to bedrock, rock quality and the fractured zones.
2. Electrical resistivity: depth of water table, weak and saturated zones, buried channels.
3. Gravity and magnetic surveys: large buried features.
4. Aerial photographs and satellite images: lineaments, landslides and reservoir rim features.
5. Geophysical logging of boreholes.

Reservoir-specific checks

• Leakage paths through permeable beds, solution cavities in limestone, open joints and faults.
• Stability of reservoir slopes (old landslides which could be reactivated).
• Watertightness of the reservoir rim and divide.
• Silting and seismic activity, including reservoir-induced seismicity.

The data are combined into a geological report with plans, sections and treatment recommendations such as grouting and cut-off.

Petrology studies the origin, composition, texture and classification of rocks. For civil engineers it helps to identify rocks in the field, judge their strength, durability and weathering, and select rock for aggregate, building stone, dam foundations and tunnels.

Structural geology studies the deformed attitudes of rocks: dip and strike, folds, faults, joints and unconformities. It is important for judging slope stability, tunnel stability, the water-tightness of dam and reservoir sites, bearing capacity of foundations and the location of seismic faults.

Engineering geology is the application of geological principles and data to civil engineering so that works are planned, designed and built safely, economically and in harmony with the environment.

Major objectives of investigation for a hill road

1. Selection of the best alignment: compare alternative routes and choose the most stable, shortest and economical one, avoiding active landslides, fault zones and unstable slopes.
2. Identification of rock and soil types along the route, their strength and weathering, to decide cut slope angles and excavation methods (manual, ripping or blasting).
3. Study of geological structures: dip and strike of beds and joints relative to the road cut; whether discontinuities dip out of the slope (risk of planar or wedge failure).
4. Landslide and slope stability assessment: mapping existing and potential slides, rockfall and debris-flow areas; deciding treatment.
5. Groundwater and drainage: locating springs, seepage and gullies; planning side drains, culverts and subsurface drainage.
6. Foundation conditions for bridges, culverts and retaining walls: depth to bedrock, bearing capacity and scour.
7. River training: studying river erosion, flood levels and bank stability where road runs along rivers.
8. Material survey: finding quarries for aggregate, borrow areas for fill and sources of sand.
9. Hazard evaluation: seismic zone, active faults, debris flows, glacial lake outburst floods in high areas.
10. Design of stabilisation measures: retaining walls, breast walls, gabions, bioengineering, anchors.
11. Environmental protection: minimising slope disturbance and disposal of spoil.
12. Cost estimation: reliable quantities of rock and soil excavation, and of protection works.

These objectives are met through desk study, aerial photograph interpretation, engineering geological mapping along the alignment, test pits and boreholes at critical sites, and geophysical profiling.

Site investigation for a road finds the ground conditions along the alignment so that the road is safe, economical and stable. It is done in stages.

1. Reconnaissance and desk study

• Collect topographic maps, geological maps, aerial photographs, satellite images and earlier reports.
• Walk over possible routes to compare them and avoid landslides, steep slopes, marshes and fault zones.

2. Preliminary survey

• Detailed engineering geological mapping of the corridor: rock types, structures, weathering, slope angles, springs, existing landslides.
• Aerial photograph interpretation for lineaments, drainage and unstable areas.
• A few test pits and shallow boreholes at key locations.

3. Detailed investigation of the chosen alignment

• Test pits and trenches every 200 to 500 m or at changes in soil type to find soil type and depth of rock; sample for laboratory tests (grain size, Atterberg limits, CBR, compaction).
• Boreholes (auger/rotary) at bridge sites, high embankments, deep cuttings and landslide areas to get strata, groundwater and rock quality.
• Geophysical methods: electrical resistivity and seismic refraction for depth of bedrock, water table and buried slide surfaces.
• In-situ tests: SPT, plate load test, field CBR.
• Slope stability study by kinematic analysis of joints and bedding relative to the cut face.

4. Material survey

• Locate quarries for stone and aggregate, borrow pits for fill, and sand and gravel sources; test for quality and quantity.

5. Reporting

• Prepare strip maps, longitudinal geological sections, and recommendations on cut slopes, retaining structures, drainage and bioengineering.

6. Construction stage

• Inspect cuttings and foundations, and revise treatment if conditions differ.

A borehole log is a written and graphical record of the materials and conditions met while drilling a borehole, drawn depth-wise.

It gives:

• Borehole number, location, elevation, date and method of drilling.
• Depth and thickness of each stratum, with description of soil or rock (type, colour, grain size, weathering).
• Core recovery percentage and RQD, and the position of fractures, cavities or weak zones.
• Groundwater level and any water loss during drilling.
• Depths of samples, and results of in-situ tests such as SPT N value.
• A graphic column using standard symbols.

Uses: to prepare geological sections, assess bearing capacity and rock quality, plan foundation depth, tunnel support and grouting, and to keep a permanent record of the ground.