Introduction to Engineering Geology

An engineering geological map is a special-purpose map that shows the geology of an area (rock and soil types, structures, groundwater, geomorphology and hazards) in terms of its behaviour and suitability for civil engineering works such as dams, tunnels, roads and bridges.

Preparation

1. Desk study: collect topographic sheets, geological maps, aerial photos/satellite images and old reports; decide scale (1:50,000 for regional, 1:5,000 to 1:1,000 for a site).
2. Base map: enlarge or prepare a topographic base map with contours and coordinates.
3. Field mapping: traverse the area, record lithology, contacts, strike and dip, joints, faults, folds, weathering grade, springs, slope condition, landslides and erosion.
4. Sub-surface data: add data from test pits, trenches, drill holes, geophysical surveys and tunnel/adit logs.
5. Laboratory/field tests: rock and soil strength, permeability, rock mass classification (RMR, Q).
6. Compilation: transfer all data to the base map, draw boundaries, then add cross-sections and engineering zoning.
7. Interpretation and report: classify the ground into engineering units and give recommendations.

Components

• Topographic base (contours, rivers, roads, settlements, north arrow)
• Lithological units with age, grain size, hardness and weathering
• Structural data: strike/dip symbols, joints, faults, folds, shear zones
• Soil/overburden type and thickness
• Hydrogeology: springs, water table, seepage zones
• Geomorphology and hazards: landslides, rock falls, flood limits, active faults
• Engineering classification/zoning of rock mass and sites (good, fair, poor)
• Legend, scale, title, cross-sections and explanatory notes

The engineering geological study (system) of a project is carried out in successive phases, each more detailed than the previous one, so that cost and risk fall as the design develops.

Detailed design phase in short: the main tasks are to confirm the foundation and abutment conditions, define geotechnical parameters for design (strength, deformability, permeability), delineate weak zones, decide excavation slopes and support, plan grouting/drainage, and locate and quantify construction materials.

The engineering geological system (EGS) is the combination of the geological environment (rock, soil, structure, water, relief and processes) and the proposed engineering structure, treated as one interacting system. The structure changes the ground, and the ground controls the safety and cost of the structure.

Main parameters for evaluation

1. Lithology: rock/soil type, grain size, mineral composition, hardness.
2. Geological structure: bedding, joints, faults, folds, shear zones; their orientation, spacing, persistence, aperture and infilling.
3. Weathering and alteration: degree and depth of weathering.
4. Physical and mechanical properties: density, porosity, strength, deformability, slake durability, swelling.
5. Rock mass quality: RQD, RMR, Q and GSI classification.
6. Hydrogeology: water table, permeability, seepage, springs, pore pressure.
7. Geomorphology and slope conditions: slope angle, drainage, overburden thickness, landslide features.
8. Geodynamic processes and hazards: landslide, erosion, flood, seismicity, active faults.
9. In-situ stress and temperature (for deep tunnels).
10. Construction materials availability.

Rock-forming minerals are the common minerals that make up most of the rocks of the earth's crust. About a dozen groups form over 90% of rocks: quartz, feldspars, micas, amphiboles, pyroxenes, olivine, calcite, dolomite, clay minerals and garnet.

Important minerals and engineering significance

Thus the minerals decide the strength, hardness, solubility, weathering behaviour and suitability of rocks as foundation and construction material.

An engineering geological map shows geological conditions (lithology, structure, soil, groundwater, hazards) classified according to their influence on engineering works.

Types

1. By purpose:
   • General-purpose maps show basic geology for many uses (regional planning).
   • Special-purpose maps are made for one project (dam, tunnel, road, landslide hazard).
2. By content:
   • Analytical (single-factor) maps show one aspect, e.g. rock type, groundwater, slope, landslide inventory.
   • Comprehensive (complex) maps combine all factors.
   • Zoning (synthesis) maps divide the area into zones of similar engineering suitability.
3. By scale: small (below 1:100,000, regional planning), medium (1:100,000-1:10,000, feasibility), large (above 1:10,000, detailed design).

Uses

• Site selection and comparison of alternatives for dams, tunnels, roads, bridges, buildings.
• Planning of site investigation (location of boreholes, trenches).
• Design of foundations, slope cutting and support.
• Hazard and land-use zoning (landslides, floods, earthquakes).
• Locating groundwater and construction materials.
• Estimating cost and construction methods.
• Environmental and urban planning.

Calcite ($CaCO_3$) is the main mineral of limestone and marble. It is soft (hardness 3) and soluble in water containing carbon dioxide.

• Water slowly dissolves calcite along joints and bedding to form solution cavities, caves and karst channels.
• At a reservoir, these openings cause heavy leakage and seepage from the reservoir, loss of stored water and possible failure of the dam foundation.
• Sinkholes and collapse of cavities may endanger the dam and abutments.
• Therefore, a reservoir should be avoided in or near calcite-rich limestone areas, or the site needs extensive grouting, cut-off walls and treatment of cavities, which increases cost.
• On the positive side, calcite gives limestone for cement and aggregate near the project.

Quartz ($SiO_2$) has hardness 7 and is very resistant to weathering, so it is the commonest mineral in river sediments.

When sediment analysis is done for a hydropower project, the quartz percentage of the suspended load is important because:

• Quartz is harder than steel (hardness about 5-6), so quartz particles abrade and erode turbine runners, guide vanes, nozzles and needles (sediment abrasion).
• High quartz content in the Himalayan rivers reduces turbine efficiency, increases maintenance cost and shortens the life of parts.
• It decides the need for desanding basins/settling basins to remove particles larger than 0.15-0.2 mm, and the selection of turbine type (Pelton or Francis), hard coatings (tungsten carbide) and operating speed.
• Quartz-rich rock also gives strong, durable aggregate for concrete, but it is abrasive on tunnel boring machine cutters.

Engineering geology applies geological knowledge to the investigation, design, construction and maintenance of civil engineering works. Its scope in civil engineering includes:

1. Site selection and investigation for dams, reservoirs, tunnels, bridges, roads, canals and buildings.
2. Foundation studies: bearing capacity, weak zones, faults, weathering and settlement.
3. Slope stability and landslide study for road cuts, dam abutments and reservoir rims.
4. Tunnelling and underground works: rock mass classification, support design, water inflow, squeezing and rock burst.
5. Groundwater studies: seepage, dewatering, water supply and drainage.
6. Construction materials: locating and testing aggregates, sand, clay, building stones.
7. Geological hazards: earthquake, flood, GLOF, erosion, subsidence, with mitigation.
8. Geological mapping and rock/soil testing for design parameters.
9. Environmental and cost aspects: avoids failures, reduces cost overruns and delay.

Weathering is the in-situ breakdown and decay of rocks at or near the earth's surface by the action of atmosphere, water and organisms, without transport of the material.

Types

• Physical (mechanical): frost wedging, temperature changes, salt crystal growth, unloading (exfoliation) and root action break the rock into pieces without changing its composition.
• Chemical: oxidation, hydrolysis, carbonation and solution change minerals (feldspar to clay, calcite dissolves).
• Biological: roots, burrowing animals, lichens and bacteria.

Factors

Climate (rain, temperature), rock type and mineral stability, joints, topography and time. Warm, humid climate gives deep chemical weathering.

Engineering significance

• Lowers strength, increases porosity and permeability of foundation rock.
• Weathered rock needs deeper excavation and flatter slopes.
• Causes landslides and rock falls on cut slopes.
• Weathering grades (fresh, slightly, moderately, highly, completely weathered) are used in rock classification.
• Weathered material forms residual soil and clay, which may swell.