Geological Investigations for Dams, Tunnels, Highways, Railways, and Bridges | Geology Optional Notes for UPSC PDF Download

Geological Investigations for Site Selection

Geological Investigation for Site Selection of Dams & Reservoirs

Dams are barriers constructed across a river valley to impound water. They serve various purposes such as controlling floods, irrigation, electricity generation, and urban water supply. Dams can be classified based on their use, hydraulic design, construction design, and materials used.

Dams

• Dams are mainly built for controlling floods, irrigation, electricity generation, and urban water supply.
• They can be constructed for specific purposes or be multipurpose, serving more than one use.

Reservoir

• A reservoir is an enlarged natural or artificial lake created by a dam to store water.
• Reservoirs can be formed by controlling a stream that drains an existing body of water.

Geological Investigation for Dam Sites & Reservoirs

• Topography plays a crucial role in the initial selection of dam sites.
• Geological investigations are essential for assessing the suitability of sites for dam construction.
• Author: Gaurav Srivastava, Civil Engineering Department, FOET, LU

Dam Site Selection and Foundation Conditions

• Dam Site Selection:
  • A dam site is typically ideal in a valley that is constricted with steep rock slopes.
  • For instance, a narrow stream flowing between high, rock walls suggests a concrete overflow dam.
  • Conversely, a low, rolling plain would recommend an earth-fill dam.
• Geology and Foundation Condition:
  • Foundation condition depends on geological factors such as the character and thickness of the strata.
  • Foundations are more stable on igneous and hard metamorphic rocks like granite, gneiss, and quartzite.
  • However, sedimentary rocks like shale, phyllite, slate, and schist may pose challenges.
  • The removal of disintegrated rocks and sealing of seams and fractures through grouting is often necessary.
  • Rocks like limestone tend to be cavernous, with solution channels that can lead to instability.
• Foundation Types:
  • Gravel foundations, if well compacted, are suitable for earth-fill, rock-fill, and low concrete gravity dams.
  • Silt or fine sand foundations can be used for low concrete gravity dams but not for rock-fill dams.
  • Poorly consolidated sediments like silt, sand, and gravel have low bearing strength, especially when moistened.
  • Clay foundations can support earth-fill dams but require special treatment to mitigate settlement risks.

Author: Gaurav Srivastava, Civil Engg. Deptt., FOET, LU

Dam Foundation Considerations

• Unconsolidated and high moisture content may lead to non-uniform foundations.
• Dam axis should ideally be perpendicular to geological structures' strike.
• Steep beds upstream are favorable for foundation stability.
• Dams on the limb of an anticline dipping upstream are favorable.
• Fault zones are unstable areas prone to leakage issues.
• Highly jointed rocks, despite hardness, can lead to strength reduction and leakage problems.
• Valley slopes along the dam axis should be free from landslides.
• Earthquake shocks can cause various failures, necessitating seismic considerations in design.

Test Questions

1. What are the challenges of building on unconsolidated, high-moisture content foundations?
2. Why is it important for the dam axis to be perpendicular to geological structures' strike?
3. Discuss the significance of steep beds upstream for dam foundations.
4. Explain why dam placement on the limb of an anticline dipping upstream is preferable.
5. How do fault zones pose risks to dam foundations, and what challenges do they present?
6. Why can highly jointed rocks, despite their hardness, be problematic for dam foundations?
7. What considerations should be made regarding valley slopes along the dam axis?
8. How do earthquake shocks impact dam structures, and what design considerations are necessary?

Source: Gaurav Srivastava, Civil Engg. Deptt., FOET, LU

Summary

• Recognition and delineation of active faults and analysis of historical records are crucial in seismological investigations.
• Availability of construction materials like soil, rock, concrete, and aggregates nearby a dam site is essential for economic construction.
• Permeability tests of foundation materials should be conducted during site selection to ensure water tightness of the dam.
• Buried channels or valleys filled with alluvial deposits may contain large boulders that need exploration techniques like drilling and seismic sounding.
• Construction of a dam impacts ecosystems, hydrological regimes, erosion, landslides, evaporation, seismic activity, flora, fauna, and displaced populations.
• Treatment of dam foundations involves improving geological conditions, controlling seepage, grouting joints in rocks, and applying soil treatment methods.

Test Questions

1. Why is it important to assess the availability of construction materials near a dam site?
2. What exploration techniques can be used to uncover hidden topography in buried channels?
3. How does the construction of a dam impact the ecosystem both upstream and downstream?
4. What methods can be employed to improve poor geological conditions in dam foundations?

Summary Notes on Dam Area, Reservoir Area, and Tunnel Construction

• The dam area and reservoir area are made impermeable to water using natural or synthetic materials like geo-textiles.
• Tunnel:
  • Grout curtains are utilized to seal off water to a certain level around the dam ends.
  • Tunnel is a nearly horizontal underground passage open at both ends.
  • There are different types of tunnels based on their use, such as traffic tunnels, hydropower headrace tunnels, and public utility tunnels.
  • Tunnels can be driven through rock or earth mass using methods like blasting in mining.
  • In soft ground, tunnels may be excavated using boring machines with walls supported by liner plates.
• Geological Investigations for Tunnels:
  • Geological investigations are crucial for selecting tunnel sites.
  • Classifications include:
    • Selection of Tunnel Route (Alignment): The final choice of alignment depends on geological factors, with the least negative geological factors being preferred.
    • Selection of Excavation Method: The method of excavation is an important decision in tunnel construction.

Test Questions

1. Why are grout curtains used in tunnel construction?
2. What are the different types of tunnels based on their use?
3. Why are geological investigations essential in the selection of tunnel sites?
4. Explain the importance of selecting the right alignment for a tunnel.
5. Discuss the significance of the excavation method in tunnel construction.

Key Objectives of Geological Investigations for Tunneling:

• Tunneling is a complex process with high costs involved.
• Proper planning is crucial to minimize costs and ensure successful excavation.
• Excavation methods are closely linked to the type of rocks present.
• Geological investigations aim to determine the nature of rocks along the tunnel alignment.

Selection of Design for the Tunnel:

• Geological composition along the tunnel alignment influences tunnel design.
• Rock strength dictates whether circular, D-shaped, horse-shoe shaped, or rectangular tunnels are suitable.
• In self-supporting strong rocks, D-shaped or horse-shoe tunnels may be appropriate.
• Circular tunnels are preferred in soft ground or weak rocks with unequal lateral pressure.

Assessment of Cost and Stability:

• Geological investigations impact excavation method, dimensions, and supporting system, influencing project cost estimates.
• Tunnels in hard and massive rocks are inherently stable.
• Tunnels in difficult grounds may collapse without proper geological understanding.

Assessment of Environmental Hazards:

• Tunneling processes can disturb the environment through vibrations.
• Environmental impacts should be considered for all tunneling projects.

Author: Gaurav Srivastava, Civil Engg. Deptt., FOET, LU

Summary Notes on Geological Information for Tunnelling Projects

Introduction

• Issues during tunnel construction include blasting, ground cutting, drilling, dust production, and interference with water supply systems.
• Understanding the geological setup helps minimize environmental hazards near populated areas.

Methods

• Preliminary Surveys:
  • Routine geological, geophysical, and geochemical methods are used.
  • Modern techniques like aerial photography and seismic surveying are common.
  • Established geological characteristics include topography, lithology, and hydrological conditions.

Key Geological Characters

• (a) General topography: highest/lowest points, valleys, depressions, slopes, slide areas, snow-line in hilly regions.
• (b) Lithology: composition, attitude, thickness of rock formations.
• (c) Hydrological conditions: water table depth, major/minor aquifers.

Author: Gaurav Srivastava, Civil Engg. Deptt., FOET, LU

Summary Notes on Tunnel Construction

Initial Surveys:

• Identification of the two places to be connected by the tunnel.
• Determination of the approximate length of the tunnel.
• Assessment of the geological conditions along potential routes.
• Consideration of the artesian type and hydrostatic heads along different alignments.
• Evaluation of the structural condition of the rock, including folding, faulting, jointing, etc.
• Exploration of reserves of rocks suitable for construction purposes.

Detailed Surveys:

• Initiation of bore-hole drilling along proposed alignments to obtain rock samples.
• Analysis of rock samples for mechanical and geochemical properties in laboratories.
• Drilling of exploratory shafts and adits for direct inspection and exploration.
• Construction of pilot tunnels for exploratory purposes, potentially serving as the main route.
• Decision on the number, depth, and length of bore holes, shafts, and adits based on tunnel specifications.

Information gathered from these surveys aids engineers in proposing alternative tunnel routes and planning for construction.

By Gaurav Srivastava, Civil Engg. Deptt., FOET, LU

Summary Notes on Geological Profile of Tunnels

• Information supplied by them has to be corroborated with that obtained by indirect methods such as seismic surveys.
• The shafts and adit borings are costly affairs but are very necessary. Often some of these could be merged with the main project subsequently as useful elements, such as for ventilation and allied purposes.
• Geological Profile of Tunnels:
  • When all the geological information gathered from preliminary and detailed surveys is plotted along a longitudinal section, the axis of the proposed tunnel being the section line, a geological profile is obtained. It is the most important geological record available with the project engineer and in fact his single most important guideline in the tunnel project.
  • Such a profile generally provides information regarding the following aspects of the proposed route:
    • Location and depth of exploratory bore holes and shafts etc.
    • Types of rocks and their geochemical characters such as whether consolidated or unconsolidated, fissured and decayed or fresh;
    • Structure of rocks, that is, whether stratified, or massive, horizontal or inclined, and if inclined, degree and direction of inclination; folding and faulting with full details.
    • Hydrological conditions along the profile line; whether the line is above or below the water table and its relation to any aquifer that is likely to be intercepted;
    • Ground temperature conditions, projected down to the tunnel axis based on calculations and observations.
• Gaurav Srivastava, Civil Engg. Deptt., FOET, LU

Geological Investigation for site selection of Bridge

• A bridge may be defined as a structure built over a river, a dry valley, low land, or an estuary to provide a link between two opposite sides. It serves as a communication link on a road, railway track, or highway.
• Bridges, especially those over major rivers and in hilly/mountainous areas, are crucial civil engineering structures with significant roles in socio-economic development and defense strategies.
• The location of a bridge is often influenced more by socio-economic factors than geological considerations. For instance, Srinagar city has seven bridges over the River Jhelum within a 5 km distance, while there is only one bridge over the River Chenab at Ramban, Jammu Province.
• In many cases, bridges within big cities divided by rivers are placed based on necessity rather than subsurface geology. However, on highways, there is some flexibility in choosing the bridge location.
• Author: Gaurav Srivastava, Civil Engg. Deptt., FOET, LU

Key Geological Considerations for Bridge Construction

• Depth to Bed Rock:
• 
  
  • In most cases, the river bed below the water is covered by varying thickness of unconsolidated natural deposits of sand, gravels, and boulders.
  • Such loose materials are not safe as foundations for bridge piers due to instability and scouring by river water.
  • The pier must be placed on a stable foundation, preferably on rock, under a suitable thickness of cover material to prevent scouring.
  • The height of the pier from under the span to the foundation level depends on the depth of the bedrock below the river water (usually within 5 to 20 meters).
  • The availability of sound bedrock depends on local geology, which must be thoroughly investigated.
• Nature of the Bed Rock:
• 
  
  • Understanding the structural disposition of rocks is crucial for bridge stability and durability.

Reference: Gaurav Srivastava, Civil Engg. Deptt., FOET, LU

Bridge Pier Foundation Notes

• Drill Holes: Holes are made along the centre line of the proposed bridge, reaching sound rock sequence or a reasonable depth. Care is needed to distinguish between boulders and bedrock.
• Nature of Bed Rock: The foundation must bear vertical, horizontal, and dynamic loads. The selected bedrock should be strong enough to support these loads throughout the bridge's life.
• Determining Bed Rock Nature: Petrological and engineering properties, including strength values from core samples, help in selecting suitable bedrock. Geological profiles aid in understanding the rock formations.
• Rock Types: Igneous, massive sedimentary, and metamorphic rocks are stable and durable foundations for bridge piers.

Test Question:

Why is it crucial to carefully select bedrock for a bridge pier foundation?

Summary Notes on Weak Rocks in Foundation Engineering

• Group of weak rocks: Rocks that may behave unfavorably in the presence of water. Examples include cavernous limestones, chalk, friable sandstones, rocks with clayey cements, shales, clays, slates, schists, and layers of peat and compressible organic material.
• Structural Disposition:
  • Ideally, horizontal attitude and uniformly massive structure with depth are preferred in foundation rocks for inherent resistance against failure.
  • Inclined rocks can be safe under bridge piers if they possess normal strength values.
  • Folding and faulting may introduce uncertainty but are not necessarily negative factors.
  • Acute fracturing and profuse jointing at foundation levels can lead to settlement beyond allowable limits.
  • Bridge sites in seismic zones require foundations designed for additional seismic loads as per area-specific codes.
  • In glaciated areas, special attention is needed to identify drowned or buried valleys filled with heterogeneous material. In such cases, reaching bedrock through piles may be necessary.
  • Drowned valleys pose significant complications in bridge foundations, limiting design options.
  • Scouring is a critical factor to consider. Riverbed materials and rocks at shallow depths are susceptible to removal by scour, influenced by river velocity, current direction, and rock consolidation.
• Author: Gaurav Srivastava, Civil Engg. Deptt., FOET, LU

Geological Investigation for Site Selection of Roads in Hilly Regions

• Meandering:
  • Construction of roads in hilly regions is complicated due to topographic and permissible factors.
  • The principle of connecting two points by the shortest route is challenging in hilly terrains.
  • Meandering, zig-zag courses are often necessary in hilly regions.
• Aerial Survey:
  • Surveying a hilly area within a specified time is a challenge.
  • Quicker surveying methods, such as aerial surveying, may be required for timely project completion.
  • Aerial surveying can help in successful project completion within the specified time frame.

Author: Gaurav Srivastava, Civil Engg. Deptt., FOET, LU

Rock Consideration:

• If some solid and stratified rocks are encountered along the alignment, special investigations should be carried out to determine:
• Dip and strike of the bed;
• Lithological composition of the rocks;
• Presence and nature of faulting, jointing and permeability due to these secondary planes of weakness.

Geological Structures:

The structural features of rocks, especially in those of sedimentary and metamorphic origin, have a significant bearing upon the design of cuts as well as on the stability of the road as a whole. A given rock might be quite hard and otherwise sound for a cut as road foundation. However, if some planes of weakness (such as bedding planes, joints, foliation, cleavage) are present in such a way that these are inclined towards the free side of the valley, the rock could likely fail along these planes. Such structural features include:

• Dip and strike;
• Joints;
• Fault planes.

Dip and Strike:

There may be three possibilities for making a cut in the inclined beds - it can be made parallel, at right angles or inclined to the dip direction. The relative merits of the cut vis-a-vis its stability would be as follows, assuming other things are favourable:

• Cut is Parallel to the Dip Direction:
• In such a case, the layers offer a uniform behaviour on either side of the cut and as such the risk of failure is minimal on this account.
• Cut is made Parallel to the Strike:
• Gaurav Srivastava, Civil Engg. Deptt., FOET, LU

Summary Notes on Road Construction

Stability Factors in Road Cuts

• Cuts Parallel to Strike:
  • Cut made parallel to strike, perpendicular to dip direction.
  • Strata plunge across, offering different inclinations on each side.
  • Slips likely on steeply inclined, rainwater-lubricated planes.
  • Stability higher when layers dip into hill rather than road.
• Cutting Inclined to Dip and Strike:
  • Strata dip across cut, unequal slope on each side.
  • Similar difficulties as cuts parallel to strike.
  • Special measures sometimes needed for stability.
• Joints:
  • Influence stability similar to bedding planes.
  • Abundant joints weaken rock stability.
  • Continuous, inclined joints can lead to slips in moisture.
  • Jointed rocks may need breastwalls for support.
• Faults:
  • Faulting crushes rock along fault planes and shear zones.
  • Unfavorable for upper, lower, or base slopes of a cut.
  • Potential failure planes.

Groundwater Conditions for Road Construction

Gaurav Srivastava, Civil Engg. Deptt., FOET, LU

Geological Investigation for Infrastructure Projects

Importance of Water Table and Aquifers

• Thorough investigation of the water table is crucial.
• Understanding water-bearing qualities along the route is essential.
• Aquifers intersecting the alignment are potential weak zones.
• Special care and design considerations are needed for water conduits.

Water Influence on Bearing Capacity

• Water significantly impacts the bearing capacity of rocks and soil.
• Moist ground may not bear design loads without proper assessment.
• Free flow of groundwater through soil can jeopardize road stability.

Geological Investigation for Dams and Reservoirs

• Dams:
• 
  
  • Dams are barriers built across river valleys for water impoundment.
  • They serve purposes like flood control, irrigation, and electricity generation.
  • Dams can be for specific or multiple uses.
  • Classification is based on use, hydraulic and construction design, and materials.
• Reservoirs:
• 
  
  • Reservoirs are enlarged natural or artificial lakes created by damming.
  • They store water and can be made by controlling draining streams.
• Geological Investigation for Dam Sites:
• 
  
  • Topography plays a crucial role in site selection.
  • Geological investigations are vital for infrastructural projects.

Author: Gaurav Srivastava, Civil Engg. Deptt., FOET, LU

Dam Construction: Key Considerations

1. Dam Site Selection

• Choose a site where a valley is constricted with steep rock slopes.
• For narrow streams between high rock walls, consider a concrete overflow dam.
• For low, rolling plains, an earth-fill dam might be more suitable.

2. Geology and Foundation Conditions

• Foundation quality depends on geological factors like strata thickness, inclination, permeability, and relation to underlying strata.
• Igneous and hard metamorphic rocks like granite are preferable to sedimentary rocks like shale for foundations.
• Grouting may be necessary to seal seams and fractures in rocks.
• Limestone rocks are often cavernous and prone to water percolation issues.
• Weathered rocks can lead to instability and the formation of clays and gritty soils.
• Gravel foundations, if well compacted, are suitable for various dam types.
• Silt or fine sand foundations can support low concrete gravity dams but not rock-fill dams.
• Poorly consolidated sediments weaken when moistened.
• Clay foundations can support earth-fill dams but may require special treatment to prevent settlement.

Test Question:

What are some key geological factors to consider when selecting a dam site?

Explain the importance of foundation quality in dam construction.

Dam Foundation Considerations

When constructing a dam, several geological and structural factors must be considered to ensure a stable and secure foundation. Below are key points to remember:

• Uniform Foundations:
  • Uniform foundations are ideal for dam construction.
  • Non-uniform foundations of rock and soft material should be avoided.
• Geological Structures:
  • Dam axis should be perpendicular to geological structures' strike.
  • Steep beds upstream are favorable for foundation.
  • Dams on the limb of an anticline dipping upstream are favorable.
  • Dams on the limb of a syncline with a downstream plunge are unsuitable.
  • Fault zones are unstable areas prone to water leakage.
  • Highly jointed rocks may be problematic for dam foundations.
  • Valley slopes along the dam axis should be free from landslides.
• Earthquake Considerations:
  • Earthquake shocks can lead to various failures in dams.
  • Consult seismic zonation maps during site investigation.
  • In seismically active areas, design must consider additional loading and stresses.

About the Author

Gaurav Srivastava, Civil Engineering Department, FOET, LU

Summary Notes on Dam Construction

• Recognition and delineation of active faults and analysis of historical records of past occurrences are important seismological investigations.
• Construction of a dam necessitates a significant quantity of construction materials such as soil, rock, concrete, and aggregates.
• The most economical dam type is usually the one with materials found in sufficient quantity at a reasonable distance from the site.
• Availability of construction materials near the proposed site needs to be assessed.
• Permeability test of the foundation material is crucial during site selection to ensure water tightness within tolerance limits.
• Buried channels or valleys abandoned by rivers in the past and filled with alluvial deposits may contain large boulders, requiring careful exploration including drilling and seismic sounding.
• Dam construction alters the ecosystem and hydrological regimes both upstream and downstream, necessitating investigation and mitigation of potential impacts.
• Concerns such as erosion, landslides in the reservoir basin, evaporation losses, reservoir-induced seismicity, destruction of flora and fauna, and resettlement of displaced individuals should be addressed.
• Treatment of Dam Foundation:
  • Poor geological conditions can be improved by enhancing load-bearing properties and controlling seepage.
  • Grouting is typically performed on highly jointed rock foundations to seal joints and fractures, enhancing strength and preventing seepage.
  • Soil treatment methods are applied on soil foundations to increase their strength for dam construction.

Author: Gaurav Srivastava, Civil Engg. Deptt., FOET, LU

Summary: Geological Investigations for Dams and Tunnels

Dams:

• The dam area and reservoir area are made impermeable to water using natural or synthetic materials like geo-textiles.
• Grout curtains are employed around the dam to seal off water to a certain level.

Tunnels:

• A tunnel is a nearly horizontal underground passage open at both ends.
• Tunnels serve various purposes such as traffic, hydropower, and public utilities.
• Tunnels can be excavated through rock or earth by methods like blasting or boring machines.

Geological Investigations for Tunnel Site Selection:

• Geological investigations are crucial for selecting tunnel sites.
• Factors for consideration include the tunnel route alignment and excavation method.
• Choosing the alignment with the least geological challenges is essential.

Classifications of Geological Investigations:

1. Selection of Tunnel Route (Alignment)
2. Selection of Excavation Method

By: Gaurav Srivastava, Civil Engg. Deptt., FOET, LU

Tunnelling Process

Tunnelling is a complex process that requires careful planning to avoid high costs. The choice of excavation method is closely tied to the type of rocks being excavated. Geological investigations play a crucial role in determining the design and dimensions of the tunnel as well as assessing costs and stability.

Key Objectives:

• Understanding Geological Features
• Selection of Design for the Tunnel
• Assessment of Cost and Stability
• Assessment of Environmental Hazards

Selection of Design for the Tunnel

• The design of a tunnel is influenced by the geological composition of the area. The choice between circular, D-shaped, horse-shoe shaped, or rectangular tunnels depends on the ground conditions. For self-supporting rocks, D-shaped or horse-shoe shapes may be suitable, while circular outlines are preferred for soft ground or weak rocks.

Assessment of Cost and Stability

• Cost and stability considerations are closely linked to geological factors. Geological investigations determine excavation methods, dimensions, and support systems, impacting cost estimates. Tunnels in hard rocks are more stable, while those in challenging ground may require additional support to prevent collapse.

Assessment of Environmental Hazards

• Tunnelling processes can disrupt the environment through vibrations and other means. Environmental hazards must be carefully assessed when planning tunnelling projects to minimize negative impacts on the surrounding area.

Summary Notes on Tunnel Engineering

Environmental Hazards in Tunnel Construction

• Blasting or ground cutting and drilling can produce abnormal quantities of dust.
• Interference with the water supply system of nearby areas.
• A correct appreciation of the geological setup is crucial, especially near populated zones, to minimize environmental hazards effectively.

Methods for Geological Information in Tunnel Projects

The geological information required for tunnelling projects differs from that needed for other civil engineering projects. It is obtained in two stages:

A. Preliminary Surveys

• Routine geological, geophysical, and geochemical methods are used in preliminary surveys.
• Modern practices include aerial photography and seismic surveying in combination with routine surface methods.
• Established geological characters in preliminary surveys include:
  • The general topography of the area.
  • The lithology of the area.
  • The hydrological conditions in the area.

Gaurav Srivastava, Civil Engg. Deptt., FOET, LU

Summary of Tunnel Construction Surveys

Initial Surveys:

• Identification of the two points to be connected by the tunnel.
• Assessment of the nature of rocks along potential tunnel routes.
• Evaluation of potential hydrostatic heads along different routes.
• Analysis of structural conditions of the rock, including folding, faulting, jointing, etc.
• Determination of reserves of rocks useful for construction in the project.
• Proposal of alternative tunnel routes based on gathered information.

Detailed Surveys:

• Initiation of construction planning after deciding the tunnel's general path.
• Bore-hole drilling along proposed alignments to desired depths for rock samples.
• Analysis of rock samples for mechanical and geochemical properties.
• Drilling exploratory shafts and adits for visual inspection.
• Driving pilot tunnels for exploratory purposes, potentially becoming main routes.
• Determination of the number, depth, and length of bore holes, shafts, and adits based on tunnel specifications.

Test Questions:

1. What is the purpose of initial surveys in tunnel construction?
2. How are rock properties assessed during detailed surveys?
3. Explain the significance of driving pilot tunnels in tunnel construction.

Geological Profile of Tunnels

• Information supplied by them has to be corroborated with that obtained by indirect methods such as seismic surveys. The shafts and adit borings are costly affairs but are very necessary. Often some of these could be merged with the main project subsequently as useful elements, such as for ventilation and allied purposes.
• When all the geological information gathered from preliminary and detailed surveys is plotted along a longitudinal section, the axis of the proposed tunnel being the section line, a geological profile is obtained. It is the most important geological record available with the project engineer and in fact his single most important guideline in the tunnel project.
• Such a profile generally provides information regarding the following aspects of the proposed route:
  • Location and depth of exploratory bore holes and shafts etc.
  • Types of rocks and their geochemical characters such as whether consolidated or unconsolidated, fissured and decayed or fresh;
  • Structure of rocks, that is, whether stratified, or massive, horizontal or inclined, and if inclined, degree and direction of inclination; folding and faulting with full details.
  • Hydrological conditions along the profile line; whether the line is above or below the water table and its relation to any aquifer that is likely to be intercepted;
  • Ground temperature conditions, projected down to the tunnel axis based on calculations and observations.

Gaurav Srivastava, Civil Engg. Deptt., FOET, LU

Geological Investigation for Site Selection of Bridge

• A bridge may be defined as a structure built over a river, a dry valley, low land, or an estuary to provide a link between two opposite sides. It serves as a communication link on a road, railway track, or highway.
• Bridges, especially those over major rivers and in hilly areas, are crucial civil engineering structures with significant roles in socio-economic development and defense strategies.
• The location of a bridge is often influenced more by socio-economic factors than geological considerations. For example, Srinagar city has seven bridges over the River Jhelum but only one bridge over the River Chenab.
• In many cases, the placement of a bridge within a city is based on necessity rather than subsurface geology. However, on highways, there can be some flexibility in choosing the bridge location.
• Gaurav Srivastava, Civil Engg. Deptt., FOET, LU

Key Geological Considerations for Bridge Construction:

• Depth to Bed Rock:
  • In most cases, the river bed below the water is covered by varying thickness of unconsolidated natural deposits of sand, gravels, and boulders.
  • Such loose materials are not safe as foundations for bridge piers due to instability and scouring by river water.
  • The pier must be placed on stable rock foundation under a suitable thickness of cover material to prevent scouring.
  • The height of the pier from under the span to the foundation level depends on the depth of the bed rock below the river water.
  • Sound bed rocks may be found within a depth varying from 5 to 20 meters below the river bed based on local geology.
• Nature of Bed Rock
• Structural Disposition of Rocks

Geological investigation and interpretation are crucial for the design, stability, and durability of bridges.

The designer aims to place bridge abutments and piers on a strong and stable rock foundation.

Proper investigation of geological conditions is essential for the success of major bridge construction projects.

By Gaurav Srivastava, Civil Engg. Deptt., FOET, LU

Summary Notes: Bridge Pier Foundation

• Drilling Holes: Holes are made along the centre line of the proposed bridge to reach sound rock sequence or a reasonable depth. Care is essential to distinguish boulders from bedrock, as boulders cannot be relied upon as foundations for bridge piers.
• Nature of Bed Rock: The initial rock below the bed cover may not be suitable as a foundation. The bedrock chosen must withstand compressive vertical loads, horizontal loads from water thrust, and dynamic loads from traffic throughout the bridge's lifespan.
• Determining Bed Rock: The bed rock's suitability is determined by studying petrological and engineering properties, including strength values from core samples obtained during test bore holes drilling.
• Geological Profiles: Detailed profiles along the bridge's centre line can reveal rock formations, aiding in deciding the pier's placement based on rock strength, stability, and durability.

Test Question:

Why is it crucial to accurately identify the nature of the bedrock for constructing bridge piers? Provide three reasons with explanations.

Summary of Weak Rocks in Foundation Engineering

• Definition: Group of weak rocks that may exhibit poor behavior in the presence of water.
• Types:
  • Cavernous limestones
  • Chalk
  • Friable sandstones, especially with clayey cements
  • Shales
  • Clays
  • Slates
  • Schists
  • Layers of peat and compressible organic material
• Treatment: Many weak rocks can be treated using artificial methods to improve their properties.
• Structural Disposition:
  • Horizontal attitude and uniformly massive structure are desirable for foundation rocks, providing inherent resistance against failure.
  • Inclined rocks can be safe if they possess normal strength values.
  • Folding and faulting can introduce uncertainty but are not necessarily negative factors.
  • Acute fracturing and profuse jointing at foundation levels can lead to settlement beyond allowable limits.
  • Foundations in seismic zones require designs to withstand additional seismic loads as per local codes.
  • In glaciated areas, drowned or buried valleys may complicate bridge foundation design, requiring special attention.
  • Scouring must be considered, especially in riverbeds, where materials can be removed due to river velocity and rock consolidation.

Test Questions

1. What are some types of weak rocks commonly encountered in foundation engineering?
2. Why is the horizontal attitude and uniformly massive structure desirable in foundation rocks?
3. How can acute fracturing and profuse jointing impact foundation stability?
4. Explain the importance of considering seismic activity in foundation design.
5. What complications may arise in bridge foundations in glaciated areas?

Geological Investigation for Site Selection of Roads in Hilly Regions

• Meandering:
  • Construction of roads in hilly regions is complex due to topographic and permissible factors.
  • The shortest route principle for road alignment is challenging in hilly terrains, often requiring a meandering, zig-zag course.
• Aerial Survey:
  • Surveying large hilly regions within a specified time frame can be challenging.
  • Aerial surveying may be necessary to expedite the surveying process and ensure project completion within the set timeline.

Author: Gaurav Srivastava, Civil Engg. Deptt., FOET, LU

Rock Consideration:

• If some solid and stratified rocks are encountered along the alignment, special investigations should be carried out to determine:
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  • Dip and strike of the bed;
  • Lithological composition of the rocks;
  • Presence and nature of faulting, jointing and permeability due to these secondary planes of weakness.

Geological Structures:

• The structural features of rocks, especially in those of sedimentary and metamorphic origin, have a significant impact on:
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  • The design of cuts;
  • The stability of the road.
• A given rock might be quite hard and otherwise sound for a cut as road foundation. However, if planes of weakness (such as bedding planes, joints, foliation, cleavage) are present and inclined towards the free side of the valley, the rock could likely fail along these planes.
• Structural features include:
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  • Dip and strike;
  • Joints;
  • Fault planes.

Dip and Strike:

• Three possibilities for making a cut in the inclined beds:
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  • Parallel;
  • At right angles;
  • Inclined to the dip direction.
• The relative merits of the cut vis-a-vis its stability:
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  • Cut is Parallel to the Dip Direction:
  • In such a case, the layers offer a uniform behavior on either side of the cut, minimizing the risk of failure.
  • Cut is made Parallel to the Strike:
  • It is important for stability considerations.

Gaurav Srivastava, Civil Engg. Deptt., FOET, LU

Summary Notes

• Cutting Techniques:
  • Cut made parallel to the strike results in different inclinations of layers on each side due to strata plunge.
  • Likelihood of slips on the dipping inside of the cut, especially with steeply inclined planes and lubrication from water.
  • Cuts inclined to dip and strike lead to unequal slopes on both sides, causing similar difficulties as parallel cuts.
  • Special measures may be needed for stability in cuts not aligned at right angles to strike.
• Joints:
  • Joints impact cut stability similarly to bedding planes, reducing even hard rock to loosely held blocks that could fall with slight vibrations.
  • Continuous inclined joints on the free side can lead to slips in moist conditions, requiring artificial support like breastwalls in road construction.
• Faults:
  • Faulting crushes rock along fault planes and shear zones, creating unfavorable conditions for cuts on upper, lower, or base slopes.
  • Faults represent planes of potential failure in cuts.
• Groundwater Conditions for Roads:
  • Authored by Gaurav Srivastava from the Civil Engineering Department at FOET, LU.

Summary Notes on Water Table and Road Stability

Key Points:

• Thorough investigation of the water table position is essential.
• Knowledge of water-bearing qualities along the proposed route is crucial.
• Water-bearing zones (aquifers) intersecting the alignment are potential weak and hazardous zones in a road.
• Water significantly influences the bearing capacity of rocks and soil.
• Moisture-rich ground may not bear design loads without proper evaluation under moist conditions.
• Free flow of groundwater through soil can be hazardous for road surface stability.