Ground models [Lecture 5]

Tectonic processes consequences

• rock folding

• rock fracture

• uplift

• subsidence

Joints

rock fracture with no movement

Formation of joints

• tectonic stressing

• systems = group of parallel joints

Faults

rock fractures with displacement → horizontal offset followed by erosion

Throw

vertical component of displacement

Types of faults

• Normal

• Reverse

• Strike-slip

Dip

Faut angle from horizontal

Slip

Displacement

Issues with faults

• Shear strength along fault reduced

• Material in fault zone can be more compressible than in intact rock 

• Can cause stability problems

• Can find fault breccia = coarse angular broken rock debris in zone (0.1–100 m wide) along a fault (commonly zone of enhanced groundwater flow)

• Loose material in fault zone → cause caving in tunnels or reduce stability on surface 

• Fractured rock is conductive to water → contaminated water can get an easy pathway along faults

Crossrail (tunnelling) hazards identified

1. ground water

2. mixed face conditions

3. hard strata

4. aggressive ground and groundwater affecting durability

5. obstructions (deep piles and foundations)

6. abrasive ground (flint)

7. unexploded ordinance

Crossrail (tunnelling) hazards experienced

1. ground water

2. mixed face conditions → instability of ground

3. obstructions → borehole casigs

Common ground profile in London

Bedding planes

thin bands of shale / clay between units of stronger rock

boundaries between individual beds of sediment

Folds

strata bent or buckled into folds

form due to forces tangential or radial to earth’s surface

anticlines & synclines

Anticline & syncline diagram

Syncline trough

Anticline crest

Strike & dip/rake of an anticline

Unconformity

plane or break between two sequences of rocks with different dips

Disconformity

unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition

Nonconformity

unconformity between parallel layers of sedimentary rock and metamorphic / igneous rock which represents a period of erosion or non-deposition ➞ (same as nonconformity but with metamorphic or igneous rocks)

Angular unconformity

horizontal parallel strata of sedimentary rock are deposited on tilted and eroded layers of sedimentary rock

• period of earth movement between times of deposition

• major structural break → older rocks more solid

Paraconformity

horizontal parallel strata of sedimentary rocks without erosion

Strike

direction of a horizontal line on a dipping surface (angle relative to north)

Dip

angle in degrees below the horizontal relative to strike

Apparent dip

dip not measured in vertical plane perpendicular to strike line

Importance of geological structures

• influence the original location of deposits and modifies shape of previously formed deposits

• Faults and joints may contain mineral deposits = where faults were used to conduct magma

• Joints generated by folding and faulting divide rock into blocks ➞ support may be required to prevent excavation collapse

• Joints and faults provide pathways for water movement

• Gaining an understanding of structure helps predict geology and enables engineering solutions to be found 

• Distribution of rock types and their properties are influenced by structure 

• Structures can link to geological hazards

Need for ground investigation

assesses ground conditions prior to starting a construction project

Include:

• Suitability of the site for the proposed project

• Site conditions and ground properties

• Potential geotechnical/geological issues

• Ground characterisation

Stages of ground investigation

Initial stage

• Desk study of available data 

• Walkover survey and visual assessment 

• Preliminary report and scope of works 

Main stage

• Field work (geological mapping, geophysics, test pits, boreholes/coring)

• Laboratory testing (soils and rocks) 

• Final report 

• Monitoring

Trial pits

• cheapest method of shallow exploration 

• dug w/ site excavator w/ backhoe 

• 2-5 m deep 

• useful where there are variable man-made fills

• can be used to look at material that has slipped to identify shear surfaces

Boreholes

• extract a length of soil and/or rock from the earth

• overall soil/rock can be broken during the process of extraction or kept intact

• samples are examined either on site or back in the laboratory

• further characterisation tests can also be performed

• Light percussion drilling (for soil) repeatedly drop a steel shell

➞ Possible to add an auger drill (but this will disturb the soil) 

• Rotary coring = rotary drive applied with downward force

➞ air / water / bentonite pumped down 

➞ drill can be diamond tipped

3 distinctive characteristics in description of rocks

MATERIAL CHARACTERISTICS 

• those free from discontinuities

• Strength, bedding/layering, colour etc and name

DISCONTINUITY CHARACTERISTICS

• those of bedding, jointing & shear

• Orientation, spacing, roughness, strength etc 

MASS CHARACTERISTICS 

• rock material + rock discontinuities = Rock mass 

• overall structure (particularly discontinuities)

• Fracture state (TCR, SCR, RQD, FI)

Types of fractures in cores

• joints

• mechanical breaks

• rubble zones

Range of defects

heavy (weak) - moderate (strong) - minor (strong)

RQD

Rock quality designation

RQD > 70 = sound rock

RQD = sum of core lengths > 10 cm *100 / borehole length

Standard penetration test SPT

Drop hammer repeatedly

SPT resistance = total number of blows to drive sampler last 300mm

Cone penetration test

• In-situ test: wide use on and offshore 

• Cone pushed into ground at a constant rate 

• Electronic transducers record force at tip & drag force on short sleeve → tip force / A = tip resistance → sleeve force / S = sleeve friction

• Can be related to a wide range of geotechnical parameters through empirical relationships

• porous element allows water flow & water pressure measurement

Geophysics

• Non intrusive

• Infer ground properties from seismic wave speed or electrical resistance 

• Electromagnetic surveys 

• Ground penetrating radar 

• Electrical surveys (resistivity / conductivity) 

• Magnetic surveys 

• Seismic surveys

Groundwater test

Packer test → measures local permeability of rock and aquifer properties between two packer seals in a borehole

New tools

Fibre optics

• Distributed Temperature Sensing (DTS)

• Distributed Temperature and Strain Sensing (DTSS) 

• Distributed Acoustic Sensing (DAS) 

= various types of fibre optic sensing technologies which use the physical properties of light as it travels along a fibre to detect changes in temperature, strain, vibration (acoustics) and other parameters

InSAR

• allows measurement of ground deformation by detecting the phase differences between at least two SAR images 

• Synthetic aperture radar (SAR) = active data collection where an instrument sends out a pulse of energy and then records the amount of that energy reflected back after it interacts with Earth

Example:

• Impact measured by InSAR of the dewatering necessary for Crossrail’s construction in East London around the Limmo peninsula

Robots

• light detection and ranging 

• can generate 3D images

Drones / UAVS (Unmanned aerial vehicle)

Can equip UAVs with 

• Digital cameras 

• Thermal detectors

• Multispectral cameras

• LiDAR 

ODS

Ordinance Survey Digimap collection

• borehole logs

• geological maps (deposits and elevation contours)