Geotechnical Tests and Geophysical Surveys

Flashcards covering different geotechnical tests, geophysical surveys, and their associated methods, uses, advantages, and disadvantages.

What does an electromagnetic survey use to measure mean conductivity?

Non-contracting terrain conductivity meter

What materials show high conductivity in electromagnetic surveys?

Clay, basalt, and water

What materials show low conductivity in electromagnetic surveys?

Sand and limestone

What can electromagnetic surveys map?

Shallow lateral changes like clay-filled fissure zones, filled sinkholes, and rockhead steps

What can electromagnetic surveys detect?

Groundwater bearing zones, clay rich formations, and metallic objects

What are some advantages of electromagnetic surveys?

Low cost, simple to use, non-invasive, rapid preliminary assessments

What are some disadvantages of electromagnetic surveys?

Poor in conductive regions, complex interpretation, susceptible to noise

How does Ground Probing Radar work?

Records microwave and electromagnetic radar signals reflected from ground contrasts.

What is Ground Probing Radar used for?

Mapping shallow drift profiles, filled sinkholes and shallow voids

What materials is Ground Probing Radar effective in?

Ice, rock, soil, water, and pavement

What are some limitations of Ground Probing Radar?

Limited depth penetration, affected by electromagnetic noise, complex interpretation

How are electrical surveys conducted?

Arrays of 4 ground electrodes are used to map lateral and vertical changes in ground conditions

What are electrical surveys useful for identifying?

Water bearing zones, clay layers, and mineralized areas

What are some advantages of electrical surveys?

Non-invasive, relatively inexpensive, can cover large areas

What are some limitations of electrical surveys?

Limited resolution at depth, interference from power lines, non-unique solutions

What does a magnetic survey measure?

The total magnetic field using a proton magnetometer

What is magnetic survey useful for detecting?

Variations in magnetic properties of subsurface materials like ores or magnetite

What are some limitations of magnetic surveys?

Limited depth penetration, interference from manmade objects, ambiguous interpretations

What does a gravity survey measure?

Variations in the earth’s gravitational force

What creates negative anomalies in gravity surveys?

Underground voids or low-density soil or rock

What creates positive anomalies in gravity surveys?

Higher than average density material

What are gravity surveys useful for identifying?

Geological structures, rock types, and mineral deposits

What are some limitations of gravity surveys?

Complex interpretation, limited resolution for small scale features, time consuming

How do seismic surveys work?

Shock waves are produced by hammer blows, or explosions, and are reflected or refracted at geological boundaries

What are the two main types of seismic surveys?

Reflection seismic and Refraction seismic

What is reflection seismic used for?

Oil exploration and mapping geological features

What is refraction seismic used for?

Mapping bedrock topography and overburden thickness

What are some limitations of seismic surveys?

Susceptible to noise, environmental impact, complex processing

How does ground tomography work?

Transmitters and detectors in adjacent boreholes acquire data for intervening ground, creating a 2D or 3D model

What is ground tomography used for?

Identifying voids and anomalies in the ground

What are some advantages of ground tomography?

High resolution images of the subsurface

What are some limitations of ground tomography?

Requires the drilling of boreholes, hard to interpret, resolution deteriorates with depth

How does an unconfined compressive test work?

A cube or cylinder of rock is loaded uniaxially between flat steel platens

What does an unconfined compressive test measure?

Unconstrained rock strength

What are some limitations of unconfined compressive tests?

Does not account for in-situ stress conditions, high variability in results, not suitable for very weak or very strong rocks

How does a triaxial test work?

A cylinder of rock is loaded axially with equal confining stresses on radial axes due to fluid bath pressure

What does a triaxial test yield?

A range of strength parameters that can be used to find the friction angle of soils and the development of failure envelopes for rocks

What are some advantages of triaxial tests?

Simulates in-situ stress conditions, can be adapted for dynamic loading, enables simulation of short-term and long-term loading conditions

What are some limitations of triaxial tests?

More expensive and complex than other tests, time consuming, limited sample size

How does a Brazilian test work?

A cylinder of rock is loaded across its diameter between 2 flat steel platens

What does a Brazilian test measure?

Tensile strength of rocks

What are some limitations of Brazilian tests?

May not accurately represent the true tensile strength of a material, not reliable for anisotropic or heterogeneous samples, only testing strength down one axis

How does a ring shear test work?

Transverse shear on 2 surfaces across a cylinder of rock

What is a ring shear test good for studying?

The strength behavior of interfaces between different materials

What are some limitations of ring shear tests?

Restricted to soils and weak rocks, complex apparatus, may overestimate strength

How does a point load test work?

Cylinder of rock is loaded across its diameter between 2 60° steel points with a tip radius of 5mm

What are some advantages of point load tests?

Portable equipment which is easy to use, ideal for rapid, direct field testing of borehole cores, minimal sample preparation

What are some limitations of point load tests?

May not represent the true strength of a sample, may not provide reliable results for weak, ductile, or highly anisotropic rocks, results can be highly variable

How does a Schmidt hammer test work?

Handheld, spring-loaded hammer measures the rebound from a rock surface

What does the Schmidt hammer test measure?

The elastic properties of a rock

What are some limitations of the Schmidt hammer test?

Inaccurate compared to more expensive lab tests, information is limited to the surface of the rock and doesn’t take into account variability with depth or the weathering, not good for weak rocks, results are highly variable

How does a cone penetration test work?

In a borehole, a 60° cone of diameter 36mm is driven into soil at 15-25mm/second

What does a cone penetration test measure?

End resistance and sheath resistance

What are some limitations of cone penetration tests?

Not applicable for very dense or gravely soils, does not provide soil samples for visual inspection, complex equipment, depth limitations

What does a seismic cone penetration test determine?

The small strain shear modulus (Gmax)

What are some features of a seismic cone penetration test?

Comprehensive soil profiling

What are some limitations of seismic cone penetration tests?

Limitations in dense or gravely soils, no soil sampling for visual inspection, complex equipment

How does the standard penetration test work?

A 64km hammer is dropped 760mm and the number of hits required to drive the tube 30cm into the ground

What is the standard penetration test useful for?

Establishing relative densities of non-cohesive soils where it is difficult to obtain undisturbed samples

What are some limitations of the standard penetration test?

Can cause significant disturbance to the soil, limited accuracy in gravelly or clay soils, results are influenced by the skill of the operator

How does light percussion drilling work?

A steel shell is driven into the ground by a weight that is repeatedly dropped 1-2m

What is light percussion drilling used for?

Recovering soil from the ground for analysis

What are some limitations of light percussion drilling?

The percussion action can cause significant disturbance to soil samples, not effective in hard formations, cannot determine groundwater conditions

What are trial pits and trenches useful for?

Seeing more of the shallow subsurface details than in a bore hole

Where are trial pits and trenches especially useful?

In man made fills

What is a disadvantage of trial pits and trenches?

Very destructive

How does rotary coring work?

Truck mounted rig with rotary drive which is applied downwards to drill, extracting a continuous core of rock

What is rotary coring good for?

Extracting high-quality samples for detailed examination of rock

What are some limitations of rotary coring?

Expensive, time consuming, complex equipment, requires skilled operators

What does the penetration rate indicate in rock probing?

The strength of the rock, soil, or voids

What are flushed chippings from rock probing used for?

Examining the subsurface material

What are some limitations of rock probing?

No core recovery, time consuming, limited lateral coverage, complex equipment and skilled operators

What is the use of electromagneti c surveys?

Helps us to discern the subsurface lithology

What is the advantages of Electromagneti c surveys?

Various depths of imaging

What is the Disadvantages of Electromagneti c surveys?

The presence of highly resistive layers like dry sand or bedrock can limit the effectiveness of the survey

What is the uses of Ground probing radar?

Mapping soil and rock stratigraphy

What is the advantages of Ground probing radar?

Effective in many different geological settings

What is the Disadvantages of Ground probing radar?

Trade off between depth and resolution

What is the Use of Electrical surveys?

Detect changes in resistivity and conductivity

What is the Advantages of Electrical surveys?

Complementary to electromagnetic and seismic methods

What is the Disadvantages of Electrical surveys?

Rocky terrain or pavement may limit electrode placement

What is the Use of Magnetic surveys?

Can detect subtle changes in rock types

What is the Advantages of Magnetic surveys?

Airborne magnetic surveys allow for rapid data acquisition over extensive regions

What is the Disadvantages of Magnetic surveys?

Rely on diurnal variation correction

What is the Use of Gravity surveys?

Detects variations in density in the subsurface which is useful for identifying geological structures, rock types and mineral deposits

What is the Advantages of Gravity surveys?

Airborne gravity surveys allow for rapid data acquisition

What is the Disadvantages of Gravity surveys?

Sensitive to tidal effects

What is the Use of Seismic surveys?

Mapping geological features

What is the Advantages of Seismic surveys?

High resolution in detecting lateral variations

What is the Disadvantages of Seismic surveys?

Susceptible to noise