3D Discontinuity Characterization of Chalk Sea Cliffs using Terrestrial Laser Scanning Systems

A. Stavrou, J. Lawrence, B. Awakian and W. Murphy

Chalk of the Northern Province - its regional context Hull University, 10-13 September 2015

To demonstrate the utilisation of Terrestrial Laser Scanning Systems for the 3D characterisation of discontinuities in chalk sea cliff environments.

To introduce the Laser Scanning or LiDAR technology as a potential replacement of conventional field techniques.

To prove that there is a strong correlation between results obtained by LiDAR and scanline surveys.

Aim of the Project

Study Area

Brighton Marina

Saltdean Peacehaven

Peacehaven Newhaven

Geological Setting

The Coastal cliffs are dominated by the Newhaven Chalk formation

Study area

Characterised by marl seams and repeated layering of flints

The bedding is sub-horizontal and gently dips to the South

Chalk is fragmented by normal and reverse faults and steeply inclined (60-700) conjugate shear joints

Structurally Controlled Cliff Instability

Discontinuity Characterisation

GEOMETRY

Orientation

Spacing

Persistence

Termination

SHEAR STRENGTH

Surface Roughness

Aperture

Filling

Wall Strength

Scanline Surveys

Traditional method to characterise discontinuities

Brady and Brown, 2004

Disadvantages: Human bias Sampling difficulties

Quality and quantity

of measurements

Accessibility Safety risks Time consuming

and costly

Terrestrial 3D Laser Scanning

Known as Light Detection and Ranging: LiDAR

A very popular and promising tool for collecting geo-data

Applicable to many geotechnical fields such as:

Rock Slope Engineering

Coastal Engineering

Structural Geology

Mining

Tunnelling

Advantages: Detailed 3D discontinuity

characterization Large amounts of very

high quality data

Almost no accessibility limitations

Reduced safety risks Rapid and cost effective

Fundamentals and Principles

1. Transmitter 2. Target

3. Receiver

3D LiDAR

Point Cloud:

(i.e. a collection of reflection points)

2m high wall

Fundamentals and Principles

Before Data Analysis

.. Noise removal

(e.g. vegetation, vehicles)

+ Triangular Mesh

generation

3D Discontinuity Characterization

Discontinuity = A group of neighboring mesh triangles with similar normal vector measurements

3D Discontinuity Identification & Characterization

Split Engineering http://www.spliteng.com

In flat, vertical cliff faces with no/low relief,

discontinuity surfaces are not readily available and

only discontinuity traces are visible

3D Discontinuity Identification & Characterization

Methodology

• Manually identify lines to the point cloud that depict the fracture’s exposed trace

• The software will modify the digitized line and calculate a best-fit plane that fits the points along the fracture trace

3D Discontinuity Identification & Characterization

Field-based manual discontinuity measurements (scanline surveys)

9 scan line surveys ranging in length from 30 to 50 meters

Computer-based processing and analysis of 3D laser scanning data (point clouds)

Project Structure

The project consisted of two distinct phases:

Example of a cliff face near Brighton Marina where a scanline survey was carried out

Example of a manually identified discontinuities using the Split-FX software

Vs

The survey was carried out using two mobile laser-scanning systems:

1. a van mounted laser scanning system and

2. a boat mounted laser scanner

Laser Scanner Data Collection

Data provided by the Brighton and Hove City Council

Results and Comparison

Results and Comparison

Kinematic Analysis

• The results show a very good fit between the two methods

• The comparison validates the good performance of laser scanners in flat, planar, low relief, chalk cliffs of limited accessibility

• Laser scanners are therefore considered to have the potential of replacing traditional discontinuity characterisation methods

• However, with the lack of any official standards and guidelines, laser scanners cannot be recommended as a stand-alone technique

Conclusion

Thank you

Any Questions?