ENS6144 Module 3 Study Materials PDF

Subsurface investigation, Civil and mining engineering projects, Graphical technique for geological data presentation Dr Sanjay Kumar Shukla School of Engineering, Edith Cowan University, Australia 1 A view of a mining site...

Subsurface investigation, Civil and mining engineering projects, Graphical technique for geological data presentation Dr Sanjay Kumar Shukla School of Engineering, Edith Cowan University, Australia 1 A view of a mining site - Australia 2 A view of a mining site - India 3 A view of construction site 4 Site investigation Unlike other civil engineering materials, soils and rocks have significant variability associated with them. Site investigation refers to the appraisal of the surface and subsurface conditions at the proposed construction site. A typical site investigation includes preliminary studies such as desk study and site reconnaissance, geophysical surveys, drilling boreholes, in situ testing, sampling and laboratory testing of samples, and groundwater observations and measurements. The site investigation project can cost about 0.1 – 1.0% of the total construction cost of the project. 5 Stages of site investigation Collection of Available Information. Site Reconnaissance. Preliminary Site Investigation. Detailed Site Investigation. 6 Methods of subsurface investigation Test pits and trenches. Boreholes. Auger boring. Wash boring. Percussion drilling. Rotary drilling. 7 Rock core sampling: (a) single tube core barrel, and (b) double-tube core barrel After Das (2008) 8 Rock core sampling Drill core is the sample record for the subsurface geology at the borehole location, so it is preserved for some period of time varying from as short as 3 months to several years, say 10 years. The extent and type of preservation required depends on the geologic characteristics and the intended testing of the rock samples. This is best done in core boxes which are usually 1.5 m long and divided longitudinally by light battens to hold 4 to 6 rows of cores. The properties of soft rocks depend to some extent on their moisture content. Representative samples of such rocks should therefore be preserved by coating them completely with a thick layer of wax after removing the softened skin. Core photography in colour is performed on all cores to record permanently the unaltered appearance of the rock. 9 Standard size and designation of casing, core barrel, and compatible drill rod After Das (2008) 10 Core box with cores After Shukla and Sivakugan (2011) 11 Some core samples from the Australian Mines After Shukla and Sivakugan (2011) 12 Core recovery CR is a measure of how much rock has been lost during drilling. Core loss may result from weak zones being washed out by the drilling water, or grinding of the core during drilling, or the presence of an open cavity. It is a good practice to install a wooden spacer at the location of the lost core in the core box. A core recovery of 100% indicates the presence of intact rock; for fractured rocks, the core recovery will be smaller than 100%. 13 Rock mass classification [rock quality designation (RQD)] RQD is widely used as a single parameter for classification of rock mass. RQD is an index related to the degree of fracturing of the rock substance, which is obtained as drill core samples. RQD is calculated by measuring the total length of all pieces of core in a drill run with lengths greater than 100 mm (4 in.), discounting fractures due to drilling. These lengths are then added together, and the total length is expressed as a percentage of the length of the drill run. A low RQD value indicates that rock mass is closely fractured, while an RQD of 100% means that all pieces are longer than 100 mm. 14 RQD and joint/fracture spacing RQD Core quality Approximate equivalent (%) fracture/joint spacing (cm) 0 - 25 Very poor Very close (< 5) 25 - 50 Poor Close (5 – 30) 50 – 75 Fair Moderately wide (30 – 100) 75 - 90 Good Wide (100 – 300) 90 - 100 Excellent Very wide (> 300) Tutorial task: Calculate CR and RQD for the given core details, and described the quality of the rock mass. 16 Geophysical Methods Geophysical methods can be used to determine the distributions of physical properties, e.g. elastic moduli, electrical resistivity, density, magnetic susceptibility, etc. at depths below the ground surface that reflect the local subsurface characteristics of the materials (soil/rock/water). These methods may be used for the investigation during the reconnaissance phase of the site investigation programme since it provides a relatively rapid and cost-effective means of deriving areally distributed information about subsurface stratification. 17 Seismic methods Seismic methods require generation of shock or seismic waves, which are parcels of elastic strain energy that propagate outwards from a seismic source such as an earthquake, or an explosion or a mechanical impact. Seismic methods generally use only the P-waves, since this simplifies the investigation in two ways. Firstly, seismic/shock detectors which are insensitive to the horizontal motion of S-waves and hence record only the vertical ground motion can be used. Secondly, the higher velocity of P-waves ensures that they always reach a detector before any related S-waves, and hence are easier to recognize. 18 Seismic methods 19 Seismic methods: determination of depth to bedrcok 20 Tutorial task: Find the ratio of P-wave velocity to S- wave velocity. Does this ratio depend on more than one parameter? Can you plot the variation of the ratio with the related independent parameter(s)? 21 Electrical resistivity method This method is based on the determination of the subsurface distribution of electrical resistivity of earth materials from measurements on the ground surface. It may be useful in determining depth to bedrock and anomalies in the stratigraphic profile, in evaluating stratified formations where a denser stratum overlies a less dense medium, and in location of prospective sand-gravel or other sources of borrow material. 22 Electrical resistivity method: Ohm’s law and resistivity The range of resistivities among earth materials is enormous, extending from 10-5 to 1015 Ω-m. 23 Electrical resistivity method: Archie’s empirical formula where η is the porosity, S is the degree of saturation, ρw is the resistivity of water in the pores, and a, b and c are empirical constants. ρw can vary considerably according to the quantities and conductivities of dissolved materials. 24 Electrical resistivity method: generalized electrode configuration, and Wenner electrode configuration 25 Tutorial task: Calculate resistivity for the Wenner electrode configuration. 26 Orientation of a discontinuity plane/slope surface: (a) Isometric view of plane, (b) plan view of plane 27 Orientation of a plane in space: approaches Strike and dip measurement. Dip and dip direction measurement (ψ/α, e.g. 60/135 ): This approach facilitates graphical presentation 28 Orientation of a line Plunge: Dip of the line. Trend: The direction of the horizontal projection of the line measured clockwise from the north. It corresponds to the dip direction of the plane. 29 Spherical projection technique It allows the three-dimensional orientation data to be presented and analyzed in two dimensions. It removes one dimension from consideration so that lines or points can represent planes, and points can present lines. A planar feature such as joint sets, bedding planes, fault planes or slope faces is represented by the curved lines or points on the projection of a reference sphere. A line (e.g. borehole axis) is represented by a point on the projection of a reference plane. 30 Spherical projection: types Equal area projection. Equal angle or stereographic projection. Because of the ease of interpretation of data, commonly equal area projection is adopted. 31 Equal area projection of a plane: approaches Use of great circles. Use of poles: It facilitates the analysis of a large number of planes compared with the use of great circles. 32 Reference sphere and equal area projection of a plane An imaginary sphere which is free to move in space but is not free to rotate in any direction. Hence any radial line joining a point on the surface to the centre of the sphere will be a fixed direction in space. Move the sphere so that its centre lies on the plane/line under consideration. The intersection of the plane with the reference sphere is a circle called a great circle., which will define uniquely the orientation of the plane in space. Since the same information is given on both upper and lower parts of the sphere, only one of these should be considered. In engineering applications, the lower reference hemisphere is used for the presentation of data. 33 Reference sphere and equal area projection of a plane For a plane, the intersection with the lower reference hemisphere is a semi-circle (or circular arc) which defines uniquely the orientation of the plane. Finally, the circular arc is rotated down to a horizontal surface at the base of the sphere in such a way that any area on the surface of the reference sphere is projected as an equal area on the stereonet. The rotated circular arc is the unique location on the stereonet that represents the dip and dip direction of the discontinuity plane. An alternative means of representating the orientation of a plane is the pole to the plane. The pole is a point on the surface of the reference sphere that is pierced by a radial line in a direction normal to the plane. 34 Equal area projection of a plane A single point as the pole can represent the complete orientation of a plane. 35 Some important points Planes and lines with shallow dips have great circles and points, respectively that plot near the circumference of the stereonet, and those with steep dips plot near the centre. The pole a shallow dipping plane plots close to the centre of the circle, and the pole of a steep plane plots close to the perimeter/circumference of the stereonet. 36 Equal area stereonet Equatorial equal area stereonet or equal area equatorial net : Can be used to plot both great circles and poles. Polar equal area stereonet or equal area polar net: Can only be used to plot poles. 37 Advantages of equal-area projection Equal area projection helps in contouring of pole plots to find concentrations of poles that represent preferred orientations of sets of discontinuity. 38 Equal-area equatorial net for plotting poles and great circles 39 Equal-area polar net for plotting poles Dip direction is indicated on the periphery of the circle. Dip direction scale starts at the bottom of the circle and increases in a clockwise direction with the north arrow corresponding to dip direction of 180 degrees. Dip is measured along radial lines with zero degrees at the centre. 40 How to plot structural data The usual procedure is to place a tracing paper on the nets and then draw poles and planes on the tracing paper. The tracing paper is held over the stereonet with a pin to allow the paper to be rotated. Plots can be plotted directly without the need for rotating the tracing paper. Poles plotted on both the polar and the equatorial nets are in identical positions. Pole plots are commonly generated by stereographic programs. 41 Analysis of orientation Step 1: Plot poles representing the dip and dip direction of each discontinuity. Step 2: Plot great circles representing the average orientation of each set of discontinuities, and the dip and dip direction of the cut face. 42 The procedure for plotting poles Lay a sheet of tracing paper on the printed polar net. Mark the north direction and quadrant position around the edge of the outer circle. A mark is then made to show the pole that represents the orientation of each discontinuity as defined by its dip and dip direction. Poles of shallow dipping discontinuities lie close to the centre of the circle, and poles of steeply dipping discontinuities lie close to the periphery of the circle. Plot the pole of a plane oriented at 50/130. 43 An example of pole plot of 421 planes comprising bedding, joints and faults Equal area projection helps us in contouring of pole plots to find concentrations of poles that represent preferred orientations or sets of discontinuities. 44 Plotting great circles The primary purpose of plotting great circles of discontinuity sets in a slope is to determine the shape of blocks formed by intersecting discontinuities, and the direction in which they may slide. Only 5 to 6 great circles are plotted in one diagram because of difficulty in identifying all the intersection points of the circles. While computer generated great circles are convenient, hand plotting is of value in developing an understanding of spherical projections. 45 Plotting great circles Plot the great circle and pole of a plane oriented at 50/130. Take an equal area equatorial net. Lay the piece of tracing paper on the net with a pin through the centre point so that it can be rotated on the net. Trace the circumference of the net and mark the north direction of the net on the tracing paper. The dip direction scale starts at the north point at the top of the circle and increases in a clockwise direction. Locate the dip direction of the plane on the scale around the circumference of the net and mark this point on the tracing paper. Rotate the net about the centre pin until dip direction mark lies on W- E axis of the net. Locate the arc on the net corresponding to the dip of the plane and trace this arc on to the paper. Note that a horizontal plane has a great circle at the circumference of the net, and a vertical plane is represented by a straight line passing through the centre of the net. Rotate the tracing paper so that the two north points coincide and the great circle is oriented correctly. 46 Line of intersection The intersection of two planes is a straight line, which defines the direction of sliding of a wedge formed by these two planes. The straight line is characterized by trend (0 – 3600) and plunge (0 – 900). In spherical projection this line of intersection is defined as the point where the two great circles cross. The two intersection planes may form a wedge-shaped block. The direction in which the block may slide is determined by the trend of the line of intersection. 47 Line of intersection Determine the orientation (plunge and trend) of the line of intersection between two planes oriented at 50/130 and 30/250. Wyllie, D.C. and Mah, C.W. (2004). Rock Slope Engineering. 3rd edition, Taylor and Francis, London 48 Pole density and contour plot of poles By contouring the pole plot, the most highly concentrated areas of poles can be more readily identified. Contouring is done by: contouring package, or by hand using a counting net such as Kalsbeek net that consists of mutually overlapping hexagons, each with an area of 1/100 of the total area. Contouring is carried out by overlaying the counting net on the pole plot and counting the number of poles in each square. If there are 8 poles out of a total 421 poles on one square, then the concentration in that square is 2%. Different pole concentrations are shown by symbols for each 1% contour interval. The percentage concentration refers to the number of poles in each 1% area of the surface of the lower hemisphere. 49 Kalsbeek counting net 50 Identification of modes of slope instability/failure: (a) plane failure, (b) Wedge failure, (c) toppling failure, and (d) circular failure 51 Thank you!