Question 4 of 4

Question 4 (25 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Determining rock types and formations in the field

Field identification of rock type and formation extent draws on several complementary techniques, none sufficient alone. Outcrop mapping and geological hammer/hand-lens examination is the starting point — mineral composition, texture, grain size, colour, and weathering are logged directly at exposures, and a dilute-acid test distinguishes carbonate rocks; a streak plate and Mohs-scale scratch test support mineral identification where hand-specimen inspection alone is ambiguous. Structural mapping (strike/dip of bedding, foliation, and discontinuities, read with a compass-clinometer or digital equivalent) extends a point observation into a three-dimensional formation geometry and lets the geologist project a contact or unit boundary between outcrops. Borehole drilling and core logging extends this mapping below the exposed surface, where core is logged for lithology, weathering grade, and RQD exactly as covered in Q2(c)/(d); down-hole geophysical logging (acoustic/optical televiewer, sonic, gamma) supplements the core log with continuous, oriented data even through poorly recovered intervals. Geophysical surveying (seismic refraction, resistivity, or ground-penetrating radar run across the site) extends point borehole/outcrop data spatially, locating a bedrock surface, a formation contact, or a buried feature between exploration points at far lower cost than closely spaced drilling. Remote sensing (air photo interpretation, satellite/LiDAR imagery, and reference to published regional geological maps from the desk study) frames the local observations within the known regional stratigraphy and structural setting before any field work begins, letting the Engineer anticipate what rock types should be expected and target field verification efficiently.

(b) Determining stress orientation in rocks and its importance

In-situ rock stress orientation and magnitude are determined by several established methods. Overcoring (e.g. the CSIRO hollow inclusion or USBM borehole deformation gauge) installs a strain-measuring device in a pilot borehole, then over-cores it with a larger-diameter bit to relieve the surrounding rock stress; the measured strain relief, combined with the rock's elastic properties, back-calculates the full 3-D stress tensor, including its principal orientations. Hydraulic fracturing pressurizes an isolated, sealed borehole interval with fluid until the rock fractures; the fracture initiates and propagates perpendicular to the minimum principal stress direction, so the induced fracture's orientation (read from an impression packer or borehole televiewer run afterward) directly gives the minimum principal stress orientation, while the breakdown/shut-in pressures give its magnitude. Borehole breakout analysis reads stress-induced spalling zones on opposite sides of an already-drilled borehole wall (imaged with a televiewer or caliper log) — breakouts form perpendicular to the maximum horizontal principal stress direction, so their orientation gives that direction directly from a borehole drilled for another purpose, at no extra field cost. Regional/tectonic stress indicators (focal mechanisms of nearby earthquakes, mapped fault-slip and fold-axis orientations) provide an independent, lower-resolution cross-check on the local point measurements. This matters from a site investigation perspective because in-situ stress orientation and magnitude directly govern the stability and required support of any underground opening (a tunnel or shaft driven parallel to the maximum principal stress experiences less roof/wall stress concentration than one driven across it), the orientation of induced fractures during hydraulic operations, and the interpretation of discontinuity behaviour (a joint favourably or unfavourably oriented relative to the stress field governs whether it is likely to slip) — none of which can be inferred from rock type or discontinuity mapping alone.

(c) Main factors of importance in a groundwater investigation

A groundwater investigation needs to establish several distinct pieces of information, each of which drives a different part of the engineering design: the static water table elevation and its seasonal fluctuation range (design excavations and buoyancy checks for the worst-case high level, not a single snapshot reading); the number and continuity of aquifer units and any perched water above a low-permeability layer (perched water is easily mistaken for the true water table if only shallow observation wells are installed); the hydraulic conductivity of each unit (governs dewatering rate, seepage into excavations, and contaminant transport velocity if contamination is present); the direction and gradient of groundwater flow (requires at least three non-collinear wells to compute, not one); artesian or confined conditions (a confined aquifer under pressure can cause a heave/blow-out failure of an excavation base if not identified before dewatering design); groundwater chemistry (sulphate/chloride content affects concrete durability and pile corrosion; contamination affects disposal of arisings); and the interaction between groundwater and the rock and soil units identified in Questions 1–2 (e.g., whether the water table sits within a liquefiable sand layer, or governs pore pressure on a discontinuity in the rock mass).

(d) Organizing a physical groundwater investigation

The field program is built around a network of monitoring wells and/or piezometers supplemented by in-situ permeability testing. First, the desk study and reconnaissance identify likely aquifer units and expected depth to water, which sets the target completion depths. Wells/piezometers are then sited to (i) bracket the structure footprint and any excavation, (ii) span each distinct water-bearing unit identified in the stratigraphy, and (iii) form a spatial pattern of at least three non-collinear points, ideally more, so a groundwater flow direction and gradient can actually be computed by triangulation. Equipment typically required includes the drilling rig itself (auger, mud-rotary, or sonic, chosen to avoid cross-contaminating aquifers during advance), well casing and screen (slotted PVC or steel sized to the target formation's grain size, with a filter sand pack and bentonite seal above the screen to prevent surface-water short-circuiting down the annulus), a water-level indicator (electronic dip meter) for manual monitoring rounds, and, where continuous records are needed, a data logger/pressure transducer left in the well. In-situ permeability is measured with slug tests, packer (falling-head) tests in boreholes, or short-duration pumping tests where a larger-scale conductivity estimate is needed. Key factors in the borehole spacing and distribution itself are: adequate number and spatial distribution of monitoring points (three-point minimum for gradient, more where the stratigraphy is complex or multiple aquifers must be separately monitored); screened interval placement matched to the specific unit being monitored (a well screened across two aquifers gives a meaningless composite reading); proper sealing between units to prevent the borehole itself creating a new vertical flow path; sufficient monitoring duration to capture seasonal high/low water levels rather than a single snapshot; and a documented QA/QC program (development of the well after installation, decontamination between holes if contamination is suspected) so that the readings are representative of formation conditions, not drilling-disturbed conditions.