Question 3 of 5

Question 3 (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 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. Structural mapping (strike/dip of bedding, foliation, and discontinuities, read with a compass-clinometer) extends a point observation into a three-dimensional formation geometry, letting 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; 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 and reference to published regional geological maps from the desk study) frames the local observations within the known regional stratigraphy before any field work begins.

(b) Determining stress orientation in rocks and its importance

In-situ rock stress orientation and magnitude are determined by several established methods. Overcoring 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 televiewer afterward) directly gives the minimum principal stress orientation. Borehole breakout analysis reads stress-induced spalling zones on opposite sides of an already-drilled borehole wall — breakouts form perpendicular to the maximum horizontal principal stress direction, giving that direction from a borehole drilled for another purpose, at no extra field cost. This matters from a site investigation perspective because in-situ stress orientation directly governs 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 whether a discontinuity is favourably or unfavourably oriented relative to the stress field for 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 must establish several distinct pieces of information, each driving 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 (easily mistaken for the true water table if only shallow wells are installed); the hydraulic conductivity of each unit (governs dewatering rate and seepage into excavations); the direction and gradient of groundwater flow (needs at least three non-collinear wells to compute); artesian or confined conditions (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); and the interaction between groundwater and the rock/soil units already identified.

(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. The desk study and reconnaissance first identify likely aquifer units and expected depth to water, which sets the target completion depths; wells/piezometers are then sited to bracket the structure footprint, span each distinct water-bearing unit, and form a spatial pattern of at least three non-collinear points so a flow direction and gradient can actually be computed by triangulation. Equipment typically required includes the drilling rig itself (chosen to avoid cross-contaminating aquifers during advance), well casing and screen (slotted PVC or steel sized to the target formation, with a filter sand pack and bentonite seal above the screen to prevent surface-water short-circuiting down the annulus), a water-level indicator for manual monitoring rounds, and a data logger/pressure transducer where continuous records are needed. Key factors in the borehole spacing and distribution are: adequate number and spatial distribution of monitoring points (three-point minimum for gradient, more where 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; and sufficient monitoring duration to capture seasonal high/low levels. Well type is chosen to match the purpose: an open standpipe (Casagrande) piezometer is simple, robust, and adequate for a single, reasonably permeable unit where response time is not critical; a vibrating-wire piezometer, sealed within a discrete grouted interval, is required in low-permeability clay or where multiple closely-spaced heads must be resolved in one borehole, because a standpipe's large water-storage volume responds too slowly in low-K material to track a genuine head change; a fully-screened observation well (rather than a discretely-sealed piezometer) is used only where a single, composite water-table elevation — not a layer-specific head — is the actual design question; and a nested multi-level well (several independently sealed piezometer tips in one borehole) is used wherever a vertical head gradient between stacked aquifers, not just a single water table, needs to be resolved.