Question 3 of 4

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) Key factors in the groundwater component of a site 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 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 specific soils identified in the investigation (e.g., whether the water table sits within a liquefiable sand layer, or within a sensitive clay where excess pore pressure controls stability).

(b) Organizing an on-site groundwater investigation

The field program is built around a network of monitoring wells and/or piezometers supplemented by in-situ permeability testing, organized as follows. 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 (a minimum of three points, ideally more, not in a straight line) so that 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/well regime 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.

(c) Function and placement of piezometers

A piezometer measures pore-water pressure (and, from that, hydraulic head) at a specific, discrete point/depth in the ground, as distinct from a standard monitoring well, which is typically screened over a longer interval and reads a composite water level. This distinction matters because in stratified ground with multiple aquifers or vertical head gradients, a single "water table" reading is not physically meaningful — the pressure head can differ substantially between two closely spaced elevations. Piezometers should be installed: at each distinct soil/rock unit where a discrete pore-pressure reading is needed for a stability or settlement analysis (e.g., within a potential slip surface for slope-stability work, or within a soft clay layer to monitor consolidation-driven pore-pressure dissipation under a new embankment load); above and below a low-permeability layer to detect and quantify a vertical (perched-versus-regional) head difference; near a proposed excavation or dewatering zone to monitor drawdown response during construction; and, for long-term geotechnical or dam-safety monitoring, at multiple depths in a single borehole (a nested/multi-level piezometer installation) so the vertical head profile is captured rather than assumed. Standpipe (Casagrande) piezometers are appropriate in more permeable soils where response time is fast; pneumatic or vibrating-wire piezometers are preferred in low-permeability clay or where a fast response time and remote/continuous readout are required, since a standpipe in clay can take days to equilibrate.