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) Geomechanical mechanisms by which groundwater influences ground conditions

Groundwater governs ground behaviour through several distinct physical mechanisms, not merely as a static "water level" to be recorded on a log:

• Effective stress reduction — Terzaghi's principle σ' = σ − u means any rise in pore pressure u directly reduces the effective stress controlling both shear strength and stiffness; a rising water table, or undrained loading that generates excess pore pressure, can reduce a slope's or foundation's factor of safety with no change in total stress at all.
• Buoyancy and uplift — groundwater pressure acting on the underside of a structure or excavation base reduces the effective weight resisting flotation; a confined (artesian) aquifer beneath an excavation can drive a sudden base heave/blow-out failure if the aquifer pressure is not relieved before or during excavation.
• Seepage force and piping/internal erosion — flowing groundwater exerts a body force on the soil skeleton in the direction of flow; where the exit hydraulic gradient at a slope toe or excavation face approaches the critical gradient, the seepage force can exceed the soil's submerged weight and initiate piping (progressive internal erosion) or a quick/boiling condition.
• Softening, sensitivity loss and collapse — wetting a previously unsaturated soil, or disturbing a sensitive clay's structure, can collapse or soften its fabric, producing a sudden strength or volume loss unrelated to any change in applied load.
• Swelling and shrinkage — expansive clays change volume with moisture content, generating heave or settlement and associated uplift pressure on light structures, independent of the applied structural load.
• Frost action — capillary rise from a shallow water table feeds ice-lens growth in frost-susceptible silt, producing heave in winter and a sharp strength loss during spring thaw as excess meltwater cannot drain quickly enough.
• Dewatering-induced consolidation settlement — lowering the water table around an excavation increases effective stress in the surrounding ground (removing the buoyant support pore pressure previously provided), which can consolidate compressible soil and settle adjacent structures even though no load was added to them.
• Liquefaction — cyclic (seismic) loading of loose, saturated, cohesionless soil generates pore pressure faster than it can dissipate, driving effective stress toward zero and causing a near-total, if temporary, loss of shear strength.
• Chemical mechanisms — sulphate and chloride content in groundwater attacks concrete and corrodes buried steel, and where contamination is present, groundwater is also the primary transport pathway for dissolved contaminants.

(b) Main factors of importance in setting up and conducting a groundwater investigation

A groundwater investigation must establish several distinct pieces of information, each feeding a different part of the design: the static water table elevation and its seasonal fluctuation range (excavation dewatering and buoyancy checks are governed by the worst-case high level, not a single reading); the number and continuity of aquifer units and any perched water above a low-permeability layer, easily mistaken for the true regional water table if only shallow points are monitored; the hydraulic conductivity of each unit, governing dewatering rate, seepage into excavations, and (where relevant) contaminant transport velocity; the flow direction and gradient, which needs at least three non-collinear monitoring points to compute rather than assume; any artesian or confined condition, since a confined aquifer under pressure can drive a base heave/blow-out failure if not identified before dewatering design; groundwater chemistry (sulphate/chloride content affecting concrete and steel durability, and contamination status affecting arisings disposal); and how the water regime interacts with the specific soil/rock units already identified at the site.

(c) Organizing a physical groundwater investigation: process, equipment, and borehole regime

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 the likely aquifer units and expected depth to water, setting target completion depths; monitoring points are then sited to span each distinct water-bearing unit identified in the stratigraphy and to form a spatial pattern — a minimum of three non-collinear points, and more where the footprint is large or the stratigraphy is complex — so that a flow direction and gradient can actually be triangulated rather than assumed. Equipment typically required includes the drilling rig (auger, mud-rotary, or sonic, chosen to avoid cross-contaminating separate aquifers during advance), well casing and screen (slotted PVC or steel sized to the formation's grain size, with a filter-sand pack and a 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 obtained from slug tests, packer (falling-head) tests, or short pumping tests where a larger-scale conductivity estimate is required. In setting the borehole spacing and distribution, the governing factors are: adequate number and layout of points for gradient triangulation and for separately monitoring each aquifer unit; screened-interval placement matched to the specific unit being read (a well screened across two units gives a meaningless composite level); proper sealing between units so the borehole itself does not create a new vertical flow path; and a monitoring duration long enough to capture the seasonal high, not a single snapshot.

(d) Piezometer installation timing and function

A piezometer measures pore-water pressure (and, from it, hydraulic head) at a specific, discrete point or depth, as distinct from a standard monitoring well, which is typically screened over a longer interval and reads a composite level; in stratified ground with vertical head gradients between units, a single "water table" reading from a long-screen well is not physically meaningful. Piezometers should be installed: at each distinct soil/rock unit where a discrete pore-pressure reading is needed for a stability or settlement analysis (within a potential slip surface for a cut-slope stability check, or within a compressible layer to monitor consolidation-driven pore-pressure dissipation during staged construction); above and below a low-permeability layer to detect and quantify a vertical head difference between perched and regional water; near a proposed excavation or dewatering zone to monitor drawdown response during construction; and, for longer-term monitoring of a critical structure, at multiple depths within a single borehole (a nested/multi-level installation) so the vertical head profile is captured directly rather than inferred. Standpipe (Casagrande) piezometers suit more permeable soils where response time is fast and simple manual dip readings are adequate; pneumatic or vibrating-wire piezometers are preferred in low-permeability clay, or wherever a fast response time and remote/continuous readout are needed, since a standpipe in clay can take days to equilibrate after installation.