Question 6 of 6: Ground Support Systems

Question 6: Ground Support Systems (20 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.

6.1 — Expandable friction bolt

An expandable friction bolt (the Swellex-type design is the most widely used example) is a rolled, seam-welded steel tube, closed at the toe and open at the collar, that is inserted into a slightly under-sized borehole and then expanded radially by injecting high-pressure water (typically ≈30 MPa) through the open end. The water pressure unrolls and permanently deforms the tube outward until it is forced tightly against the full length of the borehole wall, generating support capacity by friction alone (no grout, no resin, no mechanical anchor point) — the bolt's rough, corrugated outer surface interlocking with the borehole roughness the moment it is pressurised.

Advantages. Full-length, immediate (within seconds of pumping) support capacity with no cure time, unlike resin or cement-grouted bolts that need time to set before they carry load — important where the excavation must be reinforced and re-entered quickly. Installation is fast and mechanised (a single hydraulic pump), needs only one hole diameter tolerance rather than a precise annulus for grout, and works in wet or dry, and in broken or weak ground where a grout column might not form reliably. The bolt also has some genuine yield capacity: once the tube is fully expanded against the hole, continued rock convergence can further deform the corrugated tube wall and let the bolt slip somewhat while still retaining load, giving it a degree of the "yielding" behaviour discussed in 6.2.

Limitations. Capacity is controlled by the borehole diameter tolerance — an oversized or rough/irregular hole reduces the frictional grip and can leave the bolt seriously under-capacity without any visible sign at the collar; corrosion protection is comparatively poor unless the tube is hot-dip galvanized or coated, since bare steel is in direct contact with groundwater along the full bolt length; and because capacity depends on friction rather than a chemical bond, ultimate pull-out capacity is generally lower than a fully-cured resin-grouted rebar of the same diameter, so friction bolts are typically used as fast primary/temporary support rather than as the sole permanent reinforcement of a critical, long-design-life opening.

6.2 — Yielding support for a seismically active hard-rock mine

A cone bolt (or the similar Garford dynamic bolt) is a purpose-built yielding rock bolt for seismically active hard-rock mines. It is a smooth steel bar with a flattened cone (or a series of deformed lugs, for the Garford version) forged or swaged onto the leading end, fully grouted into the borehole with cement grout. Because the shank of the bar is left smooth (debonded) along its length, a sudden dynamic load — from a seismic event or rockburst — does not have to break the bond over the whole bolt length at once; instead the cone/lug is progressively pulled through the surrounding grout column, ploughing a channel through it. This ploughing absorbs a large amount of energy at a roughly constant, controlled load, allowing the bolt to displace by several tens to over a hundred millimetres — far more than a conventional fully-bonded rebar, which can only stretch a small amount before it either yields at a fixed peak load or fractures — while still carrying meaningful load throughout.

This behaviour is exactly what is needed under seismic loading: a rigid, fully-bonded bolt can fail suddenly (brittle fracture of the steel or the grout bond) when hit by the strain-energy release of a nearby rockburst, whereas a cone bolt is designed so its capacity envelope matches the displacement demand of a dynamic event rather than only its static load. Cone bolts are typically installed as part of a full dynamic support system — paired with a yielding surface support (e.g. weld-mesh plus shotcrete, or a purpose-made energy-absorbing mesh/strap system) so that the surface layer can also deform with the bolts instead of tearing at the collar — and are standard practice in Canadian and international deep, seismically active hard-rock mines (e.g. in the Sudbury and Val d'Or camps) as the dynamic-capable complement to static pattern bolting.