Rotary Bored Piling Specialists

Surface Ground Settlement

Surface ground settlement may cause severe damage to surface structures and services.

Primary Reason
Pile boring operations, reduces the in-situ pressure within the bore to a point where the magnitude of the external pressure causes movement from outside of the bore into the excavated area, resulting in loosening of the surrounding ground, and causing ground settlement at the surface.
Possible Causes

Upward flighting of material by the action of a continuous flight auger.

Ingress of material from an unstable zone, without a support to the bore.

Boring in an unstable zone without lateral support casing.

Hydraulic pressure is out-of-balance, with greater external pressure to that in the bore, causing ingress of material.

Likely Scenarios
Possible Causes
Upward flighting of material by the action of a continuous flight auger.
Likely Scenario
  • In order to facilitate the digging process with under-powered equipment, sometimes the rate of penetration of the auger may be too slow, resulting in the volume of material excavated being greater than the displacement volume of the auger, resulting in side-loading of the auger, the upward displacement of material by the Archimedes screw effect, and loosening of the surrounding ground.
Possible Causes
Ingress of material from an unstable zone, without a support to the bore.
Likely Scenario
  • Boring in an unstable zone, below a casing support, without a bentonite or polymer support fluid. This is most likely to occur when (prior to flooding of the bore with support fluid) the strata changes suddenly from cohesive material to granular material.
Possible Causes
Boring in an unstable zone without lateral support casing.
Likely Scenario
  • In order to facilitate the installation of casing, the rig operator bores out material from below the casing, by either rotary bored, or sectional flight auger technique.
  • The use of sectional augers to create an open hole, where the ground conditions are variable (containing unstable granular strata). The removal of the augers allows unstable material to flood into the open bore, but this only becomes apparent upon the removal of all the augers.
  • The removal of the augers potentially then allows unstable material to flood into the open bore, but this only becomes apparent upon the removal of all the augers.
Possible Causes
Hydraulic pressure is out-of-balance, with greater external pressure to that in the bore, causing ingress of material.
Likely Scenario
  • Rig operator attempts to bore a “dry” hole within the bore, creating a hydraulic imbalance which draws in additional material.
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