Pile Design and Construction Practice, Fifth edition

Longitudinal reinforcement is provided in the piles to the extent necessary to carry vertical
loading, eccentric loads from the underpinning bearers, and lateral loading from earth and
hydrostatic pressure.
Screw or helical piles comprising solid square shafts up to 60 mm and tubular shafts up
to 90 mm with helical plates between 200 and 350 mm diameter attached at intervals along
the shaft, such as the AB Chance systems and ScrewFast Foundations in the UK, are effective
as underpinning support. They are screwed into the ground adjacent to the foundation
using rotary drives, either attached to hydraulic excavators with torque capacity up to 68 kNm
or hand-held units with around 3 kNm torque – resisted by a torque bar. The bearing capacity
is related to the area of the helical plates, four plates being typical and spaced so that overlap
of bearing zones does not occur. Shaft friction is not usually applied unless the shaft
diameter is greater than 90 mm. As for the slender steel-jacked piers above, buckling has to
be considered and corrosion protection provided (note this form of slender pile should not be
confused with the large-diameter displacement pile formed by screwing a mandrel into the
ground – see Section 2.3.5).
9.3 Piling in mining subsidence areas
Miscellaneous piling problems 445
The form in which subsidence takes place after extracting minerals by underground mining
depends on the particular technique used in the mining operations. In Great Britain the problems
of subsidence mainly occur in coal-mining areas where the practice in the remaining
working collieries is to extract the coal by ‘longwall’ methods. Using this technique the
entire coal seam is removed from a continuously advancing face. The roof of the workings
behind the face is supported by multiple rows of hydraulically operated props. As the face
moves forward the props in the rear are systematically lowered to allow the roof of the
workings to sink down onto heaps of mine dirt or ‘stowage’. The overlying rock strata and
over-burden soil follow the downward movement of the roof and the consequent subsidence
of the ground surface is in the form of a wave which advances parallel to and at approximately
the same rate as the advancing coal face. The subsidence is accompanied by very
substantial horizontal strains of the ground surface, these strains being tensile at the crest
of the wave and compressive at the trough and thus taking the form shown in Figure 9.9.
Tension
0
500
1000
1500
2000
Subsidence
(mm) Subsidence
curve
Worked-out seam
Compression
Limit angle
Figure 9.9 Profile of ground subsidence over longwall mine workings.
+0.004
+0.002
0
–0.002
–0.004
Strain
Unworked coal
Ground
surface