Pile Design and Construction Practice, Fifth edition

cost-effective alternatives for 1 371 mm diameter, steel ‘caisson’piles up to 30 m deep, in
karstic dolomite(9.40). 160No micropiles replaced forty caissons at each of three piers. Where
the conditions were highly variable with pinnacles, voids and clay-filled solution cavities,
the use of ‘Tubex’duplex drilling was necessary to install 327 mm temporary casing to
depths up to 59 m with a 1 m deep rock socket to insert the permanent pile tube. At other
locations where the karstic conditions were less variable, a down-the-hole rotary percussive
drill was used to drill 305 mm diameter holes up to 23 m deep without casing to insert the
specified 245 mm steel tube – with the assistance of a D5 pile hammer (Figure 9.30). The
pile holes in each case were grouted using a tremie pipe, ensuring that grout level was sta-
ble at the top of the hole prior to inserting the permanent tube.
Drilling ‘slim’holes, with or without simultaneous casing, or driving long H-piles in
karstic conditions can cause significant problems due to deviations compromising the axial
capacity of the piles. Concreting or grouting open holes or as temporary casing is withdrawn
runs a risk of loss of material into weak cavity infill or undetected voids requiring pre-grouting
using a low slump mix injected in several stages and re-drilling. Micropile test piles,
installed with open hole drilling methods, have failed in karst geology due to contamination
of the bond zone in the time between withdrawal of the drill tools and installation of the
permanent pile tube and concreting.
Natural overburden and decomposed debris overlying the karst formation can be treated by
various ground improvement techniques prior to piling – such as vibroflotation, compaction
grouting, and jet grouting. Fischer(9.41)describes the foundations for a nuclear power plant on
karst terrain which comprised tubular steel piles driven into relatively flat limestone bedrock,

Miscellaneous piling problems 473

Figure 9.30Micropile in Karst (Uranowski et al.(9.40)).

Grout-filled drill hole

Centralizers

600 mm minimum rock socket

Top of sound rock

Ground surface

Cross section of micropile

305 mm diameter hole

Grout placed by tremie

to base of hole before

inserting pile tube

245 mm OD × 12.5 mm wall high

tensile steel tube

Outside of tube greased and

wrapped except for bond

zone in socket

Tube topped up

with grout

Hole drilled with 'duplex'

drilling/casing up to 55 m

Fully penetrating welds

to join tube sections

Unbonded zone in overburben