Civil Engineering Reference
In-Depth Information
if contaminated, can lead to expensive disposal. Displacement methods involving
jacked-in piles or pile sections create little vibration but produce lateral soil dis-
placement and potentially, some heave. Therefore they should not be employed
close to the foundations of an adjacent structure sensitive to any lateral loads.
The noise and vibration associated with driven micro-piles may be unacceptable
in some locations and such piles have the common characteristic of causing lateral
displacement.
Micro-piles do not have large end-bearing areas although they may be installed
to depths comparable with piles of larger diameters. As a consequence, they work
primarily by shaft friction. However, should the shaft friction be fully developed,
there will be relatively little base resistance to mobilize and unless a composite action
between a reinforced slab foundation and the piles is deliberately planned, settle-
ments can be excessive. In non-cohesive soils or weak rock, higher than normal unit
shaft resistance has been noted in the case of drilled grouted-in-place micro-piles,
whilst displacement types may not develop resistances comparable with conventional
piles as the volume displacement is less. It is, however, usual practice to adopt
empirical methods of design for granular soils, modified in the light of experience
by the specialist contractor. Settlements are generally found to be similar to, or
slightly less than, those associated with traditional friction piles where the piles are
used in cohesive deposits. Cooke and Price (1978) report the magnitudes of strains
and displacements associated with small-diameter piles in London Clay. For end-
bearing micro-piles, larger than usual settlements can occur, as working stresses
within the shafts of such piles tend to be high, leading to relatively large elastic
compression.
Unless reinforced with a steel section, micro-piles are weak in shear. However, the
ability with which most micro-piles can be raked means that inclined loads can be
taken axially, to obviate the need for shear resistance in many applications.
The costs of load testing to check design assumptions can be prohibitive for a
small number of micro-piles and some systems of integrity testing may be imprac-
tical because of their small diameter. It is therefore particularly important that strict
quality control is maintained and, according to the sensitivity of the project, gener-
ous factors of safety applied. Those systems which are to some extent self-checking
(i.e. instrumented jacked piles) and those employing a permanent former such as a tube,
which can be inspected after installation may have an advantage over a drilled pile
where concreting or grouting may take place under unfavourable conditions. How-
ever there is no record of micro-piles being particularly prone to poor performance
and in general the workmanship of the established operators in this specialized field
is good.
A further important factor to consider is that of corrosion, which can lead to serious
problems more rapidly than in conventional piles in view of the small sizes of micro-
piles. Because of this, small corrosion pits in a permanent steel casing could be of
significance relative to the wall thickness of the pile. In pre-cast or pre-formed piles,
the shaft can be inspected prior to use and coatings can be applied to combat corrosion
(or to reduce down-drag or heave effects). It is however common practice to employ a
central reinforcing tube or bar grouted in place so that the steel outer casing becomes
sacrificial.
