Civil Engineering Reference
In-Depth Information
material strengths found during the site investigation works,
such as standard penetration test (SPT) counts and the sub-
sequent laboratory results providing such information as clay
shear strengths, angle of internal friction and moisture content.
All this information would be utilised by an experienced
engineer to choose the correct foundation type and design the
structure accordingly, limiting settlements due to the building
structure. Unfortunately, this work is sometimes passed to an
inexperienced engineer and it is here that deficiencies in the
design are found to occur. When the engineer does not have
the relevant experience the factual investigation should be sup-
plemented by an interpretive report by a specialist engineer,
providing the necessary information along with foundation
options and expected settlements.
Care should also be taken when extending properties ensur-
ing that the new foundations do not rest or impose additional
loads on to the existing foundations. New foundations should
be adjacent to the existing structure and founded at a lower for-
mation level. As it is common for new extensions to suffer from
differential settlement, care should be taken when excavating
the foundations, ensuring that firm strata is achieved and also
that the superstructure is detailed in such a way that any minor
settlement does not adversely affect the building fabric.
In some cases where the proposed building loads are slightly
higher than the allowable bearing capacity it may be possible to
utilise ground improvement techniques such as vibro stone col-
umns. This is a cheaper alternative to piling, especially over an
area or number of plots, and would provide an improved bearing
capacity for a shallow foundation type. Care should be taken
with this type of ground improvement giving due regard to soft
strata bands such as peat that provide little lateral restraint to the
stone column, leading to bulging within the soft band layer.
9.3.6 Seismic/dynamic foundation failure
In order to assess the seismic response of a structure when it is
located within a known area of seismic activity the knowledge
of the soil strata composition is critical as the soil layers pro-
vide differing damping conditions which affect the amplitude
of the ground motion applied to the structure.
Understanding the structural form and good knowledge of the
site is essential when designing adequate foundations to transmit
the building forces to suitable bearing strata. However, the damp-
ing conditions and amplitude also have a bearing on the design of
the foundations with the aforementioned damping effects acting
as springs against the buried structure or pile model.
The material characteristics of the strata layers are to be
suitably assessed as part of the site investigation and labora-
tory testing and considered along with the choice of foundation
system. For instance, saturated non-cohesive soils (sands and
silts) under cyclic loading, as experienced in earthquakes, can
become liquefied, a process called liquefaction (
Figure 9.3
).
Within saturated soils the pore water pressure is generally
low; however, during the earthquake shake, the pore water
pressure increases enabling the soil particles to move. As this
occurs the soil strength decreases, creating a bearing capacity
failure of the structure. This also affects pressures on buried
9.3.5 Choice of appropriate foundations
By utilising the site investigation reports including trial pits,
the appropriate foundation type can be chosen. Soils with
high settlement rates such as soft clays, silts and peat require a
deep foundation such as piles to transit the loads to more cap-
able strata at depth; whereas firm strata at shallow depths can
readily accept normal foundation loads using strip, pad or raft
foundations as long as this strata is consistent for depth and the
pressure bulb does not lie in a softer lower strata.
Figure 9.3
Liquefaction failure - Niigata earthquake, Japan 1964. Photograph in the public domain
