Civil Engineering Reference
In-Depth Information
Figure 9.2
Punching shear failure at Pipers Row car park, Wolverhampton © Dr J Wood
bearing or using a combination of end-bearing and skin fric-
tion where vertical loads are resisted by the ground/pile sur-
face friction along all or part of its length.
Foundation failures can usually be identified by the defor-
mation or other signs of distress noted in the superstructure.
Settlement of foundations (excessive ground movement) is
highlighted by out of plumb walls, sloping floors and, where
differential settlement occurs, stepped cracking in both inter-
nal and external masonry.
However, the type of foundation failure itself depends upon the
type of foundation used in construction. In the majority of cases
within the UK, failure may be caused by the lack of (or reduc-
tion in) ground-bearing capacity once the structure is complete
and fully loaded. In some cases the foundation structure itself
may fail, reducing the bearing area required to safely transmit the
forces, such as shear failure of wide strip footings or achieving a
false set when driving piles to reach a required bearing depth.
The pile member may fail due to shear failure of the sur-
rounding soil and/or excessive settlement of the pile. This can
also be confused with a lack of bearing on bedrock, as was the
case with a project in Blackpool, Lancashire. Here, a small
development of residential properties constructed on a slop-
ing site was eventually part-demolished following failure of
the piled foundations. In this case, the steel-cased piles were
driven to found on rock; however, subsequent investigations
discovered that a number of piles were bearing on a sloping
outcrop causing a rotational failure of the loaded piles.
Failure of shallow foundations is typically found at positions
of high building loads, areas of soft substrata or indeed both.
These highly loaded areas include chimney stacks/breast in
older properties and extended or refurbished buildings which
have had increased structural loads such as additional floors or
changes in load paths. On internal inspection of these proper-
ties it is common to observe that the floors, door frames and
window sills slope in the direction of the settlement with asso-
ciated structural cracking both internally and externally.
Raft foundations, which are designed to balance out the struc-
tural loads over the plan area of the building, are particularly
susceptible to increased loads or soft spots on the perimeter of
the building. In these cases the whole building may rotate on the
raft without the associated external deformations; however, these
movements may damage services into and out of the building.
Foundation settlements can occur following increases in
groundwater beneath foundations. It is normal during the design
phase when assessing the allowable bearing pressure (ABP) of
granular soils to reduce the allowable pressure under the foun-
dation by half when near to the water line. If water was not a
consideration in the foundation design, either by the designer's
choice, error or lack of investigation when assessing ABP, the
addition of water to the soil strata beneath the foundation can
reduce the effective density of the soil leading to a reduction in
the factor of safety and ultimately double the settlement.
On the same basis, leaking drainage systems, water supplies,
etc., may remove fine soil particles over a long period without
