Page 151 - Numerical Analysis and Modelling in Geomechanics

Chapter 5
Computational modelling of ground waves

due to pile driving
C.L.Ramshaw and A.R.Selby

Introduction
A number of construction processes require high energy input to achieve their
objectives. They include blasting for tunnelling and quarrying, rock ripping and
breaking-out, ground improvement by vibro-compaction, dynamic compaction
or vibratory rolling, pile-driving, the movement of heavy site plant, and
demolition. All of these processes also cause severe ground vibrations close to the
source activity. While many of these are used only in locations remote from
buildings and services, piles must frequently be installed in urban development
sites and residential areas. Here ground vibrations cause disturbance to residents
and occupants of buildings, and may create a risk of cosmetic or structural
damage (e.g. Wiss, 1967; Selby, 1991; Head and Jardine 1992; Malam, 1992;
Dowding 1996). In such situations it is important to be able to predict ground
vibrations and to monitor and control peak levels; it is important also to estimate
the effects of the vibrations on buildings and buried services, see Figure 5.1.
A variety of pre-formed piles exists which must be installed by high energy
drivers. Steel sheet piles are used for quay walls, temporary cofferdams,
retaining walls and integral bridge abutments. Steel bearing piles may be of tube,
box or ‘H’ section, and may be driven vertically or to a rake and may be 40 m or
more in length. Driven concrete piles may be precast reinforced or prestressed
segmental units or driven and cast-in-place. In Canada and Scandinavia timber
piles are sometimes used.
A wide range of hammers has been developed over the years, which can be
classified as impact, vibratory and special. The character of ground vibrations is
primarily a function of hammer type and energy.

Impact hammers
The simplest, and oldest, hammer is the drop weight, which is raised and
released into ‘free’ fall by the main lift wire of a crane. It is cheap but of low
efficiency, and may cause equipment wear. Air hammers (originally steam) are
powered by compressed air; they have a high strike rate with low energy per