Civil Engineering Reference
In-Depth Information
Table 7.3
Maximum allowable velocity, from DIN 4150
Class
Description
Max. velocity*
(mm/s)
1
Ruins and buildings of great historic value
2
2
Buildings with existing defects (visible cracks in brickwork etc.)
5
3
Undamaged buildings in technically good condition except for minor cracks
10
4
Strong buildings, industrial buildings of R.C. concrete or steel
10-40
∗
Particle velocity in soil adjacent to foundations of structure.
The use of empirical limits on velocity or acceleration necessitates the use of field
instrumentation to observe the induced vibrations. Suitable equipment is expensive to
purchase, and several levels of recording are possible. The simplest scheme is man-
ual recording of peak-to-peak phenomena, and the most complex is a full record of
the ground vibrations to enable a frequency analysis to be carried out. Siting of the
accelerometers is important, as amplitudes differing by a factor of 10 can easily occur
due to structural magnification. It is also highly desirable to use triaxial accelerom-
eters, as it is not usually clear in which direction the most damaging vibrations will
propagate. The human body is extremely sensitive to vibrations and very low inten-
sity shock waves may be detected. For this reason, people frequently conclude that
ground vibrations are more severe than measurements show them to be. To limit
unreasonable claims for damage to adjoining structures, it is highly desirable to carry
out a detailed structural survey of the buildings. Good-quality photographs of sen-
sitive areas or existing defects form useful records to counter or confirm particular
assertions.
A common situation is for piles to be driven through a soft soil into a bearing
stratum, with adjacent buildings founded on the soft material. Practical experience in
such situations suggests that the soft layer effectively damps the ground vibrations.
As a crude rule little or no structural damage is likely to be experienced if the pile
and rig is at least 5 m from an adjacent building. The occupants of such a building
may, however, suffer severe nuisance. Various expedients may be adopted to reduce
the intensity of ground vibrations caused by pile driving. These include pre-boring,
the use of low-displacement piles, or in particularly sensitive areas, the adoption of
alternative pile types (see also section 10.4.5).
7.2 Bored piles
7.2.1 Excavation of the pile bore
In favourable soil conditions, i.e. stiff clays, bored piles can be constructed without
casing, except for a short guide length. Under less satisfactory conditions temporary
or permanent casings may be used to support the pile, or bentonite drilling fluid may
be resorted to. Excavation of the soil is by augers, buckets, or a variety of grabs
(Chapter 3). Continuous-flight auger equipment, which can place piles in many soil
conditions without the use of any casing, is in common use.
