Engineering Rock Mechanics

264 Excavation principles

15.5.2 Evuluution of structurul response

The next step in establishing the effects of vibrations due to excavation

is to consider the tolerable limits of structures to the various wave char-

acteristics. These limits depend on the type of structure, the construction

materials, the history of the structure and the use of the structure. Thus,

factors such as the type of foundation, existence of any finishings and

claddings, whether other types of strain have already occurred and indi-

viduals’ perceptions are all important. Each structure will be susceptible

in different ways, but general guidelines have been incorporated into

design codes to assist engineers and to provide a basis for construction-

al operations. As an example, we list below some tolerable limits

based on PPV, emphasizing that these are examples and not specific

guidelines.

SDF model

Type of structure Tolerable PPV limits

(mds)

Residential masonry buildings

Retaining walls, bridge abutments,

Lined and unlined rock tunnels

industrial buildings

12-50

100

500-600

Further detailed discussion of this topic is beyond the scope of this book,

but we refer the reader to Hendron (1977), New (1984) and Dowding (1985)

for more information.

Another factor determining the response of a structure is the range of

frequencies present in the vibration with respect to the frequency response

of the structure itself. The frequencies can be presented either as pre-

dominant frequency histograms or complete frequency spectra. Similarly,

the response of a structure can be illustrated through a response spectrum.

In Fig. 15.25(a), we show how the predominant frequencies, measured at

a ’structure of concern’, can change with the type of blasting operation.

Figure 15.25(b) demonstrates the response of low-rise residential structures

in terms of their natural frequency.

The response of structures to ground vibrations can be quantified

through the use of mathematical models, such as single degree of freedom

models (as shown in the margin sketch). Such an approach can be difficult,

given the difficulty of adequately determining values for the various

components in such a model. Consequently, a pragmatic approach is

usually adopted.

1 5.5.3 Engineering approach to blust-induced

vib r u t ions

Faced with the complexity of information which has been indicated in

Section 15.5.2, some form of pragmatic guidance is required for engi-

neering. The four main steps in considering the effect of blasting

vibrations on structures are: