UFC 3-220-01N
15 AUGUST 2005
8-1.4.2
Rippability. Excavation ease or rippablility can be assessed
approximately from field observation in similar materials or by using seismic velocity,
fracture spacing, or point load strength index. Figure 8-1.7 (from Handbook of Ripping,
by Caterpillar Tractor Co.) shows an example of charts for heavy-duty ripper
performance (ripper mounted on tracked bulldozer) as related to seismic wave velocity.
Charts similar to Figure 8-1.8 are available from various equipment manufacturers.
Figure 8-1.8 is for guidance and restricted in applicability to large tractors heavier than
50 tons with engine horsepower greater than 350 Hp. Ripper performance is also
related to configuration of ripper teeth, equipment condition and size, and fracture
orientation.
Another technique of relating physical properties of rock to excavation ease is shown in
Figure 8-1.9, (from Logging the Mechanical Character of Rock, by Franklin, et al.)
corrected to a reference diameter of 50 mm (see The Point-Load Strength Test, by
Broch and Franklin).
A third and useful technique is exploration trenching in which the depth of unrippable
rock can be established by digging test trenches in rock using rippers (or other
excavation equipment) anticipated to be used for the project. The size and shape of the
area to be excavated is a significant factor in determining the need for blasting, or the
equipment needed to remove the rock.
8-1.4.3
Blasting. Of major concern is the influence of the blasting on adjacent
structures. The maximum particle velocity (the longitudinal velocity of a particle in the
direction of the wave that is generated by the blast) is accepted as a criterion for
evaluating the potential for structural damage induced by blasting vibration. The critical
level of the particle velocity depends on the frequency characteristics of the structure,
frequency of ground and rock motion, nature of the overburden and capability of the
structure to withstand dynamic stress. Figure 8-1.9 can be used for estimating the
maximum particle velocity, which can then be used in Figure 8-1.10 (from Blasting
Vibrations and Their Effects on Structures, by Bureau of Mines) to estimate potential
damage to residential structures. Guidance for human response to blasting vibrations is
given in Figure 8-1.11 (from Engineering of Rock Blasting on Civil Protects, by
Hendron).
Once it has been determined that blasting is required, a pre-blasting survey should be
performed. At a minimum, this should include:
Examination of the site
Detailed examination and, perhaps, photographic records of adjacent
structures
Establishment of horizontal and vertical survey control points
8-12
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