Environmental Engineering Reference
In-Depth Information
Figure 3.8.
Pillow structure in basalt, Pembrokeshire, U.K.
As stated at the start of this chapter, fresh volcanic rocks are used widely as construction
materials. However, columnar jointed rock can be difficult to quarry, especially when the
columns are almost vertical. Blastholes drilled vertically tend to jam in the open joints and
explosive gases vent into them. The columns tend to topple over rather than fragment,
resulting in a poorly graded (one-sized) quarry-run product. If the column diameter is
large (i.e. more than 1 metre) then the product may be too large for the crusher.
Regardless of the column size the poorly graded quarry product does not make good
rockfill because it is difficult to compact and large voids remain between the rock blocks
after compaction. This rockfill often has relatively low modulus and because of its large
voids, requires special attention to filter design.
3.2.2.2
Undersea flows
Lava which has been extruded from the sea-bed is often in the form of distorted globular
masses up to 3 m long known as pillows (Figure 3.8).
The pillows may be welded together, separated by fine cracks, or separated by sedimentary
detritus derived from the lava. The long axes of the pillows are generally roughly parallel to
the boundaries of the “flow”. Bell (1983b) notes that joints, vesicules, and phenocrysts in
the pillows are in some cases arranged radially. The joint pattern and the irregular shapes
of the cracks at the pillow boundaries produce an overall fracture pattern that might at
first appear to be “random”.
3.2.3
Alteration of volcanic rocks
Fresh volcanic rocks are composed of minute, strong, tough mineral crystals, generally
arranged in an extremely dense, interlocking manner. This structure results in negligible
porosity and great strength and durability within the lifetime of engineering structures.
However, as stated earlier, these rocks are often found to have been altered, probably dur-
ing the late stages of solidification. In the altered rocks some of the crystals are wholly or
partly changed to secondary minerals, including serpentine, calcite, chlorite, zeolites and clay
