Environmental Engineering Reference
In-Depth Information
- Columnar joint pattern?
-Toppling failure?
- Difficulties in blasthole drilling?
- Poor fragmentation during blasting?
-Irregular joint pattern and “pillow” structure?
-Alteration effects - secondary minerals?
-Fresh, extremely strong boulders in extremely weathered materials (high plasticity clay)?
-Very high plasticity soils, expansive, fissured?
- Unstable slopes?
- Alkali-aggregate reaction?
3.3
PYROCLASTICS
Pyroclastic or “fire-broken” deposits are those which have been formed by the accummula-
tion of solid fragments of volcanic rock, shot into the air during volcanic eruptions. The rock
fragments include dense, solidified lava, highly vesicular lava (termed scoria) and extremely
vesicular lava (termed pumice). Pumice is formed only from acidic lavas (e.g. rhyolite) and is
so porous that it will float on water. Francis (1976) provides a detailed account of the ways
in which pyroclastic materials are formed, based mainly on historical accounts of modern
eruptions. Prebble (1983) describes the pyroclastic deposits of the Taupo Volcanic Zone in
New Zealand and the difficulties they present in dam and canal engineering.
3.3.1
Variability of pyroclastic materials and masses
Pyroclastic deposits are characterized by extreme variability in engineering properties over
short distances laterally and vertically. They range from extremely low density “collapsing”
type soils to extremely strong rocks. This wide range in properties results from differences
between the ways in which they were initially deposited and also from the ways they have
been modified since deposition.
There are four main types of deposit, based on initial mode of deposition:
(a) Air fall deposits in which the fragments have simply been shot up into the air and
fallen down again. Where they have “soil” properties such deposits are termed ash
(sand sizes and smaller), or lapilli and bombs (gravel sizes and larger). Where welded,
compacted or cemented to form rocks, they are termed tuff (sand sizes and smaller) or
agglomerate (gravel sizes and larger in a matrix of ash or tuff).
(b) Water-sorted deposits in which the fragments fall into the sea or a lake and become
intermixed and interbedded with marine or lake deposits. These also may be “soils”
or rocks depending upon their subsequent history.
(c) Air-flow, or “nuées ardentes”, deposits in which the fragments are white-hot and
mixed with large volumes of hot gases, to form fluidized mixtures which can travel
large distances across the countryside at speeds of probably several hundreds of kilo-
metres per hour. The resulting materials, known as ignimbrites, range from extremely
low density soils with void ratios as high as 5 (Prebble, 1983) to extremely strong
rocks. The latter are formed when the white-hot fragments become welded together to
form rocks almost indistinguishable from solidified lavas. These rocks are called
welded ignimbrites or welded tuffs.
(d) Hot avalanche deposits which are formed by the gravitational breakup and collapse
of molten lavas on steep slopes. The deposits comprise loosely packed but partly
welded boulders, often showing prismatic fracture patterns which indicates that they
were chilled rapidly after deposition (Francis, 1976).
Search WWH ::




Custom Search