Environmental Engineering Reference
In-Depth Information
limitations that the scale and nature of the mate-
rials allow. These areas have a similar treatment
when planning geotechnical surveys.
(phases of formation of large shield volcanoes in the
early subaerial volcanism of the canary islands).
Usually they present in surface RmR
b
values from
50 to 60. They are considered as T3h terrain.
2.1
Unit I: Basal complex
The basal complex of the canary islands is rep-
resented by cretaceous sediments, submarine
lavas and plutonic rocks (gabbros and syenites).
This set is traversed by numerous dykes intrusion
with a density so high that often leave no trace of
the rock sticking. Typically, with a high degree of
alteration as rock materials are very slippery and
difficult to recognition. This gives them features
of soft rock RMR
b
values were below 40 and can
occasionally reach up to 60. They are considered
as T3e terrain.
2.4
Unit IV: Fresh lava flows
in this unit lists the basaltic lava flows that retain
their original structure due to their low state of
alteration, so it can distinguish the types “pahoe-
hoe” and “aa”.
“Pahoehoe” lavas are characterized by a smooth
and undulating surface, although in detail they are
formed by interlocking corrugated ropes. inter-
nally is to highlight the presence of large num-
bers of small vacuoles or spherical voids that give
them high porosity. however, the more remarkable
internal detail is the presence of volcanic tunnels
or tubes that can reach kilometers in length and
several meters in diameter. often during surveys
those tunnels or tubes are not detected, which does
not mean that they none exist.
“aa” lavas or scoriaceous lavas have an extremely
rough or spiny surface. Vertical section consists of
a central band of dense rock crossed by a network
of joints, limited below and above by two irregular
scoriaceous bands.
The basalt rock massif levels in general have high
bearing capacity, RmR
b
valued between 60 and 85.
however, scoriaceous levels may show low bearing
capacity and high deformation, if scorias are loose
and without matrix. Besides, they show moderate
bearing capacity and low deformability, if they are
welded or with a weakly cementation degree.
so for geotechnical surveys purposes, the Guide
considers that this unit recognizes two subunits.
2.2
Unit II: Salic lava flows and salic massif
This unit consists of highly resistant rock mate-
rials. There are two forms of outcrop: 1) as very
thick lava flows, usually with horizontal arrange-
ment or as thick tabular packages with slopes not
too steep and large horizontal extension. some-
times these packages may consist of very compact
breccias with salic fragments and 2) as domes, like
a large rock massif. in any case, for practical pur-
poses, the geotechnical features of both types of
upwelling are considered similar and should there-
fore be considered as the same geotechnical unit.
They are massifs of trachytic-phonolitic composi-
tion, generally moderate to high bearing capacity,
characteristics of hard rock and RmR
b
values from
80 to 90. They are considered as T1.
2.3
Unit III: Altered basaltic massif
composed of basaltic lava flows of small thickness
(around 1 m or less) and moderate to high distur-
bance. The remarkable peculiarity of these basaltic
lavas is a vertical alternation of compact basal-
tic levels (basalt rock) and scoria levels (granular
material).
Pyroclastic mantles and burned paleosols may
appear interspersed in these massifs.
The presence of interspersed scoriaceous levels
produces a high heterogeneity, due alternation both
vertically and horizontally. overall, scoriaceous levels
tend to behave like a granular soil, little or nothing
compact. But these features fade in Unit iii mate-
rials due to the advanced state of alteration. so,
these materials show problems as expansiveness,
high deformability and slope instabilities. in addi-
tion, there may be caves due to water circulation
and low compaction.
They are generally soft rocks and spread mainly
in the areas of outcrop of the antique series
2.4.1
Subunit IVa
“aa” lavas with compact basalt thicknesses equal
to or greater than 2 m, while retaining its lateral
continuity across the plot; with less than 0.5 m
scoriaceous levels, absence of cavities and a slope
of field less than 15º. They are considered as
T1 terrain.
2.4.2
Subunit IVb
it includes “pahoehoe” lavas and “aa” lavas with
compact basalt thicknesses less than 2 m, inter-
spersed scoriaceous levels and/or presence of cavi-
ties. They are considered as T3e.
2.5
Unit V: Pyroclastic deposits
it consists of areas of undifferentiated pyroclastic
deposits. Thickness and dip depend on the topog-
raphy on which they were deposited at the time of
the eruption. They can be subdivided into.
