Environmental Engineering Reference
In-Depth Information
2.5.1
Subunit Va
ignimbrites and tuffs: hard or medium hard rock
corresponding to highly compact pumice or cinder
pyroclastic deposits. This variety of materials occurs
when a mass of pyroclastic products are transported
in the form of gas dispersion and high or moder-
ate density of particles, the result is a material with
characteristics more or less hard rock, with a degree
of compactness and/or variable cementation. They
present in surface RmR
b
values between 60 to 75
and they are considered as T2 terrain.
They are formed in the fund or semi-closed lake
basins by sedimentation of fine or very fine clay-
size detritus. They also could have formed by
alteration by intense alteration of surface of rocky
material. in both cases, the resulting material is
usually silty or clayey nature. They are generally
soft soil type T3j.
2.10
Unit X: Anthropic filled
They are defined as soft terrains type T3f, unfit to
build buildings unless improvements or reinforce-
ments carried out to increase their properties.
2.5.2
Subunit Vb
Pyroclastic materials loose or weakly cemented:
non-compact and easily collapsible. They form
when magma fragments fall and settle near the
eruptive center. There are two types if geochemical
composition is considered: basaltic and salic. The
smaller basaltics are called lapillis (2 to 15-20 mm),
the largest are called scoria. salic deposits (trach-
ytes or phonolite composition) form pumice, light
and porous. Both are considered as T3b terrain.
3
GeoTechnical sURVeYs
once the type of building and type of land are
defined, guide criteria must apply in order to deter-
mine the minimum intensity and extent of the field
exploration activities.
Recognition or surveys points relate to bore-
holes in this guide. here, in this guide, boreholes
are exploration drilling carried out by the rotation
system with continuous extraction of intact cores.
2.6
Unit VI: Volcanic breccias
This unit is associated with violent eruptive epi-
sodes of high explosivity. The final result is a
chaotic and brecciated mass formed by blocks of
different nature, generally very sharp. Grain size is
variable. Matrix is fine and more or less cemented
and occasionally very hard. These materials are
very thick (up to hundreds of meters).
They exhibit characteristics of hard rock and
sometimes of medium hardness. They present RmRb
values of between 65 to 75 and they are considered
land type T2 terrain.
3.1
Number of recognition points
at least three points of recognition is established
and a maximum distance (dmax) between survey
points. For the purpose of this guide, the maxi-
mum distances between points of recognition are
showed in Table 4.
For T-3 terrains or when the survey is derived
from another which has proved inadequate, recog-
nition points are interspersed in the problem areas
until they are sufficient to characterize the ground
properly. absence of singularities under the foun-
dation level and excavation fronts must be checked
in order to ensure the security of the building, con-
struction process and neighboring buildings.
in geotechnical units iii and iVb, the surveys
will be carried out as Table 4 indicated, by the
rotation system with continuous extraction of
intact cores. additionally, exploration will also be
required under each load transfer element of the
2.7
Unit VII: Alluvial-colluvial deposits
These deposits are made of sand and very hetero-
metric fragments ranging in size from centimeter
to over a meter and rounded or subrounded forms.
The matrix, of detrital nature, may be abundant or
absent. They are soft or loose terrain type T3c.
2.8
Unit VIII: Coastal sands
This unit consists of beach deposits of loose basalt
dark sand or silica clear sand or calcareous nature,
extensions or sediment which have been trans-
ported by marine or aeolian process (dune forma-
tions). They appear along the coastline and ravine
mouths. They have low to very low bearing capac-
ity and are soft or loose terrain type T3c.
Table 4.
Maximum distance (dmax) between survey
points.
Building type
T-1
T-2
T-3(*)
c-0, c-1
35
30
60
c-2
30
25
50
c-3
25
20
40
2.9
Unit IX: Clay and silt soils
These deposits consist of residual soils and lake
sediments which are essentially clay and/or silty.
c-4
20
17
34
(*) apply only for geotechnical units iii and iVb.
