Environmental Engineering Reference
In-Depth Information
a large proportion of the cross section. This was
interpreted as a protruding dyke from its profile
with respect to the old cliff.
in Figure 17 can be observed that it is not a sin-
gle dyke. There is another one embedded therein
having a more acid nature evidenced by a lighter
color. The dyke is covered by the material fell down
from the cliff some time before the arrival of the
lava flows spewed by san antonio Volcano. loose
stones remain stable because of grouting and short-
crete. Through the shortcrete layer the variable
grain size distribution and the total lack of cohe-
sion can be observed. at the contact section with
the dyke, through which water from the injection
flowed through, with no traces of cement, it can be
appreciated that the nature of the boulders fails to
correspond to cinders associated to volcanic lava
flows. They are stones flows carried by the front of
the lava flow that subsequently fell down from the
top of the cliff.
The presence of the dyke gave confidence that
the Fuente santa spring was found; suddenly, all
previous assumptions that provided keys to find
out the spring were clarified upon contact with
the dyke. From a hydrogeologic point of view such
vertical volcanic structure carried water from the
interior of the island, from the aquifer to the boul-
der beach . such a dyke was the geologic structure
that separated the thermal water at the side corre-
sponding to the gallery from the water of the aqui-
fer that hardly mixed with the thermal water, that
should spring up at the opposite side of the dyke.
Up to that point, the temperature at the excava-
tions made at the pebbles progressively increased,
and it was reached the foot of the dyke with a water
temperature of 45°. if the spring was there, after
crossing the dyke, the temperature should decrease
rapidly. We did so; after the gallery crossed the
dyke and the soleplate was immediately exca-
vated where a temperature of 30°c was detected;
through the thickness of the dyke (four meters) a
temperature gradient of 15°c was obtained.
The Fuente santa spring was found: after more
three hundred years it lay below our feet in front
of the dyke.
Because the gallery should eventually display the
spring, it was decided to excavate three more pools
to allow the Fuente santa waters flow through
them.
The problem to be solved was that if the excava-
tion through such cohesionless material was diffi-
cult to perform without it being saturated, it could
be even more difficult when an attempt was made
to excavate it under the phreatic water table. it was
then decided to use injections of water, sand and
cement. The problem was that an excessive increase
of cohesion could prevent the flow of water into
the pool because permeability could be lost upon
increasing of the cohesion.
The problem was solved by drilling inclined
borings at the side walls, in a downwards direc-
tion and separated. The objective was to build
battered micropiles to hold the material together
but at the same time allowing water to flow around
them toward the pools. To execute this pile system
a mix of water, cement and sand was injected, with
the addition of setting additives. in this way, the
grouting adopted under water the shape of bulbs
surrounding the slotted injection pipe, therefore
preventing cement from seeping into the aquifer.
in Figure 19 is depicted in the background, the
old cliff covering the section at the right of the
Figure 17. heading of the gallery showing the volcanic
dyke that conveys the water from the Fuente santa to the
coast.
Figure 18. cross section of the recovery gallery of
Fuente santa showing the procedure used to excavate
below the phreatic level.
