Tímarit Verkfræðingafélags Íslands - 01.12.1964, Síða 35

TlMARIT VFl 1964
91
7. Niðurstaða.
Niðurstaða þessarar tilraunar bendir ótví-
rætt til þess, að eiginleiki aurmettaðs vatns
til þess að þétta undirlag sitt sé atriði,
sem taka má tillit til við hönnun vatnsvirkj-
ana á hraunum. Yfirleitt eru aðstæður þannig
við stíflugerð á hraunum, að varla er hætta á,
að undirstaðan grafist af lekavatni. Er áhætta
því mjög lítil að gefa ánum sjálfum tækifæri til
þess að þétta jafnvel mjög lek hraun. Reynslan
frá millilaginu í holu DI-3 bendir til þess að
sandsía þéttist betur og fljótar en hraunin sjálf.
Þetta atriði getur verið vert að hafa í huga í
sambandi við hönnun á stórum uppistöðulónum,
þar sem sandurinn í aurburðinum mun ekki ná
upp á grynningar fjarri innrennsli. Þesskonar
sandsíur eru raunverulega til á yfirborði margra
gamalla hrauna, sem sandorpin eru orðin eða
gróin.
Aurburður er réttilega talinn mikill bölvaldur,
því hann fyllir uppistöðulón og eyðileggur með
því vatnsmiðlanir. Sennilega mun aurburður í
Þjórsá og Jökulsá á Fjöllum teljast í milljónum
m3 árlega, sem að töluverðu leyti mun setjast
í væntanleg rafstöðvalón. En á móti leysir aur-
burðurinn annað vandamál jarðtæknilegs eðlis,
sem allajafna er mjög kostnaðarsamt að leysa
á annan hátt. Þetta vandamál er leki úr uppi-
stöðulónum á hraunum runnum eftir ísöld, en
einmitt á hraunasvæðunum eru margir að öðru
leyti mjög álitlegir virkjunarstaðir.
Heimildir:
Bureau of Reclamation 1960: Earth Manual. Denver.
Einarsson, í>. og Tómasson, H. 1962: Búrfell, General
Geology (skýrsla til raforkumálastjóra).
Harza Engineering Company International 1963: Búr-
fell Project, volume II, appendix B, Geology. (Skýrsla
til raforkumálastjóra).
Harr M. E. 1962: Groundwater and Seepage, New
York.
Kjartansson, G. 1953: tír sögu Helliskvíslar. Nátt-
úrufræðingurinn 23: 1—13.
Kjartansson, G. 1961: Tungnaá, skýrsla um jarðfræði-
atnuganir á hugsanlegum virkjunarstöðum. (Skýrsla til
raforkumálastjóra).
Thoroddsen, S. og Tómasson, H. 1964: Dam construct-
ion in a geology of bedded basalt lava flows, as eluci-
dated by conditions in Iceland. Eight International Con-
gress on Large Dams, Edinburgh.
S U M M A R Y
Seepage anil sediment lo:ul.
Considerable part of the water power potentials in
Iceland is located in areas where rivers are flowing on
very young and highly permeable strata, which are
mainly postglacial lava flows. Many of these rivers have
considerable sediment load of which the more fine
grained part largely comes from the glacier fed com-
ponents of the rivers. It seems that the sediment load
has tightened the river channels to such extent, that at
present the rivers have no connection with the ground
water table which may be tens of meters below the
riverbed.
The tightening effect of the sediment load might be
of great value in connection with dam projects in these
areas, but the rapidity of the tightening process is un-
known. In 1963 an experiment was performed in order
to find out whether the tightening by the river Thjórsá
is rapid enough to be taken into account when dams
are projected on postglacial lavas.
Firstly the permeability of the lava fields is described,
and it seems that their permeability is of the same
order as for open work gravel. This is elucidated both
from tests in bore holes and by the ground water flow
through the restricted area between Búrfell and Sauda-
fell, which is fairly well known. Secondly it is known
from geological evidence that the permeability of the
lava fields is different, in vertical and horizontal
direction and much higher in the horizontal one. The
vertical permeability can be decreased in two manners,
either by accumulation of sand and silt, (which is much
less permeable than the lava) on top of the lavabed or
by seepage of sediment ladden water through the lava,
where the sediment load is left behind in joints and
openings in the lavabed and gradually seals it. There
is also geological evidence of big lakes having been
dammed up by lava flows and probably in due time
then became tight. Krókslón is the biggest of these
former lakes and had an area of about 17.5 km= and
existed for an interval of about 1500 to 2000 years.
The test was performed by pumping water from the
river Thjórsá into a borehole situated in the lavafields
at Búrfell. The hole is considered as a seepage path
through the lavabeds and the tightening of the hole
as an example of how natural leakage paths are sealed
by seeping water.
The test was performed in the following manner:
1) Permeability test I
2) Tightening test I
Pumping of water from Thjórsá river into the hole
for 17 days. (Samples were taken from the water and
its sediment load analysed. The results are shown in
table IV and fig. 4).
3) Permeability test II
4) Tightening test II
Pumping of water as previously for three days.
The results of tightening test I show that the water
take of the hole decreased to 1/100 part of what it was
when at maximum. Then 3500 ms of water had been
pumped into the hole, containing approximately 950 kg
of sediment load. In permeability test II the sediment
fillings were washed out again, but still the permea-
bility was much lower than previously. In tightening
test II, the water take was similar to that of the llth,
12th and 13th day of tightening I. The interbed was
completely sealed by tightening test I and allowed a
pressure of 6—7 kg in permeability test II without