ABSTRACT
W e distinguish wide subsidences, settlements and lowerings.
Subsidences are tectonic procrsses, witk could not yet be shown in the north coastal r-gion by repeated “Feinnivellements”. The notcd changes of niveau can n o w only be explained by rising of the water levcl.
Settlement are found in limited regions, in which the relation between water and soil has been disturbed by the withdrawal of water.
Lowerings originate from soil, regional limited, which is compressed and condensed by pressure.
Our examination is rectricted to settlements in “holozäne” sea and river marshes, which are blocked by dikes against floods with salty sea and brakish river water.
The soil of the sea marshes mainly consists of pure sand with more or less organic components (silt). In river marshes organic components are dominate (moor).
These days the demands of intensive utilization of the lower grounds has caused a deeper withdrawal of water, that is obtained by the s u m of withdrawing water (bigger ditches, drainage sluice, and pumping stations). The waterlevel willbe held lower in relation to the land surface and absolutely lower than necessary for extensive utilization.
Fixed pipe supports and lowering plates are uscd to measure settlements. Fixed pipe supports are built through the stratum of “holozäne” just into the “pleistozäne” by taking care of certain demands for the purpose of avoiding them to take part in the settlement.
Plates are located 25 c m undcr the surfac:.
Each vear the height of the fixed pipe supports and plates is controlled and deter- minated by “Feinnivellements”. In diagrams of time and position the change at height will be computed. These measurements are completed by observing the ground water.
RESUME
Nous distinguerons les larges affaissements, les tassements et les dépressions.
Les affaissrments sont dus à des processus tectoniques qui ne peuvent pas être montrés maintenant dans la région côtière allemande par des nivellements répétés. Les change- ments de niveau considérés ne peuvent être expliqués actuellement que par la montée du niveau d’eau.
Les tassements se rencontrent dans des régions limitées dans lesquelles les relations entre l’eau et le sol ont été perturbées par les pompages.
Les dépressions ont leur origine dans le sol régionalement limité, qui est comprimé et condensé par la pression.
Notre étude est réduite aux tassements dans les marais fluviaux et maritimes qui sont protégés par des digues contre les crues d’eau de mer salée ou d’eau de rivière saumâtre. L e sol des marais maritimes consiste surtout en sable pur avec plus ou moins de matières organiques (vase). Dans les marais fluviaux les matières organiques ont la prépondérance.
A l’heure présente, l’utilisation intensive des sols bas a provoqué un retrait consi- dérable de l’eau obtenu par des dispositifs d’évacuation des eaux (fossés plus larges, écluses de drainage, stations de pompage).
Des supports fixes de canalisation et des plaques sont utilisés pour la mesure des tassements. Les premiers traversent l’holocène jusque dans le Pleistocene en prenant certaines précautions pour qu’ils ne prennent pas part au tassement.
Les plaques sont placées 25 c m sous la surface.
Des nivellements de précision déterminent chaque année la cote des repères. Des diagrammes présentent les résultats des mesures qui sont complétés par des obser- vations sur l’eau souterraine.
R. Dolezal and M. Peterser
GENERAL R E M A R K S
W e distinguish between subsidences, settlements and lowerings.
Subsidence is a tectonic phenomenon involving comparatively large areas. The pos- sibility of such a subsidence in this region was investigated by comparing two North Sea coastal levellings, the fìrst made in the years 1928-1931 [9] and the second between 1948- 1955. The base lines were arranged on high and dry land upon the Pleistocene “Geest ”.
Five subsoil points each about 2+ meters below the G e r m a n land level (N.N.) were considered and the levels related to a bench mark at a point on a large diluvial sand area nearly 150 kilometers distant from the coast.
Comparisons of these two surveys indicated that the observed changes in the relative elevation of water and land could not be explained by subsidence of the coastal region and must therefore be explained by a rise in water level.
Settlements are of a more limited extent than subsidences and are due to a disturbance of the relationship between the soil and water [6] due to de-watering. On moorland, bor or peatisubsoil the settlement may be very great. Such strata compressed by the dead weight of the soil above may lose substance by chemical means [3].
In the years immediately following poldering the volume of fresh marsh clay will be reduced by de-watering, de-salination, de-liming and by the deprivation of nutritive material. In comparison with this loss the loss of mass in old marshes is slight. The groundwater level is high and the loss of soluble salts occurs only in the upper two meters.
Borings have shown that deeper clay strata are íìrm and tight.
Thus large settlements in these circumstances are not possible. Thus tectonic subsidence and regional settlements can be measured and explained
[lo]
[li].Recent intensive cultivation of the lowlands involving bigger ditches, drains, sluices and pumping works have caused a deeper de-watering in these areas. Currently the water level is held lower in relation to the land surface and absolutely lower than formerly.
Lowerings are of even more limited extent. The soil is compressed and condensed by artifical loads such as buildings, streets, dams, dykes, sluices etc. [2] [5]
S E T T L E M E N T A R E A S A N D M E A S U R I N G M E T H O D S
Our examination was restricted to settlements in “holozäne” seamarches and river mar- ches protected by dykes against floods of sea water and brackish river water.
Figure 1 shows the regions involved between the North Sea, the Baltic Sea and the mouth of the river Elbe, The surface of these lowlands is generally below NNl-2.50 m but goes down to -2.50 m in the river marshes [SI.
The following two representative examples serve to explain the investigations carried out.
Seumursh [i] with relatively young sandy-clay soil, poldered since 1925 and currently de-watered by sluices.
River Marsh [2] with relatively high parts on bog, moor, peat and clay subsoi1,poldered since the middle ages and because of the low position, de-watered by pumping works [4].
Depending on different soil structures there are many different methods suitable for observing settlement. In sector 1, fixed pipes were driven through the stratum of holozäne and were surrounded by a group of three plate measuring stations located in the highest strata 25 centimeters below the land surface.
In the second sector (river marsh) the geological structure is much less regular. There- fore, an extensive system of sixteen settlement measuring stations was chosen. Three of these (A, B and C) are discussed in this paper.
The soil profiles belonging to the fixed pipe supports shows the geological section (Wilster march 2).
36
FIGURE 1.
cast plate: with fixed point bolt
sight of the: bolt
FIGURE 2.lFixed pipe support plate for level of settlement
R. Dolezal and M. Petersen
FIGURE 3. Geology section Wilster marsh It