3447
Settlement velocity measured over ten years in major-scale shallow foundations on
a preloaded 20-m thick silty alluvial layer
Velocité des affaissements mesurés sur dix ans, sur une foundation superficielle de grandes
dimensions, sur une couche alluviale limoneuse de 20 m d’épaisseur préchargée
Dapena E.
Geotechnics Laboratory, CEDEX, Madrid Polytechnical University, Spain
Román F.
Confederation Hidrográfica del Norte, Ministry for the Environment, Spain
Pardo de Santayana F., Cuéllar V.
Geotechnics Laboratory, CEDEX, Madrid, Spain
ABSTRACT: The site of the GALINDO Waste Water Treatment Plant stands on a layer of alluvial deposits between 15 and 20 m
thick which has been subjected to an average preloading of 2 Kg/cm
2
for over 20 years. The Phase 2 biodigester tanks take up two 62
x 195 m rectangles, separated by a 23 m wide central corridor in which the layer of alluvial sediment is 15 m thick. Construction of
the slabs and walls of the deposits and the load of the water during normal operation of the plant bring about an increased load of 1.1
Kg/cm
2
, giving rise to a net increase in load, as compared to that of the previous five years, of 0.14 Kg/cm
2
, while still remaining
below the preconsolidation pressure present from the beginning. According to the settlement measurements recorded, ten years after
construction, three different areas can be seen at the site of these biodigester tanks: the area corresponding to Deposits 1 and 4, which
has low compressibility, where the maximum settlement recorded was 32 and 40 mm; the area corresponding to Deposits 3 and 6,
with average compressibility and maximum settlement recorded at 114 and 108 mm and that of Deposits 2 and 5, located in the
intermediate area, with high compressibility and maximum settlement recorded at 217 and 156 mm. One obtains the settlement
velocity distribution and analyzes them for each of the four deposits that have significant movement, comparing the average velocity
corresponding to the period between 0.5 and 9 years, and the average velocity for the period between 8 and 10.3 years.
RÉSUMÉ : le site de la Station de traitement des eaux usées de GALINDO repose sur une couche de dépôts alluviaux de 15 à 20 m
d’épaisseur qui a fait l’objet d’une surcharge moyenne de 2 Kg/cm
2
pendant plus de 20 ans. Les réservoirs du digesteur anaérobie de
la Phase 2 utilisent deux rectangles de 62 x 195 m, séparés par un couloir central de 23 m de large dans lequel la couche des sédiments
alluviaux mesure 15 m d’épaisseur. La construction des dalles et des parois des dépôts et le chargement de l’eau pendant le
fonctionnement normal de la station entraînent un chargement supplémentaire de 1,1 Kg/cm
2
, ce qui donne lieu à une augmentation
nette du chargement de 0,14 Kg/cm
2
, si nous le comparons à ceux des cinq années précédentes, tandis qu’il reste au-dessous de la
pression de préconsolidation présente depuis le début. Conformément aux mesures de tassement enregistrées, dix ans après la
construction, trois zones différentes peuvent être observées à l’endroit de ces réservoirs du digesteur anaérobie : la zone correspondant
aux Dépôts 1 et 4 qui présente une faible compressibilité, où les tassements maximums enregistrés étaient de 32 et 40 mm ; la zone
correspondant aux Dépôts 3 et 6 présentant une compressibilité moyenne et un tassement maximum enregistrés de 114 et 108 mm et
ceux des Dépôts 2 et 5 qui se trouvent dans la zone intermédiaire, présentant une compressibilité élevée et un tassement maximum
enregistré de 217 et 156 mm. Nous obtenons la distribution de la vitesse de tassement et nous l’analysons pour chacun des quatre
dépôts qui présentent un mouvement significatif. Nous comparons la vitesse moyenne correspondant à une période de 0,5 à 9 ans et la
vitesse moyenne sur une période de 8 à 10,3 ans.
KEYWORDS: settlement, alluvial sediments, waste-water treatment plant, blast furnace slag
1 INTRODUCTION
The foundations of the phase 2 biodigester tanks at the Galindo
Waste Water Treatment Plant are located on top of a layer of
dark gray silty clay alluvial sediments between 15 and 20 m
thick. See Fig. 1. Beneath that is a compact layer of clay with
gravel between 1 and 2 m thick, which extends down to solid
rock.
This soil was preloaded for over 20 years with a layer of
blast furnace slag, which transferred an average preload of 2
kg/cm
2
. However, the distribution of the preload on the site of
the phase 2 biodigesters was not known for certain, and this
may explain the different behavior of the pools in response to
settlement.
In addition, for the 6 years prior to construction of the phase
2 biodigesters, there was a uniform preload throughout the
entire area on a scale equal to that which would be transferred
by the biodigesters. However, when they were built, the initial
design was modified and the load transferred was 0.14 kg/cm
2
greater than the load of the soil removed.
In Figure 1, it is possible to see that in the middle section of
the site, precisely where pools 2 and 5 are located, the level of
alluvial sediments is high, at the height of the support level for
the foundation slab of pools 2 and 5. However, at the sites of
pools 3 and 6, the surface of these deposits is 2.5 m below the
foundation height, and the alluvial deposits are at an average
depth of 3.5 m at pools 1 and 4.
The influence of preloading on settlement due to primary
consolidation has been studied by Johnson (1970), Mitchell
(1981), and Statmatopoulos and Kotzias (1983), among others.
Secondary consolidation has been studied by Jamiolkowski, et
al. (1983), Magnan (1984), Kousotfas, et al. (1987), Yu and
Frizzi (1992), Alonso, Gens and Lloret (2000 and 2001), and
Alonso (2004).
