3145
Geotechnical characteristics of glacial soil deposits at Punta Arenas in Chilean
Patagonia
Caractéristiques géotechniques des dépôts glaciaires du sol à Punta Arenas en Patagonie
chilienne.
Vásquez A., Le Roux J.-P.
Universidad de Chile
Foncea C.
SRK Consulting (Chile) S.A
ABSTRACT: In the city of Punta Arenas, Chilean Patagonia, soils of glacial origin were deposited during the Last Glacial Maximum
(LGM), with soft, organic and alluvial materials dominating during interglacials and the post-glacial period. The superposition of
sedimentary environments together with glacial loading and unloading events, resulted in a heterogeneous stratigraphic succession
with a variety of mechanical properties. This study proposes a geotechnical classification for the different deposits, which include
tills, glacio-lacustrine and overbank materials observed in the urban and sub-urban areas of the city. Undisturbed samples obtained
from excavations and open pits were tested for their index properties, shear resistance and compressibility. Additional tests included
X-ray diffraction. For the geotechnical classification a composite mapping technique was used which included information of a great
number of existing borings. It was established that low quality soils are located in depressions molded by glacial action as well as in
overbank areas, which were filled by layers of normally consolidated clays and organic materials interbedded with coarser-grained
deposits.
RÉSUMÉ: Dans la ville de Punta Arenas, Patagonie chilienne, les sols d'origine glaciaire ont été déposés au cours du dernier
maximum glaciaire (DMG), dont des matières meubles, organiques et d'alluvions dominent pendant les périodes interglaciaires et la
période post-glaciaire. La superposition des environnements sédimentaires avec des événements de chargement et de déchargement
glaciaires, ont-ils abouti à une succession stratigraphique hétérogène avec une variété de propriétés mécaniques. Cette étude propose
une classification géotechnique pour les différents dépôts, qui comprennent l’amas de débris mineral de la moraine et les dépôts
glacio-lacustres. Des échantillons intacts provenant de fouilles ont été testés pour leurs propriétés d'index, de la résistance au
cisaillement et la compressibilité. Des tests supplémentaires ont été inclus comment ceux de diffraction des rayons X. Pour la
classification géotechnique nous avons utilisé une technique de cartographie composite. Il a été établi que les sols de mauvaise qualité
sont situés dans des dépressions résultant du travail du glacier ainsi que dans les zones de débordement, qui ont été remplis par des
couches d'argiles normalement consolidées et des matières organiques interstratifiés avec des dépôts de grain plus grossier.
KEYWORDS: Magellan Lobe, Patagonia, glacial clay, till, consolidation, composite maps
1 INTRODUCTION.
The city of Punta Arenas is situated on the western shore of the
Magellan Strait, (see figure 5). The pressure of rapid urban
development has led to building construction on soils that are
almost always saturated, and due to their low bearing resistance,
high compressibility and excessive variation, present
unfavorable conditions for foundation design. The origin of the
soils is mainly glacial, being due to the cyclic advance and
retreat of a glacial lobe during the LGM, which caused
stratigraphic successions that are highly variable both laterally
and vertically. Temperate conditions during interglacial events
and the post-glacial period, allowed the deposition of soft clays
and the proliferation of peat bogs. Sand and gravel were also
deposited within the channels of a drainage system.
The local physiography is one of the most important factors
in the classification or zonation of the soils. In the context of
surface hydraulics, Punta Arenas is crossed in an east-west
direction by six natural channels: the Los Ciervos, La Mano,
and Las Minas Rivers, and the D’Agostini or Pitet, Llau-Llau,
and Bitsch Creeks (see figure 5). The drainage patterns are
variable; the southern area is dominated by the relict glacial
morphology, with the La Mano River mostly confined to a
glacial valley. In the central area, the Las Minas River is
dynamic and frequently overflows its banks, with occasional
mud flows being considered a risk factor. Finally, in the north,
the hydraulic pattern is more disperse with little incision. This is
problematic with regard to the impermeable basal sediments
that increase the risk of inundation. Fine overbank sediments
and peat bogs are thus situated in areas with low gradients,
mostly in the northern part of the city, whereas coarser
sediments are deposited by the Las Minas River in the central
area.
Understanding the complexity of the geological processes
that operated in the area, allows the soils to be classified
according to their origin and loading history, which is a key
factor in their geotechnical characterization. We propose a
paleogeographic interpretation method that includes SPT
(Standard Penetrometer Test) values, and differentiates and
demarcates soils of low bearing capacity.
2 THE LGM ADVANCE AND RETREAT
In the study area at least five glacial advances (A - E) of the
Magellan Lobe are recorded, which were dated using
14
C and
cosmogenic isotopes (Bentley 2005, Clapperton, 1995,
McCulloch, 2005a). The LGM is represented by glacial advance
B and is characterized by the evolution of the Juan Mazía
Peninsula (figure 5), which happened after 31,250 calibrated
years before present (Cal yr BP) and culminated between
25,200 – 23,100 Cal yr BP, followed by advance C of lesser
extent, dated between 22,400 and 20,300 Cal yr BP. Glacial
advance D, also of smaller extent, culminated between 17,700
and 17,600 Cal yr BP, succeeded by a glacial retreat. Glacial
advance E caused the damming up of a lake between 15,500 and
11,770 Cal yr BP. The retreat of this glacier coincides with the
maximum cooling period referred to as the Younger Dryas in
