1769
Effects of buried structures on the formation of underground cavity
Effets des structures enterrées sur la formation d’une cavité souterraine
Sato M.
Graduate student, the University of Tokyo
Kuwano R.
Associate professor, Institute of Industrial Science, The University of Tokyo
ABSTRACT: The number of “cave-in” accident, a sudden collapse of the ground like a pitfall, has been increasing nowadays in urban
roads. It is often caused by an underground cavity expansion without being noticed. A collapse happens when the cavity has reached
the surface ground. Many cavities are found close to underground structures. Some of those cavities are caused due to the breakage of
buried structures such as sewer pipes. Soil is flowed out from cracks of pipes with water. However, in some cases, obvious breakages
were not found in underground structures. In those cases, it is not clear why cavities tend to be generated around buried structures. It
is considered that water flows more easily around underground structures than in other part of the ground. Soil is carried with water
flow around buried structures. In this study, a series of permeability tests and model tests was conducted to investigate the effects of
buried structure on the formation of underground cavity.
RÉSUMÉ : Le nombre d’accidents dus à un affaissement, un effondrement soudain du sol comme dans une embuche, s’est accru
dernièrement dans les voies urbaines. Ces accidents sont souvent causés par une cavité souterraine s’élargissant sans que l’on s’en
aperçoive. Un effondrement se produit lorsque la cavité atteint la surface du sol. De nombreuses cavités se situent à proximité
d’ouvrages souterrains. Certaines de ces cavités sont dues à la rupture de structures enterrées telles que les canalisations d'égouts. Le
sol se répand dans les canalisations d'eau à travers les fissures présentes dans celles-ci. Toutefois, dans certains cas, des ruptures
évidentes n'ont pas été décelées dans les ouvrages souterrains. Dans ces cas-là, la raison pour laquelle les cavités ont tendance à se
produire autour de structures enterrées n’est pas claire. On considère que l'eau s'écoule plus facilement à proximité des ouvrages
souterrains que dans d’autres parties du sol. Le sol est transporté par le flux d'eau autour des structures enterrées. Dans cette étude,
une série d'essais de perméabilité et d’essais sur modèle a été réalisée pour examiner les effets des structures enterrées sur la formation
de cavités souterraines.
KEYWORDS: cave-in, sinkhole, erosion, cavity, model test, permeability test, underground structure, pipe
1 INTRODUCTION
Cave-in accidents in urban area are mostly caused due to broken
part of old buried pipes. Soil is washed out and flown into
brakages with water. Then underground cavities are formed.
1)
However, pipe’s failure was not always found with
underground cavities. The process of expansion of underground
cavities in such cases is not clarified. One supposition is that;
water flow is concentrated at the gaps between the ground and
buried structures and soil is drained through this “water
pathway”.
In this research, influence of buried structure on the
formation of underground cavity was evaluated by permeability
test and model tests.
Figure 1. Permeability coefficient of constant head tests with/ without
the cylinder (Toyoura sand)
2 PERMEABILITY AT THE GAP OF THE GROUND
To investigate water permeability at the gap between a buried
structure and the ground, permeability test was conducted. An
acrylic cylinder was installed into the center of the constant
head peameability test apparatus. Tested:material, relative
density, cylinder’s diameter and unevenness of the cylinder’s
surface were changed. An example of the test results is shown
in Figure 1. Permeability coefficient with the cylinder was
around 10% higher than that without the cylinder. In case that
cylinder’s surface was uneaven (0.2mm thickness, rectanglar
block of the plastic tape) with crossing direction of water flow,
permeability coefficient showed larger value.
2)
Figure 2 represented the influence of slight shaking of the
cylinder on permeability. Rotation of the cylinder made
permeability increase in dense ground but decrease in loose
ground, because rotation caused disturbance of the ground near
the cylinder. It was suggested that small shaking of the ground
caused increase of permeability around buried structures.
