OPENINGS IN SOFT GROUND.
a. Ground Behavior. The method of construction of tunnels depends upon
the response of the ground during and after excavations. The stand up time
depends upon the type of soil, the position of groundwater, and the size of
opening. Depending upon the response during its movement period, the ground
is classified as: (1) firm, (2) raveling, (3) running, (4) flowing, (5)
squeezing or (6) swelling.
(1) In firm ground, no roof support is needed during excavation and
there is no perceptible movement.
(2) In raveling ground, chunks or flakes of material begin to fall
prior to installing the final ground supports. Stand up time decreases with
increasing size of excavation. With rising groundwater, raveling ground may
become running ground. Sand with clay binder is one example of this type of
(3) In running ground, stand up time is zero. The roof support
must be inserted prior to excavation. Removal of side supports results in
inflow of material which comes to rest at its angle of repose. Dry
cohesionless soils fall into this category.
(4) Flowing ground acts as a thick liquid and it invades the opening
from all directions including the bottom. If support is not provided, flow
continues until the tunnel is completely filled. Cohesionless soil below
groundwater constitutes flowing ground.
(5) Squeezing ground advances gradually into the opening without any
signs of rupture. For slow advancing soil, stand up time is adequate, yet
the loss of ground results in settlement of the ground surface. Soft clay
is a typical example of squeezing ground.
(6) Swelling ground advances into the opening and is caused by an
increase in volume due to stress release and/or moisture increase.
Pressures on support members may increase substantially even after the
movement is restrained.
b. Loss of Ground. As the underground excavation is made, the
surrounding ground starts to move toward the opening. Displacements result
from stress release, soil coming into the tunnel from raveling, runs, flows,
etc. The resulting loss of ground causes settlement of the ground surface.
The loss of ground associated with stress reduction can be predicted
reasonably well, but the ground loss due to raveling, flows, runs, etc.
requires a detailed knowledge of the subsurface conditions to avoid
unacceptable amounts of settlement. For acceptable levels of ground loss in
various types of soils see Reference 23, Earth Tunneling with Steel
Supports, by Proctor and White.