UNDERGROUND ENGINEERING - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 260

bedding

R,i

(K)

P

K

(1)

R

u

R,i

contraction

u R

bedding

LOW
STRESS

HIGH
STIFFNESS

(K)

(K)

R,i

R,i

u MAX

u MAX

contraction

contraction

u R

u R

Cavity

Figure 15.10 Tunnel shield design:
(above) soil effect simulated by a simple spring (Duddeck's method);
(below) soil stiffness: low stiffness (left), high stiffness (right); the grey box shows the
corresponding cavity (free space between soil and shield); u R is the oversize of the drill
wheel with respect to the shield .

Tertiary glauconitic sands can show much greater variation than one may
imagine. Temporarily, a void can arise, where the shield contact is released, or a
very stiff reaction is generated, where the shield contact is intensified. Similar
experience is found in pipe jacking: the boring hole is rather stable, but steering
corrections are hardly possible. The clay content and sand density determine this
behaviour. A combination of facts contributed to the excessive shield deformation:
drift, glauconitic sand and high water pressure. The soil stiffness chosen in the
design did not reflect the most critical condition at the lowest position. The extreme
bedding (cavity) and high pore pressure condition there could have been foreseen,
when the available soil data and experience had been considered with sufficient
care. However, the execution of specific on-site soil investigation was extremely
difficult and the apparent high horizontal stresses (high K 0 -value) were unexpected.
Fortunately, the project was finished with almost no delay and a successful tunnel
is now in fine operation.