230
Coastal Engineering: Theory and Practice
7.9.5 Sand bags or grout filled bags
Stabilisation of scour holes is usually done using sand bags which require
sufficient time for their installation at the desired location. Such measures
are effective only for a shorter duration since they are likely to be undermined. Sand bags or grout filled bags are also used to underpin pipeline free
spans, but may then require a protective covering of gravel or mattresses
to provide protection to the underpinning material and to stabilise the bed
around the pipeline [Van Dijk, 1980], prior to the installation of a protective
layer of rock or mattresses.
7.9.6 Concrète grout
Voids, e.g. below footings, can be filled with concrète grout. Where this is
carried out underwater the grout can be injected using tremie pipes. Formwork is needed to contain the grout, which can take the form of concrète
bags or sheet piles, as described above, as well as conventional formwork.
Grouting is carried out from the bottom up. As grouting progresses the
tremie pipe should remain embedded in the freshly placed grout. Formwork
should be watertight to prevent loss of grout and held down by dowels or
similar to prevent uplift generated by pressure of the grout. The top of the
formwork should be protected to avoid the loss of fines and cernent, but
vented to indicate when the formwork is full. A typical arrangement for the
top of the formwork may consist of a highly permeable fabric held down
with wire mesh, in turn held down with plywood.
7.9.7 Structural improvements
Laying deep foundations beyond the lirait of maximum expected scour is one
of the most sought after and effective structural solution. Structures including sills, drop structures etc. are often built in the close vicinity of bridges.
The effect of contraction scour can be mitigated by erecting new spans
facilitating additional flow area and thereby, reducing the flow velocity.
7.10
Bed Shear Stress
7.10.1 Bed shear stress due to waves
Sédiment is mobilized under combinations of wave-current flow when the
value of maximum shear stress, rmax exceeds rcr. (rcr is threshold value
Coastal Engineering: Theory and Practice
7.9.5 Sand bags or grout filled bags
Stabilisation of scour holes is usually done using sand bags which require
sufficient time for their installation at the desired location. Such measures
are effective only for a shorter duration since they are likely to be undermined. Sand bags or grout filled bags are also used to underpin pipeline free
spans, but may then require a protective covering of gravel or mattresses
to provide protection to the underpinning material and to stabilise the bed
around the pipeline [Van Dijk, 1980], prior to the installation of a protective
layer of rock or mattresses.
7.9.6 Concrète grout
Voids, e.g. below footings, can be filled with concrète grout. Where this is
carried out underwater the grout can be injected using tremie pipes. Formwork is needed to contain the grout, which can take the form of concrète
bags or sheet piles, as described above, as well as conventional formwork.
Grouting is carried out from the bottom up. As grouting progresses the
tremie pipe should remain embedded in the freshly placed grout. Formwork
should be watertight to prevent loss of grout and held down by dowels or
similar to prevent uplift generated by pressure of the grout. The top of the
formwork should be protected to avoid the loss of fines and cernent, but
vented to indicate when the formwork is full. A typical arrangement for the
top of the formwork may consist of a highly permeable fabric held down
with wire mesh, in turn held down with plywood.
7.9.7 Structural improvements
Laying deep foundations beyond the lirait of maximum expected scour is one
of the most sought after and effective structural solution. Structures including sills, drop structures etc. are often built in the close vicinity of bridges.
The effect of contraction scour can be mitigated by erecting new spans
facilitating additional flow area and thereby, reducing the flow velocity.
7.10
Bed Shear Stress
7.10.1 Bed shear stress due to waves
Sédiment is mobilized under combinations of wave-current flow when the
value of maximum shear stress, rmax exceeds rcr. (rcr is threshold value