A Closer Look at Prevailing Civil Engineering Practice - European ...

A Closer Look at Prevailing Civil Engineering Practice, What, Why and How
Vincent T. H. CHU
9. What is the significance of free length and fixed length in tiebacks
in anchored excavation
The use of tiebacks in deep excavation allows uninterrupted earth moving
within the excavation zone owing to the absence of interior obstructions.
The spacing of tieback should not be placed too close as this may impair
the capacity of tieback because of the interference between adjacent
grouted zones.
A tieback is made first by drilling a hole by a drill rig, followed by placing a
bar in the drilled hole. Concrete is then poured in the hole and the
connection of tieback with wall is made lastly. A tieback anchor consists of
an anchorage located in a bearing layer and the anchor is tensioned at the
front face of the wall. The portion of the anchor which transmits the force to
the surrounding soil is called the "fixed length". One the other hand, the
"free length" of tieback transfers the force from the fixed length through the
anchor head to the wall.
Fig. 7.1 Free length and fixed length in tieback
10. What is the typical proportioning of a retaining wall
The base slab thickness of a cantilever retaining wall is about 10% of the
total height of retaining wall. The length of base slab is about 50-70% of the
total height of retaining wall. Generally speaking, the thickness of wall
stems may vary along the stem provided that its size should not be less
than 300mm to facilitate concrete placement.
For retaining wall with a total height exceeding 8-12m, it is recommended
to adopt counterfort retaining wall. The counterforts in counterfort retaining
wall are normally spaced at about 30% to 70% of the total height. The
length of base slab is about 40-70% of the total height of retaining wall.
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