As the soil next to the piles is removed horizontal boards known
as lagging are introduced as shown in the figure and are wedged against
the soil outside the cut. As the general depth of excavation advances from
one level to another, wales and struts are inserted in the samemanner as
for steel sheeting.
If the width of a deep excavation is too great to permit
economical use of struts across the entire excavation, tiebacks are often
used as an alternative to cross-bracings as shown in Fig. 20.26(c).
Inclined holes are drilled into the soil outside the sheeting or H piles.
Tensile reinforcement is then inserted and concreted into the hole. Each
tieback is usually prestressed before the depth of excavation is increased.