FEATURE
into account before or during the design stage to avoid the type of situation where a service trench cuts into the geofabric after a wall has been built, rendering it useless, or an irrigation system is installed with similar consequential damage. Design flaws can also occur when the main contractor changes the original design
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without consulting the CRB design engineer. This could involve raising the wall or changing a kerb position, which can alter and ultimately compromise the safety and structural integrity of the wall. To avoid the scenarios outlined above, adherence to the SANS standards as
Ground conditions
well as to the CMA’s Installation Manual,
Geotechnical reports are mainly in-
Code of Practice for Gravity Walls, Design
tended for piling and building foundation
required less frequently and the results
of Reinforced Walls, Design Checklist and
designs and seldom include shear box or
can be correlated with DCP tests to
various project reviews such as Hydraulic
triaxial tests. This means that if design
ensure compaction uniformity.
Applications, should be mandatory refer-
engineers rely solely on a geotechnical
Stormwater management should be
ence material. They remove the ambiguity
report, their assumptions can be off the
the overall responsibility of the profes-
as to where responsibility for design and
mark by a considerable margin; this is why
sional team, the project managers and
construction procedure lies. One of the
every site must be inspected before wall
the main contractor. It is unreasonable
important rules covers design devia-
construction commences.
to expect a sub-contractor to manage
(Above): Sub-surface drains against a cut face.
tions and how best to handle them. In
The initial geotechnical investigation
the other sub-trades such as bulk earth-
this instance, good liaison between the
must also include undisturbed shear box
works, plumbing and bricklaying, whose
project engineer and CRB design engineer
or triaxial test results as per SANS 207
site works often have a direct impact
is a prerequisite.
if the project is to include a retaining wall
on the CRB contractor’s progress and
element. But even thorough geotechnical
performance.
Washed away
soil tests can miss hazards which could
Most walls fail because their design
undermine the integrity of a wall, such as
On-site supervision
or structural weaknesses make the fill
a large rock, underground water or poor
Temporary stormwater protection during
prone to water saturation. Irrigation
geological soil intrusions.
the works is vital. Failure to attend to it
systems, which often leak and block with
Some design engineers insist on a
debris, can also undermine the integrity
troxler test, a nuclear device which
of a wall. It therefore makes sense for
tests compacted soil density. As this is
For example, stormwater drains are
design engineers to be involved in the
expensive and involves testing at every
often blocked. In addition, wall drains
specification of irrigation systems, as
300mm, it is generally only used when
are sometimes covered with concrete
this helps lessen the threat of leaking.
the walls are very high or when there is
when a drain channel, walkway or road is
In an ideal world, the backfill of CRB
a substantial amount of fill. DCP indicator
built next to a wall. This type of mishap
walls would contain a substantial per-
testing is used extensively by experi-
demonstrates why it is crucial for the
centage of coarse material and a low
enced contractors and their appointed
design engineer not only to design the
percentage of fines (<15% minimum
design engineers to check the indicative
wall, but to oversee its construction with
passing a 75 micron sieve). This would
degree of compaction and fill quality as
regular site visits.
reduce wall failure incidence dramatically,
work progresses. It is inexpensive and
but importing selected fill is an expensive
takes little time, but can be invaluable.
option, which is why soil found on site
will cause erosion and localised collapses, sometimes requiring a costly rebuild.
In summary, if a significant reduction in the number of CRB wall failures is to be achieved, a substantial change of atti-
is generally selected, although it may
Testing required
tude from the entire professional project
require improving.
As the compaction and classification of
team, including the CRB wall contractors,
Before engineers can design a wall,
backfill is critical, all CRB contractors
is required. It is certainly an attainable
they need accurate information on the
should initiate regular DCP indicator
goal and one that needs pursuing.
internal friction angle (Ă&#x2DC;) of the soil and
compaction tests before laying founda-
its cohesion by conducting a shear box
tions and during the construction of a
or triaxial test. SANS 207 prescribes
wall. They should also take regular photos
this test, yet in the majority of retaining
as proof that the walls were built and
wall sites, it is not done. Instead, as a
detailed according to design.
fall-back measure, design engineers use
DCP indicator tests should be man-
the geotechnical report, which includes
datory up to the full height of the fill at
results on the fines content of the soil.
1 000mm intervals. Troxler tests are
PRECAST | ISSUE THREE | 2017
APOLOGY In a previous article (July issue) on failed retaining walls, Brennan Small was referred to as Vernon Small and we extend our apologies to Brennan for this error.