MCU14/0106 Appendix E - Engineering Report

August 2004 -8- 04682104(AB)

 

6.1.4 Pond Construction

It is understood that stormwater ponds / basins will be constructed in the vicinity of the existing
drainage lines. Should bund walls be proposed, the walls should include a clay ‘core’,
preferably medium to high plasticity silty clay, compacted at moisture contents within the range
of -1% to +3% of optimum moisture content for Standard Compaction to achieve a dry density
ratio of at least 98% of the Maximum Standard Dry Density. In order to prevent shrinkage
cracking and leakage through the clay core, it is recommended that the core should be overlain
by a 600 mm to 800 mm cover of low plasticity material, including a ‘topsoil’ surface which
should be grassed. Areas likely to be subject to erosion may require additional protection using
appropriate ‘rock armour’ materials.

Where sand or low plasticity clay material is encountered in the walls or floor of pond
excavations, such material should be over-excavated and replaced with a low permeability clay
liner (preferably medium to high plasticity silty clay). The liner should be placed as a 300 mm
thickness loose layer and compacted in accordance with the specifications for pond clay core
construction, as described above. The use of a low plasticity cover. to prevent shrinkage
cracking should also be considered within, and above, the water level fluctuation zone around
the pond walls.

6.1.5 Batter Slopes

For short-term construction periods, excavated unsurcharged faces could be battered at 1V:2H
in the sand and 1V:1H in the clay. Flatter batters may be required if significant ground water
seepage is encountered. Permanent batters in the sand and natural clay / compacted clay fill
should not be steeper than 1V:3H and 1V:2H respectively, or supported by an engineer designed
retained structure (ref. Section 6.2) where steeper slopes are required.

6.2. Retaining Structure Design

Retaining structure design should be carried out using a triangular pressure distribution applicable
to the wall type (ie. active pressures for a cantilever wall or at rest pressures for a
propped/anchored wall) and based on the parameters in Table 1. The values in Table 1 assume
that the walls are fully drained and no allowance is included for construction loading, up-slope
foundation loadings, the effect of sloping ground, or backfill compaction stresses.

Golder Associates