MCU14/0106 Appendix E - Engineering Report

August 2004 -6- 04682104(AB)

 

6.0 GEOTECHNICAL ASSESSMENT
6.1 Earthworks
6.1.1. Site Preparation

Prior to any earthworks activities, organic rich topsoil must be stripped and removed from the
construction area. The test pits indicate trace (5%) or some (5-12%) root matter, together with
large tree roots, to depths of up to about 200 mm across the site. This organic matter is
generally coarse material contained within surface sands. Dependent upon the method of site
preparation employed, we believe that it should be possible to achieve an average stripping
depth of about 200 mm. Large roots will require selective removal during site preparation, in
conjunction with ‘grubbing out’ of stumps. Organic material must not be used in fill to support
any structural loads, however it may be used in landscaping areas.

It should be noted that firm clayey / silty soils were encountered in test pits GA2 and GA4 (ie.
western part of site near a drainage line) to depths of 0.2 m and 0.4 m respectively. At the time
of our investigation, these soils were noted to be moist. In this condition, the proposed filling
should be achievable without requiring any additional surface treatment.

However, following stripping, proposed fill areas should be proof rolled by several passes of a
heavy (minimum 10 tonne) vibrating compactor to densify loose, near surface sandy soils and
confirm suitable subgrade strength. Dependent upon the prevailing weather conditions at the
time of site preparation (ie. should earthworks commence following a period of heavy or
prolonged rainfall), it may be necessary to construct a ‘bridging layer’ over clayey / silty soils
that occur across the low-lying parts of the site. Such a ‘bridging layer’ could be necessary to
prevent wetting up and ‘heaving’ of the initial fill layers.

If necessary, we suggest that a ‘bridging layer’ could comprise the placement of geotextile filter -
fabric overlain by a minimum 200 mm thickness of coarse aggregate or clean sand having less
than 3% fines content. Alternatively, a geotextile filter layer may not be required if the clayey /
silty layer is removed altogether; ie. down to the upper interface of an underlying stiff (or
stronger) clay, if present at reasonable shallow depth.

6.1.2 Excavation

Our investigation results indicate that the proposed bulk excavations (ie. up to about 3.5 m
depth across the ridge) should encounter a shallow layer of sand overlying very stiff to hard clay
and extremely weathered sandstone. We anticipate that such materials should be excavatable
using conventional earthmoving equipment, possibly with the assistance of ‘ripper’ attachments
for excavations in the sandstone.

The investigation results indicate that ground water seepage may emanate from sandy zones
overlying, and interlayered with, the clay strata. The rate of ground water seepage will be

Golder Associates