Geology Reference
In-Depth Information
comprehensive).
1. Unravelling the geological history at a site. This will come initially from regional and local knowledge,
examination of existing documents, including maps and aerial photographs, and the interpretation of exposed
rock and geomorphologic expression. Geology should be the starting point of an adequate ground model for
design.
2. Prediction of the changes and impacts that could occur in the engineering lifetime of a structure (perhaps
50
100 years). At some sites, severe deterioration can be anticipated due to exposure to the elements, with
swelling, shrinkage and ravelling of materials. Sites may be subject to environmental hazards, including
exceptional rainfall, earthquake, tsunami, subsidence, settlement,
-
flooding, surface and sub-surface erosion
and landsliding.
3. Recognising the in
uence of Quaternary geology, including recent glaciations and rises and falls in sea level;
the potential for encountering buried channels beneath rivers and estuaries.
4. Identifying past weathering patterns and the likely locality and extent of weathered zones.
5. Ensuring appropriate and cost-effective investigation and testing that focuses on the important features that
are speci
c to the site and project.
6. Preparation of adequate ground models, including groundwater conditions, to allow appropriate analysis
and prediction of project performance.
7. An ability to recognise potential hazards and residual risks, even following high-quality ground
investigation.
8. Identification of aggregates and other construction materials; safe disposal of wastes.
9. Regarding project management, he should be able to foresee the difficulties with inadequate contracts that
do not allow
flexibility to deal with poor ground conditions, if they are encountered.
The investigations into a rock slope failure are reported by Hencher (1983a), Hencher et al. (1985) and by
Clover (1986). During site formation works of a large rock slope, behind some planned high-rise apartment
blocks, almost 4,000m
3
of rock slid during heavy rainfall on a well-dened and very persistent discontinuity
dipping out of the slope at about 28 degrees. The failure scar is seen in
Figure B1-2.1.
The lateral
continuity of the wavy feature is evident to the left of the photograph, beneath the shotcrete cover, marked
by a slight depression and a line of seepage points. If the failure had occurred after construction, the debris
would have hit the apartment blocks. A series of boreholes had been put down prior to excavation and the
orientation of discontinuities had been measured using impression packers (
Chapter 4).
Statistical analysis
of potential failure mechanisms involving the most frequent joint sets led to a design against shallow rock
failures by installation of rock bolts and some drains. The proposed design was for a steep cutting, with
the apartment blocks to be sited even closer to the slope face than would normally be allowed.
Unfortunately, the standard method of discontinuity analysis had eliminated an infrequent series of
discontinuities daylighting out of the slope and on one of which the failure eventually occurred. Pitfalls of
stereographic analysis in rock slope design are addressed by Hencher (1985), a paper written following this
near-disaster.
Examination of the failure surface showed it to be a major, persistent fault in
lled with clay-bounded
rock breccia about 700mm thick and dipping out of the slope
(Figures B1-2.2
and
B1-2.3)
. In the pre-
failure borehole logs, the fault could be identi
ed as zones of particularly poor core recovery; the rock
in these zones was described as tectonically in
uenced at several locations. In hindsight, the fault
had been overlooked for the design and this can be attributed to poor quality of ground investigation and
