Flood Slip Recovery - Wainui Hill
December 2011 brought unexpected record rainfall and slips to the
Nelson/Tasman region of New Zealand. Wainui Hill in Golden Bay was just one
example of the downpour’s aftermath, with 25 slips over a 2km stretch resulting
in hill closure & house evacuations.
Geotech Engineers required a quick set of survey data to enable visual
extents of slips and their catchments, enable profiling along testpits, to
produce geotech reports/costs on options.
Roading Engineers required a DTM
model showing existing carriageway and structures to enable concept road design
to accompany remedial work options.
With the short time frame provided, and the steep, bush-clad environment, it was
decided to use the Council’s existing LiDAR data. This, of course, did not
include the new slips, and LiDAR point data files are huge and cumbersome (Excel
would not open them for edit in one go); many software products cannot handle
them, and the data is not as accurate in level.
Some existing road/recent slip asbuilt
data was available in older formats, but this information differed in coordinate
and level datums.
Field surveys were undertaken over four
days to gain GPS control where reception was available, and robotic total
station topographic surveys undertaken of each slip site as well as road edges,
culverts, and structures.
LiDAR level datum
was validated as good enough for purpose (prelim road design alignments) on road
surfaces and good enough though less accurate for geotechnical purposes in
vegetated areas above and below the actual slip sites.
The job was divided into two parts (each side of hill) for ease of use and
because one side was reported on first (though 12d Model software could easily
have handled it as one project).
robotic topographic survey traverses where easily adjusted to the GPS control
using Survey Adjustments Helmert 2d (advanced). Old data was imported using the
File I/O data input DXF import, then adjusted to match GPS control and converted
using NZ Conversions. This created a combined actual surveyed data set of slip
surfaces and road edges or CLs, culverts, etc. 12d Model’s ability to
display different views with different datums all in one project, and the ease
of conversion (and not having to worry about overlapping point numbers!), made
this task a relative ‘breeze’.
The LiDAR data was imported using x,y,z pt
id input. The ease of the format options was helpful as it enabled the team to
bypass the problem of editing huge files in a third-party product, by simply
ticking what columns were to be imported, and indicating in what order. These
four tiles were huge, so the team employed the FENCE routine, by only bringing
in the points required inside a drawn polygon (fence).
A polygon was also drawn around the
extents of the merged topographical surveys. Using FENCE, this time from the
Utilities menu, the LiDAR data inside the topographic polygon was easily fenced
out and deleted. By turning on the remaining LiDAR data and the more accurate
topographical models, the team gained a complete set of data which was used to
create the TIN model. This method led to an excellent TIN and contour model
without the rough edges that can occur (because of the huge number of LiDAR
points) when creating super tins between data of differing qualities.
The visual representation for the
geotechnical engineers was achieved by simply creating a perspective view and
toggling on SHADE, then turning on the appropriate models (roads, slips, culvert
strings, etc.) that show in surveyed positions on the TIN. By using TIN
DRAPE tool, the legal boundaries could even be represented over the TIN. All in
all, a successful presentation of data was achieved.
Profiling of slip sites was then
undertaken by creating a simple line string through test pit sites and extending
above and below the slip sites, then draping over the TIN (visually seen live on
screen). PPFs were then edited to show appropriate string cuts, etc.
Outputs to other programs were also performed, to be included in reports, and
the overall model used by the road designers for design alignments.
As a relatively basic survey team still learning to use 12d Model software, and
with no Visualisation module yet installed, 12d Model proved to be the ideal
answer to everyone’s needs - the simple learning curve and time saving
opportunities made what would have been a near-impossible task much easier.
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