By Billy Alan Chavers, RLS, Dale Stockstill & Associates
Even More Than Meets the Eye
Posted: April 28th, 2011 03:37 PM CDT
Equipment Today, April 2011
In September 2010, the New Orleans District Corps
of Engineers was trying to determine if a stage
control structure built near Zachary, LA, in East
Baton Rouge Parish was constructed according to
design tolerances. The construction contractor's
surveyor had collected 12,000 shots on the structure
surface with a total station without being able to
determine if it was within the strict tolerances
required. Something more powerful was needed - and
laser scanning turned out to be the answer.
The Amite River and its tributary, the Comite River,
are the major causes of catastrophic flooding in the
Baton Rouge metro area. In 2001, the U.S. Army
Corps of Engineers, the state, the Amite River &
Tributaries Study Authority and East Baton Rouge
Parish signed an agreement to develop a regional
solution called the Comite River Diversion Canal
Project. Its purpose is to divert about 50% of the
flood waters from the Upper Comite River to the
Mississippi River.
Little room for deviation
Due for completion in 2016, the Comite River
Diversion Canal Project involves construction of a
12-mile-long diversion channel from the Comite
River to the Mississippi River; a diversion structure
at the Comite River; guide levees; the Lilly Bayou
stage control structure; and several drop structures
where the diversion channel intersects with roads,
railroad bridges and bayous. As of 2008, the total
estimated cost of the project was $187 million.
The Lilly Bayou Control Structure will dissipate the
energy of water flowing between the Comite and the
Mississippi from a higher to lower elevation,
according to Rick Tillman, structural engineer with
the Corps. The energy will dissipate as water flows
Dale Stockstill & Associates
Dale Stockstill & Associates collected
several million points with a Topcon
GLS-1000 laser scanner, and determined
that the Lilly Bayou stage control
structure deviated from tight design
tolerances in many locations.
over a spillway featuring an elevated weir.
The $27.6 million second phase of the project
consists of the excavation and construction of the
concrete control structure, a stilling basin and an
outflow channel. The main surface slab was designed
to have a 1 to 5 (1v: 5h) slope, a several feet thick
substructure of mass concrete and large baffle blocks
filling the stilling basin at the bottom constructed of
3,000-psi concrete. A 1-foot-thick, 5,000-psi concrete
overlay was also placed on top of the substructure to
handle the compressive force and abrasion generated
by rushing flood waters.
At no more than 1/8 inch of deviation for every 10
feet of surface, the elevation tolerance on the
structure is uncommonly tight, notes Tillman. "That's
out of the ordinary for the [Corps'] New Orleans
District to design; however, it's not unusual for the
purpose of the structure," he points out. "We have
flow velocities that exceed 40 feet per second. At
those velocities, you can get cavitation and if you
have unevenness on the surface, it can tear a hole and
tear the structure apart. That's why you have to have
these tight tolerances."
After the structure was built, however, deviations
from the elevation tolerance on the sloped face were
plainly visible using a straight edge. Determining the
extent to which the structure was out of tolerance
promised to be quite a task in itself.
Dale Stockstill & Associates
Shown is a three-dimensional model of
the Lilly Bayou stage control structure.
The points were collected by a Topcon
GLS-1000 laser scanner.
Dale Stockstill & Associates
The structure was designed to have a 1 to
5 slope, a several-feet-thick substructure
of mass concrete, large baffle blocks filling
a stilling basin at the bottom and a 1-footthick, high-strength concrete overlay.
The difference: quantity of points
The Corps' lead engineering technician, Dwayne Blanchard, had previously experienced successful
scanning projects completed for the division by Dale Stockstill & Associates (DS&A). He
recommended that Tillman ask us about scanning the structure.
Eager to demonstrate the potential of scanning to the Corps, I agreed to perform the scan and data
analysis without a task order. After obtaining the coordinates for the PK nails that were already in
place from the construction company's project surveyor, our crew checked them with a GPT-3105W
reflectorless total station from Topcon Positioning Systems, and then started the scan using a Topcon
GLS-1000 laser scanner.
The elevation did not adhere to tolerance throughout the structure surface, but the X and Y axes were
fairly close to tolerance. So the crew performed the scans from the existing control point, which was
in place at the top and bottom of the large structure, knowing that the two scans could be corrected
during the cloud registration process.
Tillman notes that scanning provided much richer surface detail than shooting in less time than a total
station. "You can obtain those results on conventional total stations, but this is much quicker," he says.
"They scanned the entire surface of this flume - I think it took them two to four hours - and they had
something like 7.5 million points. It took about three days to collect about 12,000 points with the
conventional total station."
After the initial scan was completed at the top of the structure, the data file was downloaded for the
engineers to view in the construction trailer while the second scan was being taken. It didn't take long
for Tillman's question to be answered - the millions of points collected on the surface clearly
displayed undulations, ridges, dips and humps in the structure.
DS&A technicians used a PolyWorks routine to "color map" the deviations as related to the design
plane. DS&A technicians provided traditional cross sections at specified intervals. But because the
surface was so large and tolerances were so tight, many hundreds of sections would have been
required just to view a small portion of the slope.
DS&A's technicians went to work programming. They modified some existing routines in their
PolyWorks software to generate an ascii-type EM file - a format favored by the New Orleans District that converts the point cloud data to a grid every foot laterally and half a foot longitudinally going
down the slope. This programming had to be refined several times to deal with file size and computer
processing speed.
After the program was refined, it extracted traditional points (point number, Northing, Easting,
elevation and code) from the cloud at the specified intervals and wrote them out to an ascii file. This
file was then converted to the New Orleans District's EM file format, enabling the data to be loaded
into the district's programs for analysis by its engineers and technicians.
In addition to the EM file, the District Engineers requested a contour map - but not just any contour
map.
The Corps engineers requested a CAD drawing that depicted the 1v: 5h slope as horizontal and
contours that were related to the design plane. DS&A's technicians rose to the challenge and created a
process that flattened the data onto the slope, while maintaining the original project stationing. This
required application of a precise scale factor to the data on the slope, i.e., essentially squeezing the
hypotenuse down to the base of the triangle. The contour interval was small (five hundredths of an
inch) and directly in proportion to the original design.
Officially scanning
After the Corps received the data in a manner that was easily understandable to all Corps personnel
involved - without the need to learn new software or go to a seminar to learn how to interpret a color
map - it made the project official. It issued a task order to survey and scan the structure. Corps
policies required that the data be recollected because the previous data provided by the contractor
were accepted before the scan was performed.
DS&A's crew returned to the project site and started from scratch, checking the project Permanent
Bench Mark (PBM) with their Topcon GPS units, running precise three-wire level loops from the
PBM to the PK nails on the top and bottom of the structure with its Topcon AT-G1 AutoLevel.
I instructed the crew to use its GPT-3105W reflectorless total station in conjunction with its Topcon
DL-500 digital level to obtain data under water and mud covering the stilling basin that could not be
pumped out. The total station shot the digital level rod for the horizontal position, while the DL-500
obtained a precise elevation for each shot. The additional data obtained under the water were checked,
compared and incorporated into the point cloud data.
Again, the data reflected the deviations from the design criteria and the deliverables were remade
using the new data. As of early spring 2011, a process for correcting the structure's elevations was
being worked out.
Based on this project, Tillman is a believer in the use of scanning technology to determine adherence
to specifications on structures of such large scale. "I see other applications of its use that would be
beneficial," he says, "such as repeat-type surveys that we have to do periodically for movement or
settlement."
Billy Alan Chavers, RLS is the owner of Dale Stockstill & Associates, Carriere, MS, a leading
provider of surveying, mapping and laser scanning services on the Gulf Coast.
------------------Ten Times Project's Scanning Efficiency Available
As productive as it was, the laser scanner used to scan the Lilly Bayou Control Structure does not
represent the fastest available technology. In March 2010, Topcon Positioning Systems unveiled the
GLS-1500, which speeds up point cloud collection at a rate of 30,000 points per second and a range of
150 meters. According to the manufacturer, the unit also has an "all-in-one" design that reduces the
amount of equipment needed in the field.
A built-in 2.0 megapixel digital camera can be connected to a PC and used with the ScanMaster
software to enable a live video feed of the jobsite to be streamed to aid in scan setup and data
acquisition. It also has an onboard data collector with a keypad and LCD display that allows use as a
stand-alone laser scanner. Data collected can be stored onboard on an SD memory card or logged into
a PC. A built-in wireless LAN connection allows control of the scanner on a PC from the inside of a
vehicle.
Topcon Precise Scan Technology is designed to allow high-accuracy measurements over a wide range
of distances. Lens array optics technology maintains distance accuracy from 1-150 meters, and
additional ranging past 330 meters is available. The unit's Class 1 laser classification allows scanning
near airports, busy traffic areas and populated areas, as well as low power consumption and fewer
battery changes.
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