Precise Positioning 'In-the-Wet'
The Ohio River, named from the Iroquois word for “great river,” has long been an important waterway. Stretching from Pittsburgh, Pa., southwestward 981 miles to the Mississippi River, the river separates West Virginia and Ohio and meanders along the borders of Kentucky, Indiana and Illinois. As early as 1824, the Ohio River became a major transportation route for crops, finished goods and coal. Today, it remains a key commercial navigation channel. At its hub near Olmsted, Ill., the the river handles more than 80 to 90 million tons each year of raw commodities, tallying up a combined total value of over $17 billion.* In fact, this section of river carries more tonnage than any other place in America’s inland navigation system.
In 1928 and 1929, two wooden wicket dams (Dam #52, Dam #53) were built near Olmsted. Just as they were exceeding their life expectancy, two temporary lock and dam systems were constructed in the 1960s and 1970s, each 1,200 feet in length. These systems were only designed to last about 20 years. Now, 30 to 40 years later, they are in dire need of replacement. Delays are likely along this portion of the river due to frequent scheduled and unscheduled maintenance to keep the systems working. The failure of either system would render the river unnavigable for push boats hauling barges with weighted materials and raw goods during low water due to the profound changes in the river’s depths throughout the year.
In 1994, the U.S. Army Corps of Engineers embarked on a new locks and dam construction project in Olmsted--one of the largest civil works construction projects in the history of the Corps. The new system will feature two 110-foot by 1,200-foot lock chambers located along the Illinois shoreline as well as a dam consisting of tainter gates, a navigable pass section, and a fixed weir. The locks, completed in 2002, were built by a joint venture of Atkinson, Dillingham and Lane. Construction on the new dam, designed by Jacobs & Gerwick Joint Venture and contracted to URS & Alberici Constructors Joint Venture, began in 2004.
All observed data must be presented so that the project personnel can visually understand the progress in the simplest of terms. Additionally, both observed and processed data must be submitted in the minimum amount of time possible to map out the progress and assist key personnel in their decision making. Successfully completing such a complex project requires a combination of modern technology and erstwhile construction survey procedures.
The need for accuracy starts with the hydrographic survey. The cornerstone of the hydrographic survey operation is the Trimble SPS850 and SPS851 RTK base station. The 28-foot-long survey boat, aptly named “CBLow,” is fitted with a hydraulic bow (multibeam) transducer mount. “All measurements with regard to vessel size and relevant equipment offsets are physically surveyed in to within ¼ inch accuracy,” says Don “Woody” Woodward, URS hydrographic survey specialist. “By utilizing RTK measurement combined with the accurate vertical and horizontal measurement offsets, the errors for vessel squat and settlement are negated, thereby leaving the water velocity correction as the only mitigating factor in observing any erroneous depth data.” The team further compensates for these possible errors by observing the dips in the velocity of propogation (Vp) in the water column and cross checking these indicators with the localized river water temperature, which is constantly monitored. Calibrations for position, pitch, roll and yaw (heading) are done on a regular basis to maintain data collection accuracy.
All surveys are done in elevation mode, conforming to the basic geodetic survey parameters for the project. The riverbed topography is therefore expressed in elevation, not depth, in keeping with the topography of the land above the river surface, regardless of the river’s rise and fall throughout the year. “The survey boat is capable of mapping the full river bottom survey consisting of more than ¼ square mile in three hours with data edited to an XYZ file,” Neihoff says. “Early on, river work on the second phase used a geodometer with a Lowrance paper graph display and a prism attached to the boat, where each mark on the graph was labeled with the point number to match the instrument for elevations. Although it took some time to sort out the data, it worked for engineers on excavation and rock placement. We have come a long way in few years.”
The final processed elevation datasets (XYZ) are added the project’s central database for retrieval onsite by the project’s barge engineers, who use the data to update the Hypack Dredgepack software used by the dredging barges to compare the actual dredged elevation, in full and differential matrix, to the required templates. The final elevation datasets are also used to create 3D representations of the as-built excavation areas in true elevation using Fledermaus 3D software. All relevant survey data (raw data, edited data, XYZ, 3D, temperature, Vp, river elevation, etc.) is fully archived to provide a comprehensive database that allows for a quick retrieval of data and a visual history of the various real-world observations during the project’s timeframe.
The combined efforts of river construction and surveying create the synergy necessary to prepare the river bed for the placement of the concrete shell segments. Each river season, approximately 400 piles are hammered into the river’s bottom to provide a secure foundation for the segments. A Trimble SPS 880 Extreme Smart GPS RTK rover is installed on the piling barge for coarse positioning. Working inside a position tolerance of less than 3 inches both laterally and vertically, the survey crew uses Trimble S6 and S8 Robotic Total Stations to guide the river crew’s pile template into the exact location for driving pile.
An as-built of the pile location provides necessary information during the outfitting of the shell in the precast yard. To maintain the construction schedule, the pile driving operations run 24 consecutive hours with full survey support. “We have a very small window of opportunity to get the pile driven because we are at the mercy of the river,” says Josh Brown, PLS, a Kentucky surveyor for URS. “Our survey crews work around the clock during the river season until the pile driving effort is completed.”
Survey crews also provide monitoring support during the tremie concrete placement to ensure no movement of the shell occurs at the river bed.
In the precast yard, all embedded materials and other connective items that are under and outside of the shell must be precise and accurate within their respective location tolerances. They must match the plan locations, the as-built locations of the driven piles and the previously set shells already positioned in the river. Using the Trimble S6 and S8 Robotic Total Stations with TSC2 Controllers, the surveyors establish the proper control, layouts, support, verifications and as-built information for these shells. However, the equipment is just one part of the equation. For the crews to meet the tight tolerances that are required, they must consistently practice accurate and precise survey methods. Often, “old-school” procedures used in concert with new technology provides the best results.
Surveying free-standing designed-to-build shells on beams and shoring stilts creates challenges for control points. The movement of the steel as it is exposed to the sun and the nearby New Madrid Fault line makes it imperative to keep the control on the shell to maintain a stable location relative to the shell, even if the shell moves. By using the “rounds” procedure in the Trimble software for the TSC2, the survey team is able to set the control to a closure better than 1':300,000' and utilize a minimal amount of control for the shell to maintain true consistency on accuracy and precision.
Throughout the construction process and concrete placements, other challenges arise. The rebar mats on the free-standing shells receive a significant amount of foot traffic, making it difficult to set up the robotic total stations. To overcome this problem, a survey control stand dubbed the “Fierro Stand” was invented and designed with attachments to set the instrument. The Fierro Stand is free from the rebar mat movement and minimizes shell vibrations, allowing an optimal environment for layout and location verifications with the specified tolerances.
Once the shell is built, the outfitting sequence begins underneath and on every side of the shell. Here the survey team applies more long-standing techniques, such as “closing the horizon,” “distance precedes angles” and “three amigos procedure,” to transfer control from the top of the shell to its sides and underside. Most of the items scheduled to be placed under the shell, such as support posts, landing pad flat jacks, spiral rebar mat and guide pintles, are not in accordance to designed plans but rather are drawn from as-built information from the river piles and previously set shells. Most of these items are set within 1/16 inch in accordance to their respective specifications.
To maintain high levels of accuracy, precision and consistency, the survey department uses a wide range of checks and balances. All layouts are prechecked from office to field and coordinated with several cross checks. Verifications are post-checked from field to office and later inspected with engineering personnel to ensure a complete and confident assessment for each embed and structured piece. To help with the layout and verification inspections, the survey team meticulously documents its approach in field books and also pulls the data into an Autodesk AutoCAD Civil 3D works cadastral program. The information is then shared with the design team. “We deal with different designed dimensions, such as the ogee curve for one of the shells,” comments Tom Gifford, surveyor for URS. “It’s a parabolic curve, but we are able to use Trimble software to create a surface in order to layout and verify any embeds on the curve. It gives us better accuracy and saves quite a bit of time in our support.”
To ready the shell transport, the survey team obtains a complete and accurate as-built of the entire shell. With the 3D works CAD software, a three dimensional model of the shell is processed to calculate weight quantities and allow the engineer to determine how the shell will fit into the preceding dam segment in the river bed.
Survey support plays a key role throughout the process. The URS survey team uses the Trimble robotic total stations to collect significant as-built information prior to the primary key lift of the shell. The as-built data is gathered from the lifting frame on its horizontal plane, vertical alignment and orientation against the shell’s actual location. “Before we can ever lift a shell, we need to make sure of our control on the lift frame, that both horizontal and vertical is in check,” explains Kevin Russell, survey chief of parties for URS. “The container plates, pintle receivers, and pintles are all pieces of a large puzzle, located under and outside the shell, and has to match as-built pile locations supplied by marine survey and previously set shells. We’re operating with a small error budget.”
As the shell begins to hoist from 0 to 100 percent tension lift on the crane, the surveyors periodically check every lifting lug (25 total) and elevation changes to ensure an even ascent and avoid overstress cracking and possible breakage of the shell. Throughout the process, the crews provide the heavy lift team with complete information on the shell’s camber, the elasticity of the lifting lugs and any indication of excess strain on the surface as the shell raises from its’ original position.
Perhaps nowhere is survey support more vital than in setting each 4,800 ton concrete dam segment on a piling foundation under 70 feet of flowing muddy river water within 1 inch of horizontal design location. The lifting frame uses the strand jacks to hold the shell vertically. Eight mooring anchors position the catamaran barge in the general vicinity, and the snubbing winches guide the master pile brackets that are embedded in the shell’s wall down to the river bottom and through the master pile I-beams.
A complex monitoring and communication process was designed for this project alone. “The heavy lift aspect for the survey department requires us to be involved in not only conventional aspects of the construction and setting but also the testing,” Russell says. “Each time we are presented with a challenge, we must first practice whatever task we are about to perform. We call them dry runs. Communication is key. We can’t always be where the action is, but to compensate for this, we have machine control radios that do much for the marine aspect of setting the shell. Above that, we have to maintain clear communication among each other and between survey and field engineers.”
Using the Trimble robotic total stations, the crews locate the shell and determine its vector by targeting the MT-1000 prism locks. Digi International XTend XT-09 data modems connected to the total stations transfer the initial data, first in Olmsted Locks and Dam (station and offset) coordinates, and then processed over to the State Plane Coordinate system (NAD-27) (Kentucky South 1602).
The Olmsted Locks and Dam project is a modern engineering marvel. Setting a river dam in a 2,700-foot span on the river bed, piece by piece, without constructing temporary coffer dams to allow for a build-in-place design, is a first. Every facet of the project dictates a necessary guide and first-rate survey support. The responsibility ultimately falls on the shoulders of the survey team to guarantee an accurate location of each shell segment, sequentially set them properly and give promise that the interlocking pieces are fastened correctly underwater.
“This is definitely a one of a kind,” says Tom Goodwin, lead CAD surveyor for URS. “It is fun to work on such a complex project with the constant changes and challenging circumstances.”
Go here for more details on the old-school methods "closing the horizon,” “distance precedes angles” and “the three amigos procedure."