The trench
Over the next few posts I'll be looking at the current gate structures at the lakefront. Specifically, I'll be detailing the gates themselves (which are two feet too short), the front line storm defense for the city of New Orleans for the foreseeable future.
The basic design
The basic design of the gate structures at all three sites is similar. A large framework holds the gates (individual gate segments are called "needles") in place. At all three sites the gates are lowered and raised by hydraulic winches. They slide down between vertical guide columns until they come to rest in a keyway, or "trench" at the bottom of the canal. Here's an overall view of the London Avenue gates showing those components except the trench, which is underwater (taken from the Corps of Engineers Flickr stream):
And here's a closeup of a London Avenue needle riding in its guide rails during an actual high water event, the rainfall event of September, 2009 (also taken from Corps of Engineers Flickr stream):
And while all three sites' structures share these basic elements, there are substantial differences among them. In broad terms, the gates at 17th Street are different from those at Orleans and London because the 17th Street gates were designed by a different firm from the other two.
Here's the 17th Street design:
The force of the water at 17th St is transferred to the main structure and foundation at three points - two directly connected to the structure and the third being the trench in which the gates sit.
And here's the London/Orleans design:
The designs at Orleans and London allow for force to be transferred only through two points: the upper point on the structure and the trench. That upper point is clearly seen in this shot of the Orleans Avenue gates under construction on May 26, 2006 (via SCPR Flickr):
This difference makes it all the more critical that the gates sit properly in the trenches at Orleans and London. However, that does not obviate the need at 17th Street for full seating of the gates, especially since the 17th Street structure lies nearly directly on the lakefront, as opposed to being set back like London and Orleans.
As I mentioned above, the bottoms of the needles seat into a trench along the bottom of the canal. The trenches are lined with regular carbon steel, despite being submerged in seriously brackish water. This is a major oversight. They should have been stainless steel, but the Corps cheaped out. The Corps' consultants, Black and Veatch, picked up on this in 2006, when something could have been done. From their December, 2006 "Alternative Considerations Report," which (in part) looked at the possibility of salvaging parts of the Interim Closure Structures for use in the permanent stations, here's what they said about the trenches in a discussion of the Corps' material choices:
We know how the brackish water has treated the hydraulic pumps which also reside there: very poorly. The Corps is now pulling all those rusted pumps out from all three sites, after the pumps leak oil into the canals through the holes opened up by the rust. They are replacing all the pumps' carbon steel guts with stainless steel pieces. Details on the pump repairs are here, and details on the oil spills are here.
While the materials of the trench liners at all three sites are carbon steel, there is a slight difference between the designs of liners. Here's a detail of the trench liner from the drawings for 17th Street:
Note the trench liner at 17th Street was designed with a little rounded curb to encourage the gates to slide in:
Here's the trench liner at London and Orleans (no curb):
Without that curb, the trench design seems a little less robust at London and Orleans than at 17th Street.
Keeping the trenches clear
At all three structures force from storm surge is transferred to the foundation through - in part- the gates' contact with the trench. Thus it is very important the trench be kept clean so the needles can seat properly. And when the needles are in the trench, they need to stay there, to make sure the forces are transferred evenly through the structure, as well as ensuring the needles don't let surge through to the canals. My later posts will examine how the needles are supposed to be kept securely in the trench, as well as whether their design truly keeps water out.
To keep the trenches clean, the Corps has a standing contract with a firm called Independent Divers out of St Rose, LA, previously known as H.J. Merrihue. According to publicly available contract documents, they seem to do all the diving work for the Corps New Orleans District. Independent Divers is supposed to go down and clean out the trenches periodically, as well as immediately in advance of storms when the Corps expects to shut the gates. They're also supposed to be on call for emergency cleaning.
That's exactly how it worked in 2008 as Hurricane Gustav approached in late August. The Corps called on Independent Divers three times:
Task order 36
August 23 at London Avenue
Task order 37
August 27 at London Avenue
Task order 38
August 31 at all three sites
The last task order was exactly what we should be seeing - a preemptive task order ensuring that crews were on site if any gates needed to be closed. Good thing, because the gates at London and 17th did drop in Gustav the first week of September, and then again about two weeks later during Hurricane Ike.
And during high water events in 2009? Not so much. More on that in a later post.
So what precisely do the divers do down there, ten feet below the water's surface? Here's the verbiage from the 2009 Operating Manual for the Interim Closure Structures:
This is not the best way to do things. The trench cleaning could have been automated with a jetting system. The Corps themselves designed a jet dredging system that can at least be operated without divers nearly 20 years ago to help clean out difficult-to-get-to spots on the Empire floodgate in St Bernard Parish, so this is not a new or difficult problem.
The Dutch pointed out the problem of not having an automated trench cleaner publicly to the Corps in the pages of National Geographic in August, 2007:
It goes without saying that relying on divers is the cheapest way in the short term to clear the trench, but the repeated calls (at about $10,000 a pop) quickly make it cost ineffective. However, no one ever got rich pointing out how much foresight the Corps has.
Grout fills the trenches
In addition, the conditions of the trenches themselves are questionable. In the summer of 2006, while the 17th Street and Orleans structures were under construction, the Corps had their foundation subcontractors spray jet grout around the bottom of the sites while doing foundation reinforcements through deep soil mixing. A detailed scholarly article explaining the foundation work from International Water Power and Dam Construction magazine written by a Corps staffer can be found here.
Let me just say here that I'm no expert in soil mechanics. However, it is surprising to me that of the three sites, it is London Avenue that did not have its soil strength improved. After all, it was the London Avenue canal that had two breaches during Katrina, and a third area where the wall tipped but didn't fall. 17th Street had one breach and Orleans Avenue had none. One of those London breach sites and the tipped wall are within eyesight of the spot where the current gates sit. And the current safe water level at London is five feet, the lowest of all three canals. All of this is due to the poor soils along the London Avenue canal.
Now it is possible there is a legitimate reason for not improving the soil strength at the London gates site. It is set back quite a way from the lake, so the forces imposed by a surge are reduced. In fact, that is a primary reason for the selection of the site for the permanent London Avenue pump station, which is similarly recessed from the lakefront. This setback might mitigate the need for soil improvements. Nonetheless, it is certainly an eyebrow raiser.
Anyway, back to the grout at 17th Street...
The jet grout was supposed to be confined to the dirt at the bottom of the canal. However, the grout got everywhere, including into the trench under the needle gates. It was so bad the middle needle of the 17th Street gates was cemented into place. That is why so many photos from that time show that segment down with the others up. Here's an example from July 30, 2006:
It wasn't that way by choice - the Corps cemented it in place.
The Corps had to bring in divers, jackhammers, and backhoes on barges to beat the hardened grout out of the trenches for days. The Times-Picayune reported on it in their August 8, 2006 editions:
Here's that barge with the backhoe still stationed in front of the gates five days after the article:
When this story broke, the full implications were somewhat muted by the Corps. The full implication is that the gates would not have worked at all because the needles would not seat in the trench. They only discovered the problem when they went to lower the gates in advance of a tropical system.
During an August 17, 2006 interview on WLAE's "Road to Recovery" program, then-New Orleans District Commander Col. Richard Wagenaar mentioned during a discussion of the grout problem that they were planning to place inserts in the trench, in order to keep silt out when the gates were not operating. This never happened. Independent Divers has continued to clean out the trenches for the last four years.
It also appears that they may not have cleaned out all the grout that first go-around in 2006. In May, 2007, I reported that the gates at 17th Street still wouldn't close properly during a May 15, 2007 drill. A year later, they were still having the same problems. Task order 26 was issued to Independent Divers on April 14, 2008 and includes the following verbiage:
This was almost two years after the Corps assured the public the trenches were cleared of grout.
Grout fills the site
One other matter: the T-P article on the trenches really doesn't give a true sense of how much jet grout there was all over the place. Fortunately, the contracted engineering designers detailed everything in a presentation they gave to the Corps' 2007 Infrastructure Conference:
Here's pictures of the jet grout far above the bottom of the canal at 17th Street, also from that presentation:
If that's what it looks like on shore, imagine the what happened under the water. One hint is how the middle needle looked right after they broke it free from its grouty shackles in the late summer of 2006:
Taking the amount of grout on the gate as a guideline, there was about 3 feet of solidified grout sitting on the bottom of the canal over the trench:
What a mess.
Conclusion
So that's the trench. The design of the 17th Street version was better than the ones at Orleans and London Avenues. However, the condition of the 17th Street one (and the Orleans Avenue one) is questionable at best after getting beaten up by jackhammers in 2006.
The next few posts will look at the needle segments, the winches, and the dogging pins. There's a reason I'm doing this, and it's not nice.
The basic design
The basic design of the gate structures at all three sites is similar. A large framework holds the gates (individual gate segments are called "needles") in place. At all three sites the gates are lowered and raised by hydraulic winches. They slide down between vertical guide columns until they come to rest in a keyway, or "trench" at the bottom of the canal. Here's an overall view of the London Avenue gates showing those components except the trench, which is underwater (taken from the Corps of Engineers Flickr stream):
And here's a closeup of a London Avenue needle riding in its guide rails during an actual high water event, the rainfall event of September, 2009 (also taken from Corps of Engineers Flickr stream):
And while all three sites' structures share these basic elements, there are substantial differences among them. In broad terms, the gates at 17th Street are different from those at Orleans and London because the 17th Street gates were designed by a different firm from the other two.
Here's the 17th Street design:
The force of the water at 17th St is transferred to the main structure and foundation at three points - two directly connected to the structure and the third being the trench in which the gates sit.
And here's the London/Orleans design:
The designs at Orleans and London allow for force to be transferred only through two points: the upper point on the structure and the trench. That upper point is clearly seen in this shot of the Orleans Avenue gates under construction on May 26, 2006 (via SCPR Flickr):
This difference makes it all the more critical that the gates sit properly in the trenches at Orleans and London. However, that does not obviate the need at 17th Street for full seating of the gates, especially since the 17th Street structure lies nearly directly on the lakefront, as opposed to being set back like London and Orleans.
As I mentioned above, the bottoms of the needles seat into a trench along the bottom of the canal. The trenches are lined with regular carbon steel, despite being submerged in seriously brackish water. This is a major oversight. They should have been stainless steel, but the Corps cheaped out. The Corps' consultants, Black and Veatch, picked up on this in 2006, when something could have been done. From their December, 2006 "Alternative Considerations Report," which (in part) looked at the possibility of salvaging parts of the Interim Closure Structures for use in the permanent stations, here's what they said about the trenches in a discussion of the Corps' material choices:
"Stainless steel is often used for embeds/wearing surfaces/seal plates/seal bolts and nuts for a permanent structure. There are no special materials for these components on the gates. An example is the needle trough embedded in the diaphragm seal cap. This submerged feature is fabricated of steel but not equipped with cathodic protection and will degrade and have maintenance issue[s] over time. Stainless steel is often used for this feature."
We know how the brackish water has treated the hydraulic pumps which also reside there: very poorly. The Corps is now pulling all those rusted pumps out from all three sites, after the pumps leak oil into the canals through the holes opened up by the rust. They are replacing all the pumps' carbon steel guts with stainless steel pieces. Details on the pump repairs are here, and details on the oil spills are here.
While the materials of the trench liners at all three sites are carbon steel, there is a slight difference between the designs of liners. Here's a detail of the trench liner from the drawings for 17th Street:
Note the trench liner at 17th Street was designed with a little rounded curb to encourage the gates to slide in:
Here's the trench liner at London and Orleans (no curb):
Without that curb, the trench design seems a little less robust at London and Orleans than at 17th Street.
Keeping the trenches clear
At all three structures force from storm surge is transferred to the foundation through - in part- the gates' contact with the trench. Thus it is very important the trench be kept clean so the needles can seat properly. And when the needles are in the trench, they need to stay there, to make sure the forces are transferred evenly through the structure, as well as ensuring the needles don't let surge through to the canals. My later posts will examine how the needles are supposed to be kept securely in the trench, as well as whether their design truly keeps water out.
To keep the trenches clean, the Corps has a standing contract with a firm called Independent Divers out of St Rose, LA, previously known as H.J. Merrihue. According to publicly available contract documents, they seem to do all the diving work for the Corps New Orleans District. Independent Divers is supposed to go down and clean out the trenches periodically, as well as immediately in advance of storms when the Corps expects to shut the gates. They're also supposed to be on call for emergency cleaning.
That's exactly how it worked in 2008 as Hurricane Gustav approached in late August. The Corps called on Independent Divers three times:
Task order 36
August 23 at London Avenue
"An emergency dive is required due to a possible obstruction keeping the gate from properly closing."
Task order 37
August 27 at London Avenue
"Dive team is required to investigate the concrete sill, including gate troughs, by using underwater video equipment to determine if any problems exist during opening and closing procedures."
Task order 38
August 31 at all three sites
"Divers are required to be on-site and be equipped with a water jet pump in the event that sills require cleaning, so that gates can properly close."
The last task order was exactly what we should be seeing - a preemptive task order ensuring that crews were on site if any gates needed to be closed. Good thing, because the gates at London and 17th did drop in Gustav the first week of September, and then again about two weeks later during Hurricane Ike.
And during high water events in 2009? Not so much. More on that in a later post.
So what precisely do the divers do down there, ten feet below the water's surface? Here's the verbiage from the 2009 Operating Manual for the Interim Closure Structures:
"For each structure having one or more gates that will not lower to the 100% seated position, the notches should be checked by a diver. The diver should go along the entire length of each notch of the structure. A high pressure hose should be available for the diver to use to blow out silt that may have accumulated in the notches. The diver should locate foreign objects that may be in the notches and if they are small enough, he should remove them. If the diver encounters an object that is larger than he is capable of moving the diver should secure the object with cables that will allow it to be removed by a crane"
This is not the best way to do things. The trench cleaning could have been automated with a jetting system. The Corps themselves designed a jet dredging system that can at least be operated without divers nearly 20 years ago to help clean out difficult-to-get-to spots on the Empire floodgate in St Bernard Parish, so this is not a new or difficult problem.
The Dutch pointed out the problem of not having an automated trench cleaner publicly to the Corps in the pages of National Geographic in August, 2007:
"A Dutch engineer recently visited some of the new floodgates and pumps installed at the mouths of the city's three main drainage canals. His verdict: They may be 'doomed to fail' in the next big storm.
The engineer, who asked not to be named because he sometimes collaborates with the corps, noted that the gates have no mechanism to remove sediment and other debris that might keep them from closing as a storm approaches. Instead, the corps says it will rely on divers to check for obstructions and clear them away."
It goes without saying that relying on divers is the cheapest way in the short term to clear the trench, but the repeated calls (at about $10,000 a pop) quickly make it cost ineffective. However, no one ever got rich pointing out how much foresight the Corps has.
Grout fills the trenches
In addition, the conditions of the trenches themselves are questionable. In the summer of 2006, while the 17th Street and Orleans structures were under construction, the Corps had their foundation subcontractors spray jet grout around the bottom of the sites while doing foundation reinforcements through deep soil mixing. A detailed scholarly article explaining the foundation work from International Water Power and Dam Construction magazine written by a Corps staffer can be found here.
Let me just say here that I'm no expert in soil mechanics. However, it is surprising to me that of the three sites, it is London Avenue that did not have its soil strength improved. After all, it was the London Avenue canal that had two breaches during Katrina, and a third area where the wall tipped but didn't fall. 17th Street had one breach and Orleans Avenue had none. One of those London breach sites and the tipped wall are within eyesight of the spot where the current gates sit. And the current safe water level at London is five feet, the lowest of all three canals. All of this is due to the poor soils along the London Avenue canal.
Now it is possible there is a legitimate reason for not improving the soil strength at the London gates site. It is set back quite a way from the lake, so the forces imposed by a surge are reduced. In fact, that is a primary reason for the selection of the site for the permanent London Avenue pump station, which is similarly recessed from the lakefront. This setback might mitigate the need for soil improvements. Nonetheless, it is certainly an eyebrow raiser.
Anyway, back to the grout at 17th Street...
The jet grout was supposed to be confined to the dirt at the bottom of the canal. However, the grout got everywhere, including into the trench under the needle gates. It was so bad the middle needle of the 17th Street gates was cemented into place. That is why so many photos from that time show that segment down with the others up. Here's an example from July 30, 2006:
It wasn't that way by choice - the Corps cemented it in place.
The Corps had to bring in divers, jackhammers, and backhoes on barges to beat the hardened grout out of the trenches for days. The Times-Picayune reported on it in their August 8, 2006 editions:
"Contractors jetted a special grouting compound into the sediment beneath both gates between July 20 and July 31 as a way of hardening and strengthening the foundations, St. Germain said.
"'Last Wednesday, they were testing the gates and the gates wouldn't go down because of the grout, which had oozed into the sill the gate sits in,' he said. 'The material was injected in front of the sill and when they started putting the material in there, it expanded and swelled up, and made its way onto the sill.'
Contractors used a backhoe working from a barge, with assistance from divers, to break the hardened grout from the sills."
Here's that barge with the backhoe still stationed in front of the gates five days after the article:
When this story broke, the full implications were somewhat muted by the Corps. The full implication is that the gates would not have worked at all because the needles would not seat in the trench. They only discovered the problem when they went to lower the gates in advance of a tropical system.
During an August 17, 2006 interview on WLAE's "Road to Recovery" program, then-New Orleans District Commander Col. Richard Wagenaar mentioned during a discussion of the grout problem that they were planning to place inserts in the trench, in order to keep silt out when the gates were not operating. This never happened. Independent Divers has continued to clean out the trenches for the last four years.
It also appears that they may not have cleaned out all the grout that first go-around in 2006. In May, 2007, I reported that the gates at 17th Street still wouldn't close properly during a May 15, 2007 drill. A year later, they were still having the same problems. Task order 26 was issued to Independent Divers on April 14, 2008 and includes the following verbiage:
"Provide professional diving services in accordance with the attached performance work statement. Inspections required, with grout/material specically [sic] to be removed at 17th Street Canal (which is over 1 foot thick in many areas), and will be cleared with a water jet pump. Note: the dive contractor stated that the dive team will not use a dive boat at these particular floodgates, based on past experiences. Dive services are estimated for 2 days."
This was almost two years after the Corps assured the public the trenches were cleared of grout.
Grout fills the site
One other matter: the T-P article on the trenches really doesn't give a true sense of how much jet grout there was all over the place. Fortunately, the contracted engineering designers detailed everything in a presentation they gave to the Corps' 2007 Infrastructure Conference:
Here's pictures of the jet grout far above the bottom of the canal at 17th Street, also from that presentation:
If that's what it looks like on shore, imagine the what happened under the water. One hint is how the middle needle looked right after they broke it free from its grouty shackles in the late summer of 2006:
Taking the amount of grout on the gate as a guideline, there was about 3 feet of solidified grout sitting on the bottom of the canal over the trench:
What a mess.
Conclusion
So that's the trench. The design of the 17th Street version was better than the ones at Orleans and London Avenues. However, the condition of the 17th Street one (and the Orleans Avenue one) is questionable at best after getting beaten up by jackhammers in 2006.
The next few posts will look at the needle segments, the winches, and the dogging pins. There's a reason I'm doing this, and it's not nice.
posted by mcbrid35 at
7:35 PM
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