Hi everybody. A surveyor friend of mine has been approached to undertake an interesting survey job: A local boat builder requires stakeout for the twin rails of a newly constructed slip-way. At their farthest point, the rails are approximately 8.0m below water level (salt water). The requested tolerance is +/-2mm. I would be keen to hear from anyone who might be able to offer suggestions/past experience etc. Regards and thanks in advance.
Ask the chap / chapette who requested the tolerance.
Is it stakeout or as-built?
Richard Imrie, post: 440056, member: 11256 wrote: Is it stakeout or as-built?
I understand they require stakeout. And regarding the tolerance, apparently it's taken from a UK standard.
The underwater sections of railed slipways I have seen have been precast reinforced concrete longitudinal/cross-beam frames (on insitu piles) so a +/-2mm tolerance may have been achievable when the frames were cast on land. Then presumably something like a string line could be used to project the correct vertical alignment over the piles and shim the frames to the correct level.
To check/stakeout I guess as a suggestion one could use a very tight line to a symmetrical (spherical?) surface float (like a reverse plumb bob) that has a 360 prism to a robotic total station, and in the case of stakeout a diver with underwater comms, all in zero sea conditions. In the shallower parts, low tide and walk out with a prism pole.
barneyb, post: 440044, member: 9775 wrote: Hi everybody. A surveyor friend of mine has been approached to undertake an interesting survey job: A local boat builder requires stakeout for the twin rails of a newly constructed slip-way. At their farthest point, the rails are approximately 8.0m below water level (salt water). The requested tolerance is +/-2mm. I would be keen to hear from anyone who might be able to offer suggestions/past experience etc. Regards and thanks in advance.
is the tolerance mainly for gauge distance or for alignment as well? I'd think that if the distance between the rails is sufficiently close, a rod cut to length would serve to maintain gauge for marking. Making a 9m plumbing pole would be a challenge, but might work. it would have to be very rigid, non-buoyant, and have a positive "foot" to engage some underwater mark. adequately sensitive spirit level vials above water might do the trick once SOP was worked out. Maintaining plumb is another matter, but lines from anchorage points on shore might to it.
Richard Imrie, post: 440058, member: 11256 wrote: The underwater sections of railed slipways I have seen have been precast reinforced concrete longitudinal/cross-beam frames (on insitu piles) so a +/-2mm tolerance may have been achievable when the frames were cast on land. Then presumably something like a string line could be used to project the correct vertical alignment over the piles and shim the frames to the correct level.
To check/stakeout I guess as a suggestion one could use a very tight line to a symmetrical (spherical?) surface float (like a reverse plumb bob) that has a 360 prism to a robotic total station, and in the case of stakeout a diver with underwater comms, all in zero sea conditions.
Kent McMillan, post: 440061, member: 3 wrote: is the tolerance mainly for gauge distance or for alignment as well? I'd think that if the distance between the rails is sufficiently close, a rod cut to length would serve to maintain gauge for marking. Making a 9m plumbing pole would be a challenge, but might work. it would have to be very rigid, non-buoyant, and have a positive "foot" to engage some underwater mark. adequately sensitive spirit level vials above water might do the trick once SOP was worked out. Maintaining plumb is another matter, but lines from anchorage points on shore might to it.
As I am unaware of the proposed construction methodology i.e. precast & placed our built Insitu, it would be difficult to say whether tolerance also relates to gauge. Precast units seem to be more common (from what I have read myself), so one could suppose gauge might be taken care of already.
If you do really want to get a genuine 2mm accuracy I would be thinking coffer dam and some big pumps
Apart from the obvious question of why the lines weren't marked when the slipway was being constructed (I'm assuming that since it is smooth enough to have rails placed on it to that accuracy then it was constructed in the dry), have you asked the client how they will work to these lines once they have been marked? A line to that accuracy presumably means somebody being down at 8 metres depth to ensure things go to it - ie a diver. In that case the diver could also hold one end of a steel wire which runs well up onto low friction supports on the bank and move sideways until the wire alignment was correct. Plumbing 8 metres isn't going to work unless there is NO current and NO air movement, if you need 2mm.
I've done deeper ones before (pinning a pipe outfall) but had the benefit of a piling barge as a plumb bob. Lower pile to bed, lift 1 foot. Adjust barge cables to bring pile to line, check pile is plumb. Check distance to pile face (stuck on targets at 6 foot spacings. Lower pile. Move to next one.
jim.cox, post: 440069, member: 93 wrote: If you do really want to get a genuine 2mm accuracy I would be thinking coffer dam and some big pumps
That's a pretty good thought if the ways will need to be dry to install the rails in the first place. It makes the entire exercise much simpler.
jim.cox, post: 440069, member: 93 wrote: If you do really want to get a genuine 2mm accuracy I would be thinking coffer dam and some big pumps
Yeah, that's exactly what I suggested, but then ran the numbers for an 8.0m deep coffer.!
jim.cox, post: 440069, member: 93 wrote: If you do really want to get a genuine 2mm accuracy I would be thinking coffer dam and some big pumps
To get the accuracy you are probably going to need the shore end open, so you are looking at a U shape dam, rather then a closed box. A U would need piling or a huge amount of earthworks and, as you indicate, the cost would be out of the question. A coffer box, sitting down on a sloping slipway would cause immense problems in getting a decent seal around the base and you would still have to project the lines you put in it back up to the shore once the dam was removed.
This is either a very expensive job or one your friend doesn't want! There used to be something called project planning...
Should have asked this earlier - if it is salt water is the 8 metre depth at high or low tide and what is the tidal range?
I don't know what kind of budget they'll be working with so I'll just toss out what I've done years ago. We drove temporary H beam piles on either side of the construction limits and welded beams between the piles. Then a steel plate was welded to the beams and control was painted/etched onto the steel plates. Control was maintained on the shore to restore if templates were disturbed.
This was a cool project to work on. The contractor used sheet pilings and dams to hold the water back on the lagoon side. On the Cook Inlet side the tide difference was about 30 feet. If Washington Roebling was still around he could design a cofferdam that would work.
https://www.constructionequipment.com/westchester-lagoon-project
chris mills, post: 440076, member: 6244 wrote: Should have asked this earlier - if it is salt water is the 8 metre depth at high or low tide and what is the tidal range?
Hi Chris. From what I can gather, the 8.0m is low tide. Not sure of the range sorry.
If the 8 metres is low tide then the water depth is probably going to be 12-15 metres at high tide. That's not going to be practical to pump out between tides if you only use an 8 metre coffer, so the enabling works are going to be massive. I just find it hard to believe that somebody would construct the slipway first and only then think about how to put in the rails.
I can't think of any other practical way except a U shaped piling and you then have the problem of extracting all the piles - no good leaving cut off bits to possibly wreck any boat launch.
Well, The CORPS is constructing a dam in the dry and setting the giant LEGO block size pieces in the water with 1" or less accuracy. I guess we can do that with a 2.3 billion dollar budget. So, what's a millimeter or two worth? See http://www.waterpowermagazine.com/features/featureinnovations-at-olmsted-dam/
