My crew will be staking a sewer outfall in the near future, however there is a catch: The outfall lies 30 feet (9.14m) underwater in a lake and we have never undertaken a job like this. We are excited at the opportunity to expand our capabilities and are looking for creative ways to make it happen for our client. Our tolerance is within +/- 0.10 feet (30.5mm). We will most likely be staking this conventionally, but we do have GPS capabilities as well. The contractor has hired divers so we will have a man underwater, albeit not a surveyor. Any ideas, experience or not, are greatly appreciated. Thank you!
+- one tenth? Wow. How are they gonna check you? And, is that tolerance for both horizontal, and vertical? Are you gonna set offset stakes?
What is to be constructed down there? Is it big, or small? Just the outlet for a pipe?
Hmmmm For that tolerance, you might need an empty tube, verticle, in the water, with a plumb bob inside it, with a sight window, for the diver. If it were a piece of 4" or 6" PVC pipe, with a plexiglass window, at the bottom, and a stabilization pole in several dimensions, and a big sand bag tied to it, to sink it you could achieve this level of accuracy. If it had a big metal plate on the bottom, that would let it sit flat, on flat ground, then you could move and tweek it, until it was at the actual point. Then, the middle of the pipe would be good. Now, your diver can measure offsets, and drive rods into the bottom.
Strange problem!
N
Your post raises a lot of questions in my mind.
For starters, how is the pipe to be placed? If it is conventional excavation, I would assume that some sort of cofferdam or piles would be placed for dewatering. At that time there's a good number of ways to provide line and grade. Or if the line is to be bored under the lake, the entrance and exit pits can be located in a standard fashion. If the placement of the new sewer will be performed in any other way I have no suggestions.
I have worked with pipe and cable placement across bodies of water. We had instruments on the shore and it was, at times, frustrating. Stuff moves around A LOT when it's floating...
good luck, sounds like your going to need a 40' rod...:pinch:
How are they going to get the pipe in there? Coffer dam? If so let them do that first.
If this is a gravity flow sewer, Paden has hit the nail on the head. Ask the contractor how he is going to install the pipe. If he does not give you a logical answer, run like the wind.
This one sounds really fun. I've always been amazed as some of the bore contractors for sewer & water going a couple hundred feet under highways, busy roads etc that they could not open cut. Always within less than a tenth at the other end. Some used a big long hose where they could monitor the level from the water at the bore site, some just used a laser beam and a hip shot.
My guess is your guys will bore from dry land and hope for the best.
I would think your scuba crew could come up with something so you could verify things post construction. Even a long heavy chain, or piece together a bunch of 1/2" conduit.
In terms of staking, I'd be inclined to give them a couple BM's nearby and some type of horizontal guidance via offsets. Don't over evaluate things IMO, and let the contractor do his job.
Have fun & good luck.
I did one in Atka in about 60' of ocean. Had the coords loaded in the GPS, bouy with an anchor (and enough line on it of course), staked out to the plan location and when I was on it, kicked the anchor overboard (I used an old 6" gate valve) Divers checked it during the as-built, within 2 feet of plan. Good enough for turds!
-JD-
Sewer in the lake...the good old solution, hopefully in this day of age, the outfall is treated water...
Google the St. john's bubble: the entire city of St. john's was sending its raw sewer in the harbour. Tourists attraction, come see the bubble.
Offset buoys.
Level run by raft.
Demo the new waterproof total station from a dealer from far away.
To be serious, how about dropping some blocks with ropes attached. Put a mark on the rope at a specific distance (say 50') and pull the rope in various directions and locate the same with *gulp* RTK or Robot. Do it enough and do a 3-d D-D-D-D-D-D-D...-D intersection to find the location of each block. Have the diver run the 3-d location to the pipe by pulling every rope at the desired distance. (Could be a good use for a bunch of fiberglass tapes...)
How are they gonna check you? And, is that tolerance for both horizontal, and vertical? Are you gonna set offset stakes?
What is to be constructed down there? Is it big, or small? Just the outlet for a pipe?
Good questions. I am unaware of any plans to as-built, but will look in to it. Vertical tolerance has not been specified, but I imagine it is the same, +/- 0.10' (30.5mm). It is an 8" (20.32cm) dim. HDPE pipe. So far all the contractor has requested is the outfall location however I am sure more staking will be necessary. We are on very good terms with the contractor and are currently inquiring about their requisites. According to the plan set concrete anchors will be placed and the pipe will be installed on temporary rollers through the anchors.
According to the plan set concrete anchors will be placed and the pipe will be installed on temporary rollers through the anchors; no cofferdam unfortunately.
Or a really heavy duty plumb bob. Friend of mine has one that must weigh ten pounds that I've used for sounding.
As I understand your post, the outlet is 30' under the surface. How far out from the bank? If this is a directional bore, couldn't you give the contractor points on line in stable ground with the depths at these locations calculated from the amount of fall called for on the pipe? Once they have reached your second or third offset, they should be able to determine how accurate their gauge is on the machine.
Been there done that
But if I told you how I did it, I'd have to kill you 😀
The small 8" (20.3cm) pipe actually rests atop the lake bed, held in by concrete anchors that will need to be placed by the contractor. There is an "s" curve in the pipe that begins about 60ft (18.29m) offshore, and the outfall lies approximately 200ft (60.96m) offshore. The contractor has not yet requested we stake the anchor locations, only the outfall. Either way the challenge is the same; how to place stakes accurately 30ft (9.14m) underwater..
Then may I recommend a spud barge and GPS to put the corner of the barge directly over the point. Your tolerances aren't very realistic IMHO, but then again, I haven't had to navigate to a point with that precision. Maybe someone else can enlighten us on the tolerances.
Good luck.
Do you have any suggestions on where to find a stable 40ft rod?
> My crew will be staking a sewer outfall in the near future, however there is a catch: The outfall lies 30 feet (9.14m) underwater in a lake and we have never undertaken a job like this. We are excited at the opportunity to expand our capabilities and are looking for creative ways to make it happen for our client. Our tolerance is within +/- 0.10 feet (30.5mm). We will most likely be staking this conventionally, but we do have GPS capabilities as well. The contractor has hired divers so we will have a man underwater, albeit not a surveyor. Any ideas, experience or not, are greatly appreciated. Thank you!
Hey...
Wow!!! There is so much to address in your post. The follow up posts show why dry land types should stay far away from advising on any marine construction operations. The first thing is that the contractor has no business hiring divers - a recipe for disaster - the prime should be sub contracting to a professional underwater contractor. A diving supervisor who is in complete control of site operations will determine the type of diving operation to conduct and be responsible for personnel (your insurance carrier and legal department will be pleased). A certified commercial diver has post secondary education in areas such as physics, hyperbaric medicine, pipeline construction, and yes, topographic surveying. It will sound like a lot of money at first - like anything else on a pipeline - but you are less likely to lose sleep and in the long run, money. Divers can provide you with real time voice and video communication and use tools to survey well within your tolerances. If you aren't a sub sea engineer - hire a professional.
I assume the 40 foot rod you are looking for is so you can "touch bottom" - an anchored buoy released from depth will naturally seek the zenith in a straight line. Also, you could try using a fish finder to produce a hydrographic survey on the cheap. If you are still determined to hire your own diver, stay away from eager local scuby-doos and make contact with a maritime museum who can put you in touch with a diver that has underwater archaeology experience.
I hope I don't seem offensive - I came to surveying through diving and construction so I didn't have much of a chance to develop a lot of social skills.
Thank you for your reply! Do not worry, constructive criticism is appreciated; I am not offended. The contractor does specialize in marine construction and has their own professional divers. We ended up passing along the surveying contract because we did not feel we could accomplish what needed to be done within their time constraints. The buoy idea is great, we will keep that in mind for future surveys. Cheers!
