What's the acceptable construction line / base line

[ngneer]

In designing a sanitary sewer for an existing neighborhood, I wanted to know the standard way to determine stationing. For example, using the roadway centerline is difficult since the streets are very narrow (15'), lack curbing, and constantly change direction. Also, the property lines (some of the roads are private) do not meet in the roadway center. I'm thinking I can just use construction lines (straight runs at a certain directions and distances)and give angles to turn at the line intersections. I don't want to use the pipe layout to base the stationing since I would have to continually change the stationing for each different pipe trial layout. What is the best method for the contractor to follow? Thanks.

 

[snafuman]

Why use stationing at all? Stationing works great for roadways, but with modern EDMs, CAD drawings and coordinate based layout programs, stationing as a layout procedure is becoming obsolete. You can show the lengths of each run of sewer line on your plans. The contractor can base his price on these lengths. The manhole locations, fittings, curved runs, everything else can be field staked by radial stakeout of the coordinates. The contractor just follows the stakes.

 

[KRSServices]

Offset your grade stakes (from the alignment of the pipe) at a distance which will minimize disturbance by the contractor's operations. Have it specified in the contract that the contractor will be responsible for the cost of replacement. The offset distance will be a function of the type of cut, depth of the new line and the contractor's desired working area. For sewer pipe installation, I usually place the stations at every 15 m (50') and will coincide with the grade sheet. 15' is too close and needless work. These stations will be used for the contractor's quality control, and determining spot elevations.

snafuman, I hate to break it to you, but for layout, setting control points, laying out the BC an EC of the pipe curves and appurtenance layout, your method works great, however, to get spot elevations once the contractor is happily working, or to reset the laser, still requires grade stakes and therefore layout staking is still required. Initial layout with the EDM saves a phenomenal amount of time, but offset (grade) stakes will still be necessary.



KRS Services

http://www.krs-services.com

 

[snafuman]

Offset stakes can be set from the radial stakeout points as described in the post. The offsets can be set by the surveyor or by the contractor (the contractor can then put them where they work best for him). Of course grades must go on the offset stakes, or be moved to them by the contractor.

Offset stakes can also be laid out and set directly from the coordinates by radial stakeout, the problem with this being that you set them up on paper but once in the field there will always be things in the way or unstake-able spots, then you have to recalculate the location or field adjust the offset. Offsets often times cannot be set at a consistent distance (for instance a 15' offset ends up in someone's driveway). Better to mark the actual locations and then set offset stakes where they can be driven in and where they will be out of the contractor's way.

Still think that this method would work better than layout through stationing for the situation he describes. Contractors tend to follow stakes and would have difficulty with a stationing procedure that involves numerous station equations.

 

[KRSServices]

snafuman, I suppose it comes down to the level of comfort and experiences. I guess we both have our preferred methods.

In an open field, with little obstructions, setting up and locating by EDM coordinates is great. There still is the grade stationing hiccup though. However, in staking rehab or infrastructure in established urban areas is another task unto itself, particularly if there is little or conflicting legal evidence and constructing (and turning) in city blocks. I prefer to utilize offset stakes, relative to property lines. Straight line (either by transit or total station and level shots from an existing monument. In most cases, the existing infrastructure is there, so there really is not any mystery to location, but depth is always checked, hence the need for some stationing. Further, in existing areas, the stationing is required to assist in the locating of tie-ins, utility locates, conflicting services and critical location/elevation changes. In most cases the (stakes) are in the form of a concrete nail with a ribbon/flattened bottlecap and/or paintmark on concrete/asphalt.

Either way, none of us are absolutely wrong/right, we are used to or comfortable with different methodologies.

KRS Services

http://www.krs-services.com

 

[ngneer]

On the profile sheet, I don't want to use the grade above the proposed pipe since it will continually change each time I change the pipe layout. I have often seen other drawings using the road centerline grade, but as discussed, for these roadways the centerline is difficult to determine and I don't have any property lines to follow.

 

[KRSServices]

ngneer,

Do not your design drawing specify defined alignments relative to a property line or something? I ask because it it that reference to which the new line is to be stationed. The grade stakes, nails or paint points are shot to provide the reference elevations for the contractor to determine the cut and/or check the pipe and set the laser. The offset is whatever distance is decided. As long as one or two property pins define the road allowance width, the pipe can easily be staked by eye, theodolite or total station. I assume the profile sheet is the same as my definition of grade sheet.

The profile sheet needs four pieces of information, the grade of the pipe (in %), the hub elevation, the calculated invert elevation (or top of pipe) and the cut at that hub (station). It matters not if the hub is a stake, a paintmark, a nail in the asphalt or a nail pounded in the side of a power pole. The cut (depth to pipe) has to be relative to a known benchmark, in this case the hub. If the distance between the hubs (stations) are too freat, there is too much room for errors in verifying the as-constructed elevation (as per the design), particularly if the laser distance is getting to long and the dot is bouncing too much. Further, each pipe should be checked for grade prior to backfilling. I do not know how else it can be done without increasing the probability for too many errors.

KRS Services

http://www.krs-services.com

 

[KRSServices]

P.S.

As you can probably tell, in my early days with the City utility replacements and rehabs, I was responsible for the design, construction, survey, layout and inspection of water, sewer and storm pipe replacements in established neighbourhoods and in new subdivisions. I also had to prepare and review the as-built records as well. I have used many different techniques but have always selected the methodologies that were the simplest, efficient, fast and minimized the chance of errors. Therefore, I provide advice and thoughts based on experience.

You are free to contact me for information or specific details as to resolving some unusual layout or related issues. There is not too much that I have not encountered before.

KRS Services

http://www.krs-services.com

 

[BobPE]

ngneer:

Its common practice to design the sewers with the stationing above them and based directly from the sewer lines. They are sewer design drawings so its important to show the stationing that way. Yes the branches will have to be restationed, but I usually dont start the stationing from zero, rather a higher number and use the lower numbers for the branches. The datum will then have the added benefit of showing the grade above the sewer centerline to check depth of cover. I usually reject designs for sewers based on road centerline since its too confusing to tell exactly what is going on with the sewer design.

I hope that helped some...

BobPE

 

[ngneer]

BobPE,

Thanks for answering the reply. I guess there's no way of avoiding having to re-station the grade above the sewer. I was thinking if I kept the road centerline, I would not have to change the profile grade line for each sewer design trial. When I arrive at the final sewer layout, I will do one profile of the grade above the proposed sewer. I understand also that it's deceiving if the road centerline length can be substantially shorter/longer than the actual pipe length if the road or pipe continually changes directions.

I also appreciate the other posts concerning the actual construction stake-out.

 

[KRSServices]

ngneer,

I incorrectly assumed when you referenced the contractor your query was primarily premised on field work. However in reading the Bobpe's post, I understand your query to be more fundamental. As I understand it, you are referencing the profile drawings and the stationing and a method through which the contractor will understand.

Your stationing will involve two considerations:

1) Real Ground Distances (chainage)
2) Sewer Pipe Length (Profile Distances)

The real ground is important because that is how your legal evidence is posted. All the reference locations, centrelines, offsets and Fd.I's will be premised on this evidence. This will also be the reference for the contractor and the surveyors.

The sewer profiles drawings will show the actual (laid) pipe length (in profile). These lengths are important for the estimates and quantity takeoffs. It is also important to note that these lengths will never, ever match the surface profile distances, and should never be used as the reference for stationing.

Your plan/profile design drawings should show the surface stationing of the sewer pipe and appurtenance locations, in whatever stationing methodology you wish to choose. The profiles will either show the pipe referenced to the surface stationing (manhole tie-ins, etc.) or will be plotted as an actual length. In most cases, due to the different vertical and horizontal scaling, the pipe will be plotted and shown as being longer than the surface stationing.

In my experience, most contractors prefer the cleaner plan/profiles where the pipe is plotted in prfile to coincide with the surface stationing and are knowledgable to know that they cannot "scale" the profile pipe for quantities. In this instance, the calculated pipe lengths and proposed grade should be shown on the drawing.

I hope this clarifies this for you.


KRS Services

http://www.krs-services.com