survey and control techniques to ensure pile installment accuracy 1

[TeeNan]

To All the experts,

I am a new starter in the construction industry. The project that I am now working on is upon the completion of diaphram walls, structural auger piles are now required - positioning and vertical accuracy is very much required at this stage. What surveying and control techniques can be used to ensure that pile installment is as planned?? Previous marking may be distorted as a result of the diaphragm wall construction and heavy machineries that are used.

Please advise.

Thanx...

 

[Qshake]

Usually the pile locations are marked just before work begins so as not to destroy the markers. Markers range from "hubs" (2"x2" driven stakes) to orange spray paint. The hubs are set from offsets of the centerlines for the element.

To ensure alignment, I've seen a number of methods used and most are nothing more than developed barnyard devices. Moreover, most methods were based on a sounding line (really heavy weight on a tape). The weight on the sounding line should be heavy as you may encounter water or the process may use drillers mud. Anyway the weight usually will have some known distance from the center to the outside edge and based on this distance and the distance the line reads (depending how much line you lower in to the hole) will give you an idea of plunbness.

 

[TeeNan]

Tnx Qshake,

ur suggestions are helpful, but I still hav a few guerries:

would the hubs disrupt the machinery movement / or be destroyed by machineries? (as this site invovles 100+ piles)

what would you suggest the site engineer to ensure the accuracy of the marking since the ground may shift. (i.e. check points required and system)

Tnx (|>

 

[ishvaaag]

It the accuracy required exceeds that normal, a surveying able team better be in place. Otherwise, making the foreman or one his deputy of being alert about the persistence of what detailed, ordained and put in place for the work should be enough. The reviewer party liable for such works or its review should be at hand in a case like yours, wanting stringent accuracy.

 

[Qshake]

Ultimately the hubs will be destroyed by the drilling equipment. Normally the hubs wil not be destroyed by vibrations of the ground due to machinery! Once the auger is over the hub and begins drilling - its all over. Its just not that accurate of an operation. Usually the tolerance specified in the Job Special Provisions will note this and it is not uncommon to have a 3" or 6" max off center. Steel piling and drilled shafts are set pretty well by the foreman.

During the operation, you'll want to set "tattletales" or benchmarks that will allow you to make a quick check (usually within a 6' distance for a 6' folding rule) for location checks. So in other words you have a mark for the center of pile and two offsets from that mark in each orthogonal direction to make quick measurements. However, please keep in mind that once the pile or auger begins there is little you can do to check anything which is why it is critical to place it right the first time. Moreover, you don't want to make the crew move a pile for anything less than allowed tolerance! Too much fight!

Suggest that if you can get a copy of Engineering of Pile Installations by Fredrick Merrit. This reference discusses all the steps and mostly from the inspector's viewpoint.

By the way, never set hubs where the contractor may run equipment over them. If this is the case then too many operations are going on to accurately set the hubs. Tell the contractor this and ask him to watch out or you won't set another hub and that if a pile is located incorrectly it will come out of his pocket. That has always burned me up. The contractor asking way too soon to have some type of control set only to have it run-over by an earthmover!

 

[Ron]

TeeNan...QShake and ishvaaag have covered the technical aspects quite nicely (as usual for both of them!).

Since you are new to the construction realm, I'm going to offer something outside the technical question you asked but is included in your question...

The term "ensure" has been used several times in this discourse. While I certainly understand the context in which you are using the term, let me offer that this is one of those words that attorneys just love to hang engineers with! While it is not quite as bad as "insure", it is quite close in meaning and is used synonymously in many instances. In short, it implies certification and guarantee, both of which are taboo in the liability lexicon of engineering.

As Qshake pointed out, this is not and does not have to be an exact location exercise. Tolerances are permitted and necessary. If you tie the term ensure to this, you tend to negate the effect of having tolerances, thus locking yourself into an accuracy that you probably cannot control nor deliver. "Down the road" it could become an issue, relevant or not, but nonetheless and issue. Issues often become problems.

Be careful of the use of words in our business. We often mean them in a different light than they will be interpreted by others.

 

[Qshake]

Well stated RON!! I knew there was that ominous lawyer thing hanging around but I had forgotten all about it!! Excellent. There should be something about those nuances in construction inspection for new graduates or otherwise employees new to that vocation. I nominate you, RON, to begin a seminar tour!

 

[Ron]

Hey Q...if we go on tour do we get Groupies?

 

[Qshake]

Of course. the question is...remembering back to school dazes is what do they look like!

 

[Ron]

Yeah, and probably the same ones...just older!!

 

[DRC1]

Prior to doing any work on the site the survey team responsible for layout should establish the survey working points. These are coordinated points shown on the drawing which are established in the field. All layout is referenced off lines established by these points. After locating the workig points, the surveyor will establish a series of his own control points in locations whre he can see them from the project, yet will be safe through the life of the project. He will then run a traverese to be sure the coordinates are correct.Having done that he can set two lines of points off set from the pile location by
some distance, say 10 ft and 5 ft. He will ussually set hubs past the end of the line. This will allow him to quickly set up and restablish the line if some of the offsets are destroyed. Often times a hub or paint mark will be set at the actual pile location. If the actual pile location is not set or destroyed, the pile drilling foreman can hold his tape at the 10 ft offset, line it up over the 5 ft ofset ( to ensure line) and mark the pile at 10 ft. from the outside hub.
First a comment on destroyed markout - Hubs will be lost durring construction. Some projects it is tough to find places where the wont be lost. Some contractors do not have much regard for mark out, but that is rare. Most contractors do recognize the importance of trying to maintain stakes, however, some amount of remarkig will be inevitable. The trick is to establish control to allow remarking with a minimum of rework.
Second, If you are checking in installation of the piles is what do you check? tolerances for piles are typcally 1-2 inches for a pile that may have a diameter of 6 to 12 inches. On the plans, the piles are a perfect cile with an eaisly defined center. In the field th tops of the piles are anything but round (unless they are cased) making the center impossible to find. This brings into question the need for exact tolerances when you can't define what must be in tolerance. However, the best way I think, is to confirm the off set locations and the drill marks so that you know the drill was in tolerance to start. Note that obstructions, especcially near the surface may move the drill, and obstructions may or may not be the contractors responsibility.
Third, with auger cast piles, especcially uncased, the concern is not the tolerance usually, but the shape of the pile in the ground. GRL has equipment that can give you the pile shape in the ground. (

http://www.pile.com

for more information)
Finally if you are doing the inspection, it is a lot easier to correct a pile location prior to installing it than after, so get involved in the layout and be sure it is what you want. Also talk to the drillers and understand what it is that they are doing. If you have a good relationship with the crew, it will be easier to resolve problems when they arise.

 

[ayfarm]

Use a total station w/ EDM or an accurate DGPS system wheteher you are on land or in water!