Subsurface Materials Found to be Poor 2

[JAE]

We recently designed a small addition to an existing sewage treatment facility that involved a deep (20 ft) concrete vault situated next to an existing pumphouse/vault. Both the existing and the new vault are set on a thick floor mat of concrete at the same elevation. The geotech gave us design guidance that was followed and the contract was bid.

On the contract plans, we included boring logs from the geotech report for information only. The contractor who won the bid assumed that he could successfully cut his excavation vertically the 20 feet around the vault addition and shore this excavation with a soldier pier wall system. He saw on the boring logs that from 20 ft. to 30 ft. was a "hard" lean clay that should adequately hold his piling/piers.

Now that he has begun driving his piling, he has discovered that the actual soil conditions seem to be different - once the piling hits 25 feet, the piling takes off downward through a very soft material. He now believes that he cannot use soldier piers and must lay back the excavation.

Our specs say that he cannot rely on the soil data and then hold the owner for extra costs based on that reliance. This is what we agree as well.

Our concept is based on an important distinction:
1. If the contractor finds bad soil and this requires a change in the DESIGN of the structure, then he should be compensated for those changes and time costs.
2. BUT, if the contractor makes an assumption of soil quality that affects his MEANS AND METHODS of building the structure, then he is taking his own risk.

Do you think this distinction is consistent with "standard" industry practices? (the project is in the U.S.)

 

[dlew]

JAE,

We generally include your second note (with different wording) in the general conditions of our design projects.
Normally, the soil reports have also a disclaimer saying that the borings show the soil conditions ONLY at the exact locations where the borings where drilled and to the shown depths.

But in cases where a condition is discovered after the job is bidded that affect the job (per example, roots and organic material that need to be removed; or underground utilities that interfer with the foundation layout; or boulders or rubbish that make pile driving more difficult) the contractor has been paid for the extra work.

The question in your case, I think, is whether the layer of soft material encountered was an unforeseen soil condition, and whether the contractor's assumption that he could use shoring for that excavation was a reasonable assumption given the soil information provided.

Does the layer of soft material extend under the new vault foundation? And if so, will that change the foundation recommendations?

Regards

AEF

 

[jheidt2543]

Why is it that the designer who orders the soil borings and uses them for his design of the structure can then tell the contractor that he can't rely on them for bidding the work? This has always stuck in my craw. If the engineer uses the boring information for the design, the contractor should be allowed to use them for bidding the work.

If the conditions change they change. The contractor doesn't pick the site of the work, the Owner does. The contract documents should provide a contingency not only for changed conditions, but for unforseen conditions. The plain truth is that most (not all) designers don't think the job through from both a design standpoint, but also a construction standpoint. Designs have to be buildable.

 

[sc]

I agree with jheidt2543 if the designer has used the bore log information for design, then it must good enough for construction. Whilst the designer is not responsible for changes in condition he is responsible to ensure that the design is adequate for the site.

Given the conditions encountered I would (if I was the contractor) ask why didn't the bore logs pick up this change? or perhaps he needs to check with his wall designer to see why the change in soil wasn't picked up as problem.

Knowing that the softer material exists in the foundation area, I think that you as the design needs to check that footing is also suitable, just as the contractor is now revising the work method.

Generally in my contracts I allow for a 10% increase in cost of the subgrade/footings construction to allow for problems such as this. That way the owner or the contractor don't get caught out. If the owner doesn't like this advice then let them know that it is better than spending the extra 10% on litigation costs at a latter date.

regards
sc

 

[JAE]

Thanks for the replies!

Yes, its true that we engineers use the soil borings for our design. But its not true that we deny the contractor the right to use the borings (we put them on the plans and make the geotech report available). Both the engineer and the contractor use the boring data and geotech recommendations to accomplish their work.

The difference:
The engineer bases his/her design on it and then, or should, have the geotech review the conditions if some unforseen condition arises. Changes in the design might be necessary and the contractor would be paid for those changes. The risk of unforseen conditions is placed on the owner (as you mentioned jheidt2543, its the owner's site after all).

The contractor, on the other hand, assumed the risk of CHOOSING a soldier pier method of construction vs. a lay-back excavation. It was his initiative to do this, the owner didn't force him into this risk. Neither did the engineer.

Therefore, it seems to me that its his problem and my reason for posting this question was to see if others agreed with this interpretation.

 

[Trussdoc]

While the owner doesn't "force" the contractor into choosing a particular approach, the reality is that the owner will give the job to the low bidder who will use the cheapest method and may skimp on the contingencies just to get the job.

 

[JAE]

Trussdoc, that was actually discussed here - that the contractor's use of a "good idea" for his means and methods directly benefits the owner in that it lowers the cost of construction. But the counter comment that came out was that all contractors, all the time, are expected to bring to the table their expertise, knowledge and skills in building the project at the desired level of quality for the least cost.

By bidding the project the contractor is exclaiming: "I can build your design for X amount of money." If the contractor makes a mistake in his take-off, and it ends up costing him X + Y, that is his risk is it not? Not the owner's risk. In this case, he took a calculated risk based on the limited soil information. But I still think its the contractor's risk, not the owners.

Again, I appreciate the comments above....

 

[jheidt2543]

The cost of any design is fixed at the time the plans go out for bid. Each contractor puts his price on the engineer's design, but the actual cost of the work to the Owner is fixed by the design. Ninty percent of any cost savings in a project are found upfront in the preliminary design stage. To say that a contractor is "hungry" for work or "too busy" to be competative is simplistic.

The reality is that both the designer and the contractor are under tremendous pressure to keep costs down. If the engineer's design is too costly, then he doesn't get future work with that Owner. If the contractor's costs are too high compared to his bid, then he goes out of business.

It is true that the contractor "chooses" his methods of construction, but many times the site and design dictate those choices.

I think the prudent designer prepares his Owner for changes and provides guidance before bid day on how to handle allowances and contingencies. Uneducated Owner's only see dollar signs, they must be guided through the process to see the total picture. A quality building doesn't get built without at team approach and both the designer and contractor surviving the process!

 

[CTW]

JAE -
I think your distinctions are correct. The contractor chose to use soldier piers. Therefore he made an assumption that the soil quality in this area would be consistent with the results of the borings. He was wrong.

As a comparison, if an engineer designed a foundation based on the assumption that the soil quality at a site was consistent with several borings, and found out during construction that the soil quality was different than what the borings indicated, then the engineer is at fault for making the wrong assumption. The engineer would have to redesign the foundation at his cost because he made the mistake.

The contractor made the mistake, not the owner, engineer, or geotech. He is responsible since he made the soil assumption and the decision to use solier piers.