Settlement of hotel on fill material 4

[charliealphabravo]

Hi all,

A 4-story wood frame hotel (2 years old) is experiencing localized settlement along some shallow interior monolithic strip footings. Our working hypothesis is that the interior footings in question were constructed on about 4 feet of poorly compacted fill or possibly that there are localized problems with the natural soils. The exterior stemwall footings are about 4 feet deep and so are near natural grade. There are no indications that the exterior footings are experiencing excessive settlement and most other interior footings seem to be performing well. We have a fair set of construction plans but don't currently have access to any of the original testing reports if they exist.

As the structural investigator on the project I have some geotechnical background and am working closely with an experienced geotechnical engineer in the area to put together a testing plan. I would appreciate some additional opinions as to what tests might be informative in testing our hypothesis.

The current plan includes monitoring wells and soil sampling for laboratory density testing. We are also considering nuclear densometer testing which will need to be intelligently located and require a larger disturbed area. If the soils are not highly consolidated clays then i would like to consider some type of cone penetrometer testing through slab cores since more of them can be used and performed less intrusively.

My concerns center around the type/depth/number/location of the tests. On the interior we are working with a low overhead environment (8 ft). Also more test locations would seem to be better than fewer unless we can locate them intelligently. I think the issues are fairly intuitive but I would just like to have confidence that we are allocating our resources well and have the best chance of learning something.

Thanks in advance.

 

[fattdad]

use a dilatometer to directly measure soil modulus among other parameters.

f-d

¡papá gordo ain’t no madre flaca!

 

[Ron]

I agree with F-D; however, access is a problem, I'm sure. I would not depend solely on density testing, particularly with nuclear density testing in a confined excavation of 4 feet deep. I would use more direct measurements, such as sand cone or drive sleeve density testing.

I would also do some dynamic cone penetrometer testing. This is portable and correlatable to other properties.

Check the slab and footings for actual settlement...measured against something that is not moving relative to the objects of measure.

 

[charliealphabravo]

Thanks guys for the direction on testing methods, especially the dilatometer and sand cone. I'd like to ask a follow up question.

Would the more invasive pit-type tests (ND and SC) be better located outside the influence cone of the footings in question in order to encounter soil densities most like those at the time of construction OR should they be advanced down through the cone of influence for the footings? I am concerned that the soils near the footing may have compacted/consolidated due to the superimposed structural loads and settlement and possibly result in inconclusive results so to speak. Based on relative floor elevation mapping and damage patterns I estimate the local differential settlement near the footings is something like 1 or 2 inches more than the rest of the structure. Unfortunately I don't believe any absolute floor elevations were taken following construction to serve as a baseline.
Thanks again all.

 

[fattdad]

I don't like pits 'cause you're adding a defect to the site. They never get backfilled properly.

If you mobilize with a dilatometer (or cone) - study near the footing and away from the footing at multiple locations. It's quick.

f-d

¡papá gordo ain’t no madre flaca!

 

[Ron]

Agree again with F-D, assuming you can get access with limited headroom and door entries.

 

[Ron]

Oops! Pushed button too fast. Another option is to use a hand-held dynamic cone penetrometer through a core hole in the concrete slabs, accompanied by an auger boring.

 

[BigH]

As part of your thought process, consider the nature of the soil deposition in the area. There must be some available information - US Conservation Authority has mappings. Find out if there is any local "history" of organic filled depressions, streams or rivlets. This might give you some insight as to whether the foundation materials are potentially blamable or is it the fill.
I've heard of two cases - one of each; a building up near Barrie Ontario where after construction, it was found that organic pockets had been left within the footing footprint - lead to settlements. A second was fill placed in a very cold region industrial building (6 m thick) that was only compacted at the surface layer - and people wondered why they had huge settlements of the thin floor slab (including disjoint cracking) when the ground "warmed" up.
On several occasions, I along with some burly drillers, carried out, by hand, standard penetration tests (yes, using the 140# hammer dropping 30 inches). This could be done and might provide a bit better information than just a dynamic cone as you can obtain a sample as well. - Two 2ft drives (using the longer spoon) would get you down to 4 ft. Don't need any head room. The test pits are, in my view, useful - but as F-D states, they must be backfilled properly and that is the "bear" of the problem; a cement rich backfill might be an answer to this, though.
Just some thoughts to the always excellent advice given by the two other responders!

 

[charliealphabravo]

This exchange has been very helpful. I am hoping that the consultants I am working with will be open to potential value of the DCP and dilatometer. If my past experience is any guide I expect some reluctance to consider the DCP. I have yet to encounter a geotech other than my old gaffer at university who recognized the value of the cone penetrometer. I'm afraid I will get a blank stare when it comes to the dilatometer.

So it looks like there are options for the DCP and even the SPT for use in low overhead environments. Can anyone confirm if some form of the dilatometer is feasible in these circumstances? I imagine it might require the use of a skid steer or something similar. Also, if we decide not to use open pits what might be the next best option for density determination in lieu of the sand cone and ND?

btw since BigH has some experience in the great white north i will mention that this particular project is located in central Alberta. I am currently working here on a NAFTA visa having just moved from FL so I am not so familiar with the geology as of yet.

thanks again all.

cab

 

[oldestguy]

I'd set up any field work with the idea that it should fit with a fix method if possible. Let's say you start with some very simple testing, say by hand, maybe not even so elaborate as SPT, maybe with simple rod probes. That might be sufficient, but it may require subsequent more elaborate tests, maybe not. If you are working near the footings, with holes cored in the slab, then might not those holes be used for a fix later? Where is the underground plumbing? Is it leaking? Is it in the way of my proposed testing? What is the likely fix, assuming the ground is lightly compacted fill? Most likely a compaction grouting technique would be one of the fixes immediately coming to mind.

Then, let's say you do a grout compaction job and want to know how well you did. Well, first off I'd see if I raised those existing footings. That might be sufficient testing. Do I want to raise the floor slabs also?

However, if plumbing under the slab is likely to be damaged by compaction grouting, I might go for a chemical stabilization technique. How do I test work that later? Will my testing method in the beginning still work to check the differences for chemical grout results? Will the fixing method result in variable resulting ground conditions, but still suitable for the job and if so, will the subsequent tests be affected by that variability?

You see I'd go a step at a time, not jumping in with a big exploration program that may not be needed. You don't need to get real fancy to get a good idea of the ground conditions.