Flooded Caisson

[httc78]

I am involved with a project to construct vehicular viaduct. the contractor was not able to finish the foundation works and there are currently a few caissons that has been fully excavated and flooded with water. a combination of high water table and rain.

as the caisson has been completed for some time, it has been left flooded and i am concerned about its overall capacity after being exposed to such prolonged period of exposure to water.

is there a genuine concern for thinking that there is a tendency that the capacity has reduced substantially? if so, is there a way of ascertaining the capacity of the soil around the completed caissons?

the caissons varies between 3 to 6m in diameter and toe levels at 50-60m beneath ground level. soil around the site are generally sand and sandy clay. beneath 30m, the ground consist of stiff clay which is used for socketing as the top half is debonded for future works.

 

[TDAA]

Interesting dilemma.

How does the level of water in the hole compare to the GW levels? Is the GW perched, leaving some soils below normally unsaturated that are now saturated?

What were the design parameters? Was all end bearing assumed, or are you relying on the skin friction over a majority of the caisson?

You can get some initial indications of the clay with a pocket penetrometer, but I would try to obtain samples for testing in a similar way the soils were investigated. Also for the clays, try to determine how saturated they have become. If the wetting has only occurred over a thin portion, you can likely scrape off the muck and continue, or it may not be an issue at all. The saturation of the sands may not be too much of an issue, depending on the GW tables. If the design assumed they were, then it should not be an issue.

That is a pretty good sized foundation. How is it shored? What options do you have available to investigate the soils? Simple observations be the geotech may be all that is required, if there has been little impact, or you could have a problem on your hands.

 

[SlideRuleEra]

One way that we have dealt with this type situation, when there is not time for a detailed review of existing conditions, is to require the Contractor to drill deeper. Just a little bit (a foot or two) for an end-bearing caisson, or the necessary additional depth to develop 100% of required skin friction. If this is appropriate for the soils on your project, construction can proceed with confidence & minimal delay.

As TDAA says, problems usually don't happen because the supporting soils are wet... or dry. Things go wrong when conditions alternate between wet & dry.

http://www.SlideRuleEra.net

 

[httc78]

due to the duration that the caisson has been left lying as it is. .the water table is more or less the same as the gwt. .if not slightly higher.. depends on weather.. if it rains.. it will definitely be so..

the pile capacity is a combination of skin and breaing resistance.. based on my calculations.. a loss of 5% SPT N would have caused the caisson to lose its design capacity.. esp those completed ones. The difficult bit of drilling deeper now is that we have already cast a concrete base layer. to go down deeper, we would need to draw off the water.. hack away that base concrete and continue excavation..

the situation is that there are a few caissons all in line.. about 6 caissons (about 30 - 40m apart from each other, carying span lengths) in different stages of completion they are all flooded.. 2 has been completed with the base concreted. these are located at the beginning and the end of the series of 6. there is also one in the middle also completed. but the base not yet cast. so in terms of global effects, the water could have soaked all the soil along this "imaginary" caisson strip.

the location of the site is also such that its very tight and to cater for future works, its difficult to shift the alignment and redesign the piles..

i was thinking of sinking a few boreholes just next to these flooded caissons and between the caissons to determine the N value and use that to recalculate the capacity. in that way, i will not need to pump off all the water.. would this be good enough?

 

[dgillette]

".. a loss of 5% SPT N would have caused the caisson to lose its design capacity.."

5% is well within the "noise" for the best-quality SPTs, ignoring the uncertainty in any empirical correlation you might be using. There are NUMEROUS sources of uncertainty in the SPT measurement, energy transmission being the worst. The energy transmitted to the drill string can vary tremendously from rig to rig, from hammer to hammer, from driller to driller, and even from new rope to old rope and from one rod size to another. Then there are issues with the sampler ([space for a liner but no liner used] vs [no space for a liner or liner used]), and of course the uncertainty in the correlation used to estimate undrained strength or bearing capacity. (Did your drilling use the same methods as were used to generate the correlations?)

If a 5% variation in the SPT (say 19 blows vs 20) is enough to reduce the capacity below design capacity, you should take a hard look at the reliability of the foundation, as a few of those things going the wrong way could eat up your safety factor. If nothing else, look at how much variation in energy would be needed in order to drive the safety factor below 1.0, and see if that's plausible.

The literature on liquefaction analysis can provide some guidance. (See, e.g., Seed, H.B., K. Tokimatsu, L.F. Harder, and R.M. Chung, "Influence of SPT Procedures in Soil Liquefaction Resistance Evaluations," ASCE Jnl of the Geotechnical Engineering, v. 111, no. 12, December 1985, pp. 1425-1445.)

Bon chance!

dgillette