Compensated Rafts Below Water Table (Oldestguy Answer is preferred. :)

[bdbd]

My question is about the compensated rafts and piled rafts. First of all I have reviwed some papers on compensated piled rafts.

In a project there is a really weak clay with ground water level 1.5m. 9m will be excavated. We are planning to use jet grouts. Sides will be supported by anchored diaphragm walls. My questions are:

1) Uplift force on the raft will be 75 kpa, right? So any part of the building that weighs less than 75 kPa will try to float. And I shouldnt consider the excavated soil during this checks. Because this is a load comparison, soil's memory does not affect this. Are there anything wrong in these comments?

2) To compare unimproved bearing capacity of the foundation and structure load, usşng Terzaghi we will include the foundation depth. So is there a contradiction aith my comments in 1? It kşnd of means that I am comparing net pressure with bearing capacity. Is this correct?

3) so to calculate the load on the jet grouts, which load will I use? Directly structural losd, structural minus uplift, structural minus uplift minus excavated soil.

Say cu = 20 kPa gw at surface, excavation 10m unit weight 20 structure 120 kPa

Bearing capacity approximately 5.14*20+10*(20-10)=220 kPa. Due to excavation my bearing capacity increased by 100 kPa. My structure is safe agaşnst bearing failure.

Uplift check: uplift force at the raft 10*10=100 kpa. My structure is 120 kpa. However during the construction uplift will affect the structure. Poulos reccomends prior installing of piles in this case. But, I dont understand if we have groundwater level above the base, we will have uplift, thus continious uplift force, will our raft float before the structure complete in every case.

And last, loads on jet grouts, will I calculate my jet grout spacing with 120 - 10*10 or directly 120 kpa??

Thanks in advance.

 

[oldestguy]

OK here comes OG. First, my background is in English units and I'd need to do an equivalent conversion. Also a cross section sketch would be helpful, showing full depth of soil profile. What is the facility being built?

 

[bdbd]

WELCOME!

Do not bother with calculations, please.

Maximum depth of investigation is 40meter full clay.

A simple idea will be enough. Will I use net foundation pressure or directly structural load to distribute load to jet grouts?

 

[oldestguy]

Seems like a very deep "basement" and if you need jet grouted piles, it is rather heavy structure. The buoyancy should help, but I'd think the a more complete description of what this is would help all to generally comment on what is proposed and how it will be built. Problems with a potential quick condition in the excavation and in installing the piles may overshadow concerns about floating, etc.

 

[aeoliantexan]

You may need to dewater to construct, especially if there are sand lenses in the clay. Sometimes in thick continuous clay a little pumping from sumps is all that is needed, but I believe the pore water pressure beneath the excavation will be reduced in any case. You need to look at the effective stress in the clay throughout the construction process. Initially, there will be heave. When the building weight exceeds the buoyant weight of the excavated soil, there may be settlement. The water will not help support the building until you let the water pressure build up under the floor (which must be very strong). You will have to manage the water pressure to keep the partially-constructed building from floating or settling. Perhaps you need a granular drainage layer under the floor connectd to a sump with controlled pumping to maintain the water pressure in a safe range.

How about the future? Might other construction or a drought lower or raise the water table?

Other designers have dealt with this problem - think Mexico City and Houston. Check the literature and communicate with the authors.

 

[BUGGAR]

Sounds similar the drydock at NASSCO in San Diego. We went the underdrain route.

 

[BigH]

Do not understand use of jet grouting. If the buoyant raft cannot take out enough soil for compensation, then I would think a more robust pile system would be used. See Golder's Chapter in the 1st edition of Winterkorn and Fang.

 

[bdbd]

I think you misunderstand.

Weight of the building changes so much. Water level is at -1.5. 9 meter will be excavated. At the base of the building after construction, there will be 7.5 x 10 = 75 kPa water pressure. So, in some parts of the building, weight is smaller than 75 kPa. We will think something for these parts. Maybe make the building heavier for those parts.

Due to deep excavation, no bearing capacity problem, even though the soil is so weak. But, as net load is not zero everywhere, and as soil is very compressible, settlement is a big problem. Also, liquefaction in some parts.

Why I use pile instead of jet grout? 20m length Ø80 jet grout carries 1000 kn = 100 tons. Only reason for me to use pile is connecting them to raft via reinforcements, but I do not need. I just need to improve the soil in order to not have excessive settlements.

So, what I ask is this: Jet grout will cary load, I will assume this soil will not carry, due to liquefaction, and it is already weak. But, while I calculate the load to each jet grout should I use directly the building weight or should I use the net foundation pressure which is building weight minus effective weight of the soil excavated? Or building weight - water pressure acting on the raft which is 75 kPa.

By the way, as always, other parts are discussed here. Aeoliantexan, of course we will dewater the construction, we will use retaining system (diaphragm wall) that goes quite below the excavation. Also, during excavation, water will be pumped out. So, as per your comment, I shouldn't include the water pressure, OK, I am leaning that way already.

 

[bdbd]

Buggar,

How can you drain 7.5 meter water to eleminate water pressure? If you pump it out, what about neighbouring structures?

 

[BUGGAR]

For the underdrains, we used the first stage of the drydock dewatering pumps. These pumps were huge. It was pumped directly into the bay. We had cutoff walls that prevented dewatering from damaging adjacent structures. We only had to dewater when a ship was in the drydock. The document required for the dewatering procedures was three pages long. But the drydock did not float.
I have faced the OP's question and have gotten different answers so I'm no help here except to say that's a valid question.

 

[oldestguy]

OG back. If you are planning on figuring what unloading is present to be used partly for building support, you only can make use of the submerged unit weight of that soil, not any water long term. However, during construction, before excavation, but after dewatering, you will place the moist unit weight of the soil part, minus is submerged unit weight, as a pressure increase at the base of the excavation on that soil which will remain. Keep in mind the several stages of construction and the differing load situations. You might install some form of settlement gauges to monitor these soil movements up or down to consider and then guide what is going on. It wold be assumed that your structural engineers are designing for VARYING WITH TIME uneven loads and uneven support from place to place.

Jet grouting assumptions of improving the ground should be discussed. That technique may or may not work as you intend. Has anyone tried a test jet grout pile and tested for uplift or downward load? If your don't need these loads, there may be other methods for improving the soil. Some discussion here on that may help you.

Your method of dewatering should be a topic here also. Done the wrong way, you can destroy any support there and negate any benefit from other plans.

Again, if you would post a sketch of a section through the building, you are more likely to get meaningful comments here. Providing smittance information does not help us. A typical boring log, with test data would help also. This is not a simple, one solution job.

What is your position in the organization doing the design? Are there more senior geotechs?

 

[bdbd]

OG,

Really? I know you are the most experienced engineer here, but you continue to bring the topic somewhere elsewhere, in every single topic you have commented.

I have thought about the capacity of the jet grout, except few cases where there are bearing capacity problems and high moments, there is no need to connect the piles to raft. Thus, a jet grout and pile does not make such a difference.

There will be a load test, after our design, and we will see if our capacity estimates are correct.

About the load that will act to the jet grouts, I am not done. do you really rhink that I should consider the effective weight of the soil excavated and subtract that from the building load? Ok we do that with settlements but I dont think load will be like that. Even after 10-20-100 meter of excavation, load will be distributed to the ground with same pile-raft interaction ratios.

What do you do OG? Do you calculate the settlements with an imaginary footing when you support the raft in order to reduce settlements? How do you calculate the loads on each pile? I really want to see some of your design calculation reports, I can give you my mail if you want to share them.

For tour last question, I think this is disrespectful. I have no willing to ask any other question, I will continue to dig literature and papers.

 

[oldestguy]

No need to be sassy here. You can take comments or leave them. I think you misinterpreted my statement.

Read my last post again. This is quoted to assist you. The word "is" should have been "its"

its
"after dewatering, you will place the moist unit weight of the soil part, minus is submerged unit weight, as a pressure increase at the base of the excavation on that soil which will remain. "

This statement refers to the condition BEFORE THERE IS ANY EXCAVATION. Then the dewatered condition applies an over-load to the soil that is present below your future placed bottom of raft WELL BEFORE YOU START TO DIG using the increased density of the dewatered soil yet to be excavated. That is helpful in reducing later settlements, if any, from the building LATER. Nothing in my statement quoted above refers to the building load. I am assuming that you are dewatering before you dig, which is common for most jobs with shallow ground water depth. If you digging out saturated soil and plan to pump any water from the excavation as you proceed down, my statement does not apply. If so good luck with that!!!

The comment about the jet grout is that if your building loads are light and floating is possible, one might anchor the raft to those piles. I've seen this form of anchoring done in a few cases where floating by high water table was feared. These were driven H piles however.

It is great to be confident about how you are handling things, including items brought up that were not in your original post, but the original question on some basic geotech principles leaves the impression you lack some experience on these situations and need help from others. Only trying to help in case you missed something. Very sorry.

I appreciate your compliments.

Good luck on this rather complicated project.

 

[BUGGAR]

Jet grouted "columns" (as H. Baker called them) don't have tensile capacity. You're talking about the in situ mixing of soil and grout to depth on a grid pattern, right?

 

[oldestguy]

Install a grouted re-bar, as for tie backs, if you can't develop bond to a long re-bar group.