QA for cut off wall made of Deep Soil Mixing in Sandy Gravel Soil 4

[BigAEln]

Dear All,
In a flood defence scheme, it is proposed to construct a 650 long curtain wall using Deep Soil Mixing (DSM), under a 1.5m high flood wall . The cutoff wall will be 0.4m thick and penetrate through about 5 to 7m of sandy gravel overlying stiff fissured clay, so as to function as a cutoff wall during a flood event. The contractor proposed using Trenchmix technique to construct the cutoff wall using the wet method. He intends to use stablizing agent content of 2.5% by weight of the soil. I am not certain about the QA measures. We need to know the following:
1- The homogeneity of the soil mix in the wall;
2- Range of the Coefficient of permeability of the mixed soil in the wall:
3- Range of Strength (Unconfined compression strength) of the mixed soil in the wall;
4- Confirm that the wall penetrate in the stiff fissured clay by not less than 0.4m.

The DSM subcontractor "claims" that he can not measure the torque while penetrating the ground and use that to identify the soil at the bottom of the cutoff. He want to limit the QA measures to strength and permeability tests on samples extracted by grabbing from the freshly mixed soil, palced in U100 cylinders and cured and tested in the Lab . I think triple coring is required to extract samples from the cutoff wall, and test them in the Lab at specific periods after construction. Obviously the core quality will depend on the driller skill, i.e. the skill of a 3rd party. Given his refusal to do coring, then we have a problem of knowing that what he constructs fuldfill the required performance. I would like to ask you about the following:
1- For the proposed trench mixing technique: what are the possible QA measure to ensure the wall penetration in the stiff clay underlying the gravel layer?
2- Are we able to do field permeability tests in the cutoff wall, e.g. by inserting standing pipe in the freshly mixed soil, and doing the test at specific periods after construction? If so, what are the problems? whwere are the case studies? and How to get Specifications?
3- If we do coring tocheck quality of the mixed soil and get samples for Lab strength and permeability tests: What are the minimum core diameters to use in the sandy gravel formation?
4- What are the intervals/distance between the cores in the curtain wall? Are there any guidelines to elect these intervals?

Best regards
BigAEln

 

[BigHarvey]

You have several methods of construction for cut-off walls. Each of them present advantages and drawbacks. Technically, the choice will mainly depend on your permeability requirements. The economics is another story. Trenchmix, provided you stay in the depth range ( 10 m max )is very very economical. But you cannot impose all your QA requirements from other techniques and also claim for the economical advantages !
Strength is generally not an issue.
You can have a recording of installation parameters, but given the power of the machine, you will not see the difference between the upper materials and the stiff clay. Samples of what is coming out seems reasonable.
The main issue is permeability but since we don't know your specs, it's not possible to tell you whether the method is adapted or not.

 

[BigAEln]

Dear BigHarvey,
Thank you for the information. the sandy gravel has D10 in the range of 2mm to 0.03mm. However, most of the tested samples has D10>0.09mm. We want the curtain wall to have k of about 10-8m/s. First, I am suspecting that with stablizing agent content of 2.5%, there is a possibility of poor mixing in some areas. Further, With the traditional auger technique to construct the DSM, the soil at the tip of the column is verified by first constructing columns next to borehole locations where the foundation soil level is known. AT these locations, the contractor measure the thrust to derive the shaft in the soil and the torque exerted to rotate the blades on it in this soil, and record the range of thrust and torque when he penetrates this foundation soil.

Does the trenchmix has a similar mean to identify/verify the foundation soil of the wall?
Regards
BigAEln

 

[GeoPaveTraffic]

BegAEln,

In my opinion, you bring up the real problem with constructing cutoff walls. QA is always an issue. I've never seen a cutoff constructed that didn't have "windows" in it.

As to your specific questions,
1. Sample the mixed soil prior to setup to determine homogeneity.

2. I'm not sure why you want to measure the permeability, but if you really need to know, then you need to do a pump test(s) following construction of the wall.

3. Again, not sure why you need to know. As long as the mixture is strong enough to support your wall above. But you have two choices. Either collect samples of the mixture before or after setup. Both have pluses and minus as discussed previously.

4. Unless you drill back through the cutoff wall, everywhere, there is no way to 100% sure. The best bet is to understand and map the formation, then establish your cutoff wall bottom elevation(s) with an appropriate factor of safety.

Good luck. As you can probably tell, I'm not a fan of cutoff walls and I really don't like them for flood control structures.

If you want to prevent piping, install sheet piles. If you want to control underseepage pressures, install berms, relief wells, toe trenches, etc. To me there are just too many unknowns with cutoff walls to use them in flood control situations.

Mike Lambert

 

[motorbiketocrank]

First, to clarify, is this just a short duration flood loading? Overall I agree with GPT. I personally would lean toward sheetpiling and/or toe drains/relief wells, etc (maybe too late for that decision?).

Why are all the specifics are being left to the contractor? I'd address these things during design....take soil samples and mix with varying amounts of stabilizing agent to determine what % will give the desired strength/permeability, etc. The field testing should then be to verify the appropriate mixing is being achieved. I wouldn't recommend letting the contractor just decide how much stabilizing agent he wants to use and keep your fingers crossed hoping field permeability tests will turn out ok. Also, the specific types and frequencies of QA testing should be dictated to the contractor.....not allow him to decide what he wants to do.

As far as ensuring you are extending to the clay layer, I'd do closely spaced borings ahead of time and then set elevations to which the wall should extend (being a little conservative of course to account for variability). And since you describe the clay as fissured, any chance that pressure against the cutoff wall will cause a shear failure and lateral movement?

My main worry with this sort of soil modification would not be the main wall production but the sorts of localized construction problems that arise. A breakdown or screwup in the rate at which stabilizing agent is being added. A seam between subsequent days placement, etc. So careful, continuous visual QA would be critical.

I'd recommend samples during production for control/adjustment of the process and cores of the in place wall for verification.

 

[cvg]

4.5 feet high floodwall with a 20 feet deep DSM cutoff wall, keyed into clay? Assuming you have some freeboard in your design, seems a bit extreme. A more shallow cutoff (constructed of concrete on the toe of your floodwall) might be enough. Not sure a relief well is warranted here for this very short floodwall...

 

[BigHarvey]

10-8m/s can be reached either with DSM or Trenchmix, or CSM. The main advantages of Trenchmix are :
- continuous operation ==> no "windows" left in the wall
- speed
- cost

Using the wet method will help in getting a better homegeneity in the mixed ground. The material grain size distribution is also adequate for the method.

I don't believe that mixing with an auger is more efficient tha Trenchmix. If you really want the best in mixing technique, you should look at CSM ( it is an adaptation of diaphragm wall hydraulic cutters ) but the cost is about twice the cost of Trenchmix.

 

[BigAEln]

Dear All, Many thanks for all of your contributions.