Diaphragm wall extension 1

[JackTrades]

I've been presented with a problem concerning an existing tied-back diaphragm (or slurry) wall in which an additional excavation to 15 feet below the bottom of the wall is required. One solution put forth is to place an additional diaphragm wall at the face of the existing, extending down to a sufficient depth below the proposed new excavation elevation and adding soil anchors through the new and existing wall as necessary.
I am concerned about two issues:
1. That the bennonite slurry will not provide any passive resistance pressure prior to the placing and hydration of the reinforced concrete in the new wall possibly allowing the existing wall to fail at the toe, shearing off the rock ledge upon which it sits, due to its weight and the existing vertical component force of the anchors.
2. That the soil angle failure plane, moving down to the tip of the new wall toe from the existing wll toe, will intersect the existing anchor grout zones.
(The attached sketches might provide a better explanation of the situation than I can describe.)
My questions are:
1. If I limit the width of the new slurry wall segments, say to 6 feet, would that allow for the redistribution of passive soil resistance from the trenched area to the adjacent wall areas to each side?
2. Is there some way to adjust or modify the theoretical failure angle that will originate from the toe of the new wall so as to see if it might miss the grouted anchor zones?
3. Is there a better way to tackle this problem?

(I'm a structural engineer, not a geotech, so please speak slowly and in Sesame Street terms if you reply.)

Any advice will be greatly appreciated.

Jack

 

[csd72]

The safest way would be to do 2 new rows of ground anchors.

 

[wmta]

jack,

I would consider rebracing the wall at the bottom while the new slurry wall is placed. This could be providing additional tieback close to the toe as one option.

You need to be concerned about both lateral movement of the toe and vertical support of the wall.

This work is tricky and need to make sure the sequence is correct.

wm

 

[irawanfirmansyah]

Jack,
There is a possibility for toe kick off while you excavating trench for the new wall. This because of the reduction in passive resistance.
To solve this problem I would suggest the following:
- to use as small as possible panel width (see attached figure for definition of panel width). In my country the least width is about 2.7 m
- to use trench excavation sequence as shown the attached fig.

I believe the wall toe will be stable, but you still need to check deformation of the wall toe. You may use 0.5 to 2/3 of the original soil lateral strength. I suggest not to use manual calculation, but using a computer program,preferably the one which use lateral subgrade modulus or p-y curves, in such that you get the wall deformation.

 

[FixedEarth]

JT;

The passive resistance is a concern behind the existing wall. Your proposed wall is in front or on the active side, so there is no disruption of passive resistance.

You show only one tieback for a 33 ft cut-that is a concern (from Elv 1603 to 1570.)

You can solve the soil failure plane issue with the new wall by just lengthening the unbonded length of the lower wall.

The best way to tackle this job is to bring in a specialist-it is a complex wall and a complex soil profile.

 

[hokie66]

FE,
You may have misunderstood the sketch. The new wall is inside the existing, and the OP is worried about loss of passive pressure at the toe of the existing while the new diaphragm panels are constructed.

 

[InDepth]

Ok, so there are several options, but let's talk restraints:
a) in order to resist the passive pressure you can add a row of tiebacks at the base or a certain distance below the existing bottom of excavation. You can do this in slot cut fashion. I would recommend 6 rather than 4, with one on either side of the new panels for redundancy. I would then check the existing panel reinforcement for cantilever moments for the "retrofitted" support conditions. (you could even check the existing reinforcement without the tiebacks ...ie the base cantilevers from last existing tieback...only issue is that the last tieback will take more load than what it probably was designed for). An alternative and maybe more expensive system is to provide a horizontal raker system.

b) vertical load (from tiebacks and other axial loads): depending on the detailing you may be able to consider the existing diaphragm wall as a deep beam...but then check the increase in soil bearing pressure on the adjacent panels can be supported. An alternative and more expensive system is to consider a raker system to take the vertical load.

Other notes

1) Use a secant pile wall and provide tiebacks as you excavate. This may alleviate concerns about loosing passive pressure... And you may be able to connect the secants through doweling above the excavation line to transfer axial load.
2) you should also verify what the equivalent fluid pressure is of the slurry or polymer mix that is used to stabilize the diaphragm wall slot. There are papers that recommend certain specific gravities for these conditions. A minimum head should also be provided.
3) Panel size is critical and 6 ft sounds reasonable and allows redistribution of load.. But the problem is that you typically want tiebacks spaced at least 6 ft apart.
4) For tiebacks in the existing wall you need to make provisions that have them accesible during construction to verify load and possibly retension...you don't want the new diaphragm wall obstructing access.
5) you can also go the fancy route and use a reticulated micropile system (or variation of) where you provide lateral and axial support near the top of existing excavation (doweled into existing) and then as you excavate, shotcrete between the micropiles.

Ok, enough random thoughts.

 

[InDepth]

Oh, for question 2,
1) you can provide ground improvement of the soil behind the existing diaphragm wall in order to change the failure angle. Contact a ground improvement specialist about jet grouting or chemical grouting. You may have to extend this to the bottom of the new wall.
2) second option to change the failure angle and soil slope stablity is to provide soil nailing from the top of the existing diaphragm wall (in the zone above where you want to excavate) to the existing top of excavation or even to the bottom of new excavation. By using soil nails, you are esssentially creating a rigid block of reinforced soil. This may be the cost effective way to go if you can get a geotech to make it work (analyze without the current diaphragm wall and tiebacks for simplicity). The cost of a small segment of diaphragm wall seems prohibitive.

 

[JackTrades]

Thank you all for these valuable comments. I'll provide these suggestions to the design team geotech for consideration.

-Jack

 

[InDepth]

Jack, what is your dimension of the excavation in the perpendicular direction? You may be stuck with an internal bracing system because the drill rig may not be able to install tiebacks on the new wall...or the perpendicular shoring walls. Constructibility should be considered.

 

[48as52]

I believe the wall toe will be stable, but you still need to check deformation of the wall toe. You may use 0.5 to 2/3 of the original soil lateral strength

Eng Tips Rocks

thanks,
kk

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