lateral earth pressure between concrete wall and sheet pile wall 1

[Davidmat]

I am presently working on a bridge replacement project where the existing abutment will remain. However, in order to increase its stability under the new loading, a sheet pile wall is proposed to be driven behind this existing wall to reduce lateral earth pressures.

The sheeting will need to be driven beyond the toe of the existing wall; therefore, a reduced lateral loading will still be felt by the existing wall. How should I be approaching the development of the reduced lateral load since the triangular shape of the pressure envelope is no longer triangular? Any suggestions would be appreciated.

 

[PEinc]

I don't think this is a very reliable or efficient method to reduce the earth pressure on the abutment. In order for the sheet pile wall to pick up and support the lateral load, it has to deflect. I expect the sheet pile deflection would need to be significant in relation to the abutment wall. The sheet pile wall should be more flexible than the abutment. Therefore, in my opinion, the sheet pile wall will not help much until the abutment deflects or rotates the same as the sheet piling.

What needs to be improved? Sliding resistance? Overturning resistance? Tension steel in the stem? Can you install tieback anchors through the stem or footing? Can you install tie rods and deadmen? Can you increase the weight on the heel of the abutment footing?

 

[PSlem]

Do a search for silo pressure as the same rules apply.

Engineering is the art of not constructing...of doing that well with one dollar what any bungler can do well with two after a fashion.

 

[GeoPaveTraffic]

I agree with PEinc, the sheet pile wall will have little if any effect on the lateral pressure felt by the abutment.

 

[Davidmat]

More information - The sheet pile wall to be installed behind the abutment will be designed to support a height equal to the height of the adjacent abutment. This includes installing a line of tensioned tieback anchors for reinforcent. The sheeting wall deflection was estimated at up to 1.25 inches. There will be a 4.5 foot distance between the back of the existing abutment and the closest face of the sheeting (1.5 ft wall stem + 3 ft beyond).

 

[VoyageofDiscovery]

Hi Davidmat,

If you have pressure on both sides of the sheet pile where do you get a substantial net pressure? Thus I do not see how you can reduce earth pressure on the abutment even with tiebacks.

Another item is that pressures and distributions for flexible verse rigid retaining systems are different.

Regards

VOD

 

[GeoPaveTraffic]

Will the area between the sheet pile wall and the abutment be backfill or left open? If it is backfilled, then eventually the pressure will reach the abutment.

 

[Davidmat]

The area between the existing abutment and the sheeting will be filled with soil. The sheeting will be vibrated in through the existing fill behind the wall, followed by excavations in front of the wall to install the wales and tiebacks (if anyone would like details of the constructability, I would be glad to ellaborate).

I agree that at time = 0, the full pressure will be felt at the existing wall; however, since the sheeting is estimated to deflect up to 1.25 inches, the existing abutment would need to deflect that much before reduced loading kicks in. With the existing abutment approximately 19 feet high from the bottom of footing to the top, 1.25 inches of deflection is not considered significant.

The bridge will remain open during a phased construction process with the two abutments and two piers rehabilitated to support the new superstructure.

PEinc - Based on a stability analysis performed for the proposed rehabilitated abutments, factors of safety against overturning and sliding do not satisfy the minimum requirements and the maximum calculated bearing pressures under both existing abutments are greater than the allowable bearing capacities with the vertical force resultant outside the middle third of the footing.

We've looked at installing tieback anchors directly through the stem; however, the required anchor force will overstress the stem. One other feasilble option was to use Controlled Low Strength Material (CLSM) behind the abutments. The cost to temporarily support the roadway to install the CLSM cause this alternative to not be one of the top ranked alternatives.

 

[Focht3]

How about reinforcing the area behind the wall with a drilled shaft wall? This could reduce the down time, while increasing the stiffness of the new wall.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[BigH]

Could you not, as Focht3 suggests, install a bored pile wall behind. Then you could relieve the pressure in the original abutment wall by excavating out the material in stages between the two walls - put in your anchors on the bored pile wall (if needed) as you excavate down. Then fill the void with a lightweight fill? This would reduce the pressures on the original abutment wall. Instead of bored pile continous wall, of course, there are other possibilities - but of similar ilk.

 

[DRC1]

If you assume the sheeting defines a vertical face of the failure plane after backfilling you can define the failure block, Using Culmann's Solution, the active load can be determined graphically.
Although tis solution I feelis technically feasible, my concren would be long term corrosion of an important system that you would not be able to inspect. Also achiving sufficent compaction along the wall and especially at the wale will be difficult.
the previosly noted bored pile wall is certianly one option. Retriculated minipiles are another option.
Good Luck!

 

[PEinc]

It sounds odd that the existing abutment stem would not be strong enough to accept the tieback anchors. Maybe you need more, lower capacity anchors. Maybe you could drill and grout in some additional vertical rebars, from the top of the abutment to the footing, where the tieback anchors would be located. What loads would the tieback anchors be designed for? Would the tiebacks be designed for the entire earth and surcharge loads or just enough capacity to increase the safety factors for overturning, sliding, and bearing. Remember, the abutment is currently stable so it does have the ability to hold most of the new loads on its own. Can you check the existing abutment, for the new increased loads, with an applied load (the tiebacks) against the stem?

 

[Davidmat]

Tieback Alternative Overview - It was determined that a load of 1.65 K/ft applied at 5 feet below the top of wall would be required to bring the factors of safety for sliding, overturning, and bearing capacity to meet the state specified limits. The analysis included all the usual forces on the wall with the addition of the tieback force. Without any stabilization, the inclination angle of the abutment's resultant (with respect to vertical) was about 25 degrees, which lowered the allowable bearing capacity dramatically. The additional force added by the tiebacks reduced the inclination to about 11 degrees.

The top 6 feet of abutment was proposed to be demolished and rebuilt to support the new superstructure and reinforced enough to act as an internal waler for the tiebacks. The existing plans for the abutments indicate a deficiency in steel reinforcement to carry the bending moments when the tiebacks were tensioned up. Drilling and grouting additional rebars below the location of the tiebacks will be an option to look into (thank you PEinc).

 

[PEinc]

1.65 KLF is a very, very, low load for tieback anchors. If the anchors were to be installed at 10 feet on center and at 30 degrees below horizontal, the design load would be only 19 Kips. Drilled and grouted anchors usually are installed with much greater design loads. Your abutment must not be very tall. I still have trouble understanding how such a small tieback anchor force can be too great for the existing stem.

For example, I recently designed temporary tieback anchors for the stem of an existing, 18' high, concrete retaining wall. I designed the tiebacks for the full earth and surcharge pressures. The work was completed a few months ago with no problems. The wall needed to be tied back so that the contractor could make a deep (17' to 27'), soil nailed excavation immediately in front of and below the wall's footing toe. I may be wrong but couldn't your problem be similar to, if not less critical than, mine?

 

[Davidmat]

PE-Inc, The existing abutment wall is up to 19 feet tall with a 2.25 foot thick stem, a 3.5 foot wide toe and a 2.25 foot wide heel (toe and heel dimensions seem backwards, but that is what the existing plans indicate). I'll get back to you on the tieback force versus stem capacity (need to check with my structural engineer).

That soil nail supported cut below the existing abutment footing is quite impressive! What was the purpose of such an excavation?

 

[PEinc]

The support systems were for a building excavation at Temple University Hospital in Philadelphia. There were three adjacent buildings (two on caissons with some caissons not deep enough to be below the proposed excavation; one on spread footings above subgrade). The third side was bordered by the concrete retaining wall with its footing well above proposed subgrade. The new basement foundation walls were constructed "wall-line" against the support system walls. This was an interesting project. It had tieback anchors, tiedown anchors, soil nail walls, and pit underpinning. Bedrock elevation varied from well above to below subgrade.