Secant Piles Construction

[Doc09]

We're getting ready to start our first secant piling job. We have done a decent amount of contiguous piles, underpinining, sheet piles, etc.. but never a secant piling job. So just to be sure we did not leave anything unchecked you thoughts and profesional comments are appreciated.

The cut is 15m deep, 6m of fill (at least 15-20 years old)overlaying the Marl/marlstone/limestone (in layers). Unconfined compression results on intact cores varied between 5 Mpa and 40 Mpa. The water table is at 3.5m from the top. We've proposed 80cm piles, actually 88cm over the top 6-7m, because we need to use a casing to retain the soil, then we proceed down using 78cm drilling tools. The overlap between the piles is 20cm at the top becoming 14.5cm below the casing. The pile is embedded 6m below the dredge line.

The site is surrounded by roads, so normal surcharge of 15kpa was accounted for in the design. The design indicated the need for a single row of anchors located at 4m from the top, spaced 1.4m c-c and about 20m long. We used Phase2 (finite element) by Rocscience to simulate the problem at hand and the results indicated a maximum moment on the pile of 45 t-m which could be easily covered by 16 bars 25mm dia, and the expected deflection is about 2cm.

Your thoughts/comments on the following:

1. Any coments on the design approach ?

2. Although the analysis returns reasonable results, and takes into consideration a reaonable factor of safety on the loads, reinforcement, etc.. However, I am still concerned about the behavior of the female pile. The assumption is that it will deflect the same amount as the male pile (~2cm); will that create potential uncontrolable leakage problems, and how does it deal with the moment given that the primary piles were designed to carry the full 1.4m c-c of loads, thus theoretically, designed to carry the fraction of load to be carried by the female pile.

3. For the female (secondary) piles, we are considering using a concrete mix comprising 4 cement bags (50kg each) per cubic meter, which should result in a 28day strength of 10 to 15 Mpa. Any suggestion on the mix design, drilling tips and prefered drilling tools that provide best verticality.

4. Drilling the primary piles would take place within a day or two after casting the female piles. Any concerns ?

thanks in advance for your thoughts and contribution

 

[jdonville]

Tony Suckling has a great CFA secant pile paper out there

http://www.dfi.org/pubdetail.asp?id=1296

- unfortunately not free. I have to wonder what you are using for your tieback anchorage? We normally use structural steel sections as our tangent pile reinforcement, which makes the anchorage easier.

The female piles act essentially as lagging.

One of Briaud's graduate students (Cassandra Rutherford) wrote a soil mixing wall manual as a master's thesis at Texas A & M. I don't have a citation handy, but it appears that much of the work is boiled down here:

http://casehistories.geoengineer.org/volume/volume1/issue2/IJGCH_1_2_1.pdf

Hope this helps.

Jeff

 

[Doc09]

Jeff, thanks for the info, appreciated. I do have a copy of Cassandra's thesis.

For the tie backs we're using cable strands (5 per anchor) with a waler beam bridging between them. For the pile reinforcement we're using a regular rebar cage.

 

[InDepth]

You should also get the papers:

1) Secant Pile Walls - Design, Construction and Case History by Trivedi
2) Secant Pile Walls - A consistent Approach to Risk management by Suckling
3) Secant CFA-Pile Walls - a Risk Management View by Korff
4) Construction Dewatering and Groundwater Control by Powers has a great discussion on secant piles.

As for the female piles they should deflect with the male piles, since the females (primary) serve as lagging and are locked into the male (secondary piles) by overlap. You could also consider using jetgrouting secant pile walls, deep soil mixing walls, and cutter walls (CSM). The deep soil and cutter soil may be cheaper.

For the secondary piles drilled to overlap the primary piles, you will need to specify a concrete or slurry strength such that it doesn't cause the contractor to delay his project because his machine needs more time to drill through harder concrete.

 

[InDepth]

ICE specification for piling and embedded retaining walls 2ed is also a good reference.

 

[Doc09]

thanks

 

[rosalitambarde]

dont have any idea bout secant piles could you please give me a detailed explanation bout this?cause ill be dealing steel sheet piles in few months i dont have any background on this.