Soldier Pile and Lagging as Permanent system with Concrete Planks

[EngMan40]

22 story building is to be constructed with 15 ft deep basement. the structure engineer's original system is temporary soldier pile with timber lagging. after both wall footings and interior foundations are constructed a 12" thick reinforced basement wall will take place at the perimeter beside the soldier piles. the basement wall takes load of about 2 stories only while the major load is taken by interior footings.
The new idea is to make the SOE as permanent, that is to make the soldier piles and timber lagging take the short term loads and then after the excavation is done the reinforced concrete basement wall will be cast in place between soldier piles and becomes as lagging to the long term permanent system. there will be a beam runs on top of the soldier piles and the concrete planks which will provide support for the 2 story loads.
I am trying to compare the two systems:
system (1) temporary shoring system with 12" thick basement wall and basement wall footings
system (2) permanent shoring system with 8" thick basement wall and not basement footings
Note: No tiebacks permitted since there is a nearby basement wall of structure runs 4' away along the property line.
- Do you think there is any savings in foundation system vs increase in cost while converting from temporary to permanent.
- would 8" thick planks support lateral load on the long term with the help of soldier pile work rather than original 12"
- would a 12" section soldier pile be able to take vertical loads from the system and transfer it to say 36" sockets embedded 10' into very dense saprolitic soil.

 

[PEinc]

Precast lagging is very hard to install properly without a sloped open cut (15' deep) and stacking the precast from the bottom of the the excavation up to finished grade. Precast lagging cannot be installed very well in safe lifts from the top down as excavation progresses to subgrade. You could install temporary soldier beams and timber lagging from the top down, on wall line, and then pour the concrete basement wall and footing against the front face of the soldiier beams and lagging.
Fifteen feet is rather high for an economical, cantilevered soldier beam wall and its deflection needs to be checked.
If you are 15 feet deep and only 4 feet away from the adjacent building, you better check the depth of the adjacent building and see if the building needs underpinning. You should not try supporting an adjacent building with sheeting, especially cantilevered sheeting.
There have been other threads on ET about difficulties installing precast lagging for high walls. Precast walls should be built from the bottom up, in front of a safely shored excavation.

http://www.PeirceEngineering.com

 

[EngMan40]

PEinc,
Thanks for your reply. I am aware of the difficulties in installing concrete lagging alone with top to bottom excavation. That is why I mentioned that first the temporary system will be installed traditionally as soldier pile and timber lagging and then the concrete basement wall is going to be poured against the lagging ( with drainage strips and insulation in between of course). the soldier piles will be socketed deeper than the bottom of adjacent wall footings so as to avoid surcharging their basement wall. the encroachment of those sockets out of the PL is not more than 4".
I am assuming that the lateral earth pressure will be reduced because the adjacent structure which extends from the the bottom of the socket to about 6' below surface and thus the failure plane behind the soldier piles is no longer straight line.
Do you think this assumption is valid and if yes would that make the loads and deflection less critical?

I am still questioning whether or not I should permit the concrete planks to take loads from the superstructure( maybe a masonry wall with grade beam) and the soldier beams to take part of compression loads from ground level slab. Note that there are interior footings about 6 ft away from the soldier piles