Do I need another retaining wall here?

[7788_011]

I have this job that the ngl is higher than the ground floor and I am facing this issue that whether to add another retaining wall near the site boundary or not. Please see below.
I am thinking cut & fill - batter the soil shown in purple and construct the piers. Then backfilling after the first floor slab has cured. But I am concerning about the stability of the soil above the ground floor once starting to construct the piers. So instead of using batter & backfill, do I need another retaining wall along the boundary? Any comments?

 

[Zahait]

You may want to check your local health and safety requirements. For example my local code indicates the 1.4m deep excavation would require anywhere between 0.4m (for cohesive soils) horizontal distance up to 4.2m (for cohesionless soils) horizontal distance. Ultimately if you have concerns about the stability of the soil and you cant increase the horizontal dimension you will require earth shoring of some description.

 

[PEinc]

7788 011, you have not given enough information about what, when, or how you are trying to build the new structure. Are the piers designed to also function as a retaining wall so you can install shotcrete? What will support the shotcrete? Do you need sheeting to build the pier caps? What is the total depth of excavation to the basement slab subgrade? What structures, utilities, or surcharges are just across the boundary line? Is there a ground water table to consider?

http://www.PeirceEngineering.com

 

[7788_011]

I am happy with the horizontal distance if there is no basement structure to be retained. ie I am comfortable with the structure as shown below.

I am also comfortable with the structure like this.

But I am not comfortable with the combination of these two. I am concerning about the stability of soil highlighted in green once starting the construction of piers.

@PEinc, piers work as retaining members and shorcrete walls just span in between and supported by piers, transferring loads horizontally to piers. There will be capping beams on top of piers as well. Just vacant lands across the boundary with 5kPa surcharge. Retaining height (basement story height) will be from 3m to 5.5m (where anchor will be installed). No groundwater to consider.

 

[PEinc]

It seems to me that you will need to excavate a safely sloped, open cut from original grade to the bottom of the pier cap. You will need to get permission to temporarily slope across the boundary line. Construct the piers, excavate and install the shotcrete, and build the new building up to original grade. Then backfill to restore the grade outside the building.

http://www.PeirceEngineering.com