Installing Wall Pit Next To Footing

[applebit42]

I am needing some opinions on the best way to install a reinforced concrete wall next to a footing.

In the interior slab of a building I have to install a 13'-6" deep pit that mechanical equipment will be installed in. The pit roughly needs to be 16'- 9" feet wide and will be centered between two interior columns. The column spacing is 20'-0" bays. This leaves me 10" to the edge of the base plate to the inside face of the wall for column line "A" and 1'-1" from the edge of base plate to inside face of wall for column line "B".

I want to install a 10" thick concrete wall that will be 13'-6" deep. We are going to put sheet pile down around the outside perimeter of the pit. The sheet piles will be left in place and are going to pour the outside face of the wall up against the sheet piles.

My question is that I am excavating so close to the footing. Would it be best to temporarily support the load of the column and then excavate down to the pit footing elevation (13'-6") and re-pour a pedestal under the temporarily supported footing (ie 12'-0 pedestal)?

The interior columns are all tied together with 2'-4" deep x 18" wide grade beams on four sides of the footing like a waffle. The soil type is clay sand with a bearing pressure of 4,000 psi. Maximum column load is around 45kips.

 

[applebit42]

Previously the company installed another similar pit in a different bay. For this installation they removed about half of the footing drove sheet piles down next to the column base plate and poured the pit wall against the sheet piles. They then doweled the pit wall into the existing column footing.

They assumed the sheet piles would support the soil pressure under the footing till they doweled the existing footing into the pit wall.

 

[FixedEarth]

When I drew a sketch, it seems that you have a strip load of 3 ft 3" that is loaded with 4,000 psf max. soil pressure and is setback a 3 ft 3" from a 13 ft 6" excavation.

If we take a 1:1 (H:V) slope from the inside edge of the footing, it will intersect the shaft at 3 ft 3" below top of shaft excavation.

To get the influence of the footing load, take the 4,000 psf bearing pressure and divide it by (3.25 ft(B) x 3.25 ft(Z)) and I get 95 psf. Where B is the footing width and Z is the depth. In real life it is 0.5:1, but conservatively we took it as 1:1. This is a uniform vertical stress of 95 psf. Assuming ka of 0.35 for clayey sand, we get a lateral stress of (0.35*95)= 33 psf/ft acting on the lower 10 ft 3" of your excavation. This is too small a pressure for a sheetpile job.

According to OSHA, your excavation height can not be higher than 5 ft unsupported, so you do need some temporary support until you pour your wall.

 

[oldestguy]

Is this soil granular where you could chemically stabilize it to minimize the problems of losing ground, etc.???

 

[applebit42]

The soil is granular.

We went with driven piles to had the soil back. There is a fairly large shaker system next to the pit and wanted the reassurance of using steel pile to hold the soil back. We also spoke with the equipment supplier and we agreed that we could offset the pit around the footing.

Thank You for you time.

 

[sunny7uc]

you probably have your solution already. Jet grouting is another way to underpin and provide temporary shoring at the same time.