Shoring 17'Lx14'Wx17'D excav. in sand, inside building, No sheet pile.

[wds1]

We have a project to install a pit inside an existing building. The soil is loose sand and gravel. The clear height inside building is 18'. It's possible the soil may be very wet. Due to the limited clear height, it would be difficult to drive sheet piles or I-bms. Any recommendations.

 

[Dirtguy4587]

How deep are you digging? I have seen ground treatment/improvement schemes adopted for similar (although much more height restrictive) applications. Might be pricey - 18' might be enough to get some smaller scale shoring in place.

 

[PEinc]

You could drill in pipe pile soldier beams in short, coupled sections using small micropile drill equipment. Dewater and then excavate in lifts, installing lagging and bracing as required.

Can you get a Lo-Drill into the building? If so, maybe you could dewater and install 17' minimum length soldier beams and lagging with bracing at least one level of bracing. Braces would probably interfere with the pit wall construction.

After installing some dewatering wells or well points, you might be able to dig and shore the pit with hand-installed helical soil nails and shotcrete.

You said, "It's possible the soil may be very wet." Are there any soil borings or water level readings? Don't you know where the water is? Wouldn't the water level affect the design and or performance of the new pit and equipment?

Don't forget: If you are going to form both sides of concrete pit walls, your shoring will need to be several feet wider than the concrete structure. If you intend to pour the concrete walls against the shoring (form one side of the walls), then the bracing wales will interfere with the new walls.

Don't forget to account for surcharge pressures from any nearby footings.

 

[Zambo]

17" is about 5m so quite a deep excavation without sheet piles. How sure are you that the loose sand and gravel extends to the full depth of the excavation. You could cast the caisson at ground level and progressivley cast and sink by excavation.For the ground conditions you state I would recommend a cutting edge which forms a gap around the outside of the shaft wall of about 100mm. This gap is filled with bentonite or polymer during shaft sinkiing and then replaced on completion by pressure grouting.

 

[eric1037]

I have recommended tangential auger-cast piles for this application in the past. There are several advantages to this method.

1. You can install the piles in low headroom conditions.

2. You can install the piles, then excavate.

3. You can design the piles with sufficient reinforcement that you will need little bracing.

4. There is very little vibration.

5. The amount of spoils is less than secant drilled piers.

6. If the excavation is deeper than 18 feet, the low-overhead auger-cast rigs can add segments to their augers.

Secant drilled piers could also be considered, but if it is inside, there may be problems with spinning off the augers and there is usually more spoils since they are larger diameter. In addition, they are limited by the headroom.

You will most likely need dewatering wells, then cast a heavy concrete slab at the bottom of the excavation. You could consider a permanent sump and drainage below the slab if the bottom of the slab is below the groundwater level.

Be careful when dewatering inside a building. You should keep a close eye on settlements of existing foundations. I have seen the dewatering process cause settlement on previous projects.

One project I was involved in casted the reinforced concrete walls against the tangetial auger-cast piles. We recommended installing a drainage layer between the auger-cast and the wall to help drain water that makes it between the auger-cast piles. The drainage layer could consist of gravel or a manufactured drain board. This could then be connected to the below-slab drainage system and the sump.

Good Luck!