Excavation support 4

[hoshang]

Hi all,
Please find the attached image.
The building has 2 basement floors+ 2 floors (used as shows)+ 4 typical floors (used as flats). The mat foundation is 800mm thick. The basement floors are 3.6m high floor-to-floor each. My first thought is using piles for excavation support. My worry is the piles will interfere with boundary columns. What is the best practice to avoid such a condition?

 

[PEinc]

Drypack is described in Ratay's books. Drypack is a damp mixture of sand and cement, about 2 parts sand maximum to 1 part cement. It has just enough water added that it can be formed into a stiff ball. It gets rammed into place using a small sledge hammer and a piece of 2x4 lumber. You hit on one end of the 2x4 with the hammer. The other end of the 2x4 compacts the drypack. High strength, non-metallic, non-shrink, flowable grouts are unnecessary. Although steel wedges are often described, I have never used wedges and believe that they may be capable of damaging the existing foundation, especially rubble stone foundations. Even if you use wedges, you still need to drypak between the bottom of foundation and top of underpinning.

http://www.PeirceEngineering.com

 

[hoshang]

Hi PEinc,
appreciate your valuable reply.
It may be a newbie query:
Is it acceptable to use a plastic sleeve in the position of tieback while concreting the underpinning piers so it makes a hole through the concrete pier for the tieback? Is the tieback anchor through the underpinning interpier lagging or through the underpinning piers? If through the latter, what about the piers at the corners of the lot (the drilling rig can't reach this location)? In this case my thought is that the sequence of work should be such that the interpier lagging should be at the corners. If so, what detail should be used at the corner?

 

[PEinc]

The tiebacks are installed through the underpinning piers. Usually an angled boxout is installed in the piers at each tieback location before the pier is concreted. The boxout provides the proper angle for the tieback and provides a big enough flat area for the tieback bearing plate and the hydraulic jack. The hole through the pier can be drilled or be a sleeve set in the pier before it gets concreted.
If the pier is 3 to 4 feet wide (about 1m to 1.25m), it is usually possible to install a tieback in a corner pier at the end of the building. Some drills are more articulating than others. Talk to the tieback contractor about how close to the corner a tieback can be drilled.

http://www.PeirceEngineering.com

 

[hoshang]

Thanks PEinc,
can drypack take loads from the building above and transfer it to the underpinning piers in the case of excavation support?

 

[PEinc]

The drypack is usually, probably always, much stronger than the soil upon which the existing foundations are bearing, unless bearing is on very hard bedrock. The drypack almost always has the same bearing area as the existing footing.
You still need to check that the building does not slide off or overturn from on top of the underpinning. This depends on how much soil is pushing on the back side of the existing foundation wall and footing.

http://www.PeirceEngineering.com

 

[hoshang]

Hi PEinc,
Does ground anchor need casing? If so, how it's installed?

 

[hoshang]

Hi PEinc,
Does ground anchor need casing? If so, how it's installed through underpinning piers?

 

[PEinc]

It there is any chance that a drilled tieback hole might collapse, the drill hole needs to be temporarily cased. Even fractured rock can require temporary drill casing.
My 18 Feb 24 21:12 post talked about installing tiebacks through underpinning piers.
You need to give this design to someone else who knows what they are doing. You are heading into big trouble!
This is the last response I am giving to this thread. I am done.

http://www.PeirceEngineering.com

 

[hoshang]

Hi PEinc,
Your help is highly appreciated, thanks.