25 ft rock excavation below 18 ft of soil 2

[EngMan40]

Anyone has designed shoring to retain 18 ft of fill and soil, then 25 ft rock below it? I am thinking of soldier pils with one row of tiebacks for the top and rock bolts for the rock below but not sure about the soldier pile embedment, maybe I should put a tieback close to the bottom of the pile. I can't leave a lot of horizontal bench (maybe 3 ft) because the piles will be drilled in city sidewalk with utilities behind it. Any comments on the rock excavation sequence, possible impact on the stability of the pile system above.

There is one side where one building is on the property line so I am thinking underpinning with tiebacks and rock bolts below it but again, the rock excavation will be carried out after underpinning is installed and any fracturing or vibration from rock excavation will impact the building. Any concerns or comments? the rest of that side has an apartment building that is 8 ft away from the property line with 8 ft wide porch and a 10 ft deep basement. any suggestion on shoring this side? I will have to take the full foundation pressure since I can not just rip out the porch to underpin the building.

 

[PEinc]

If the rock is good (not highly fractured) at its surface and if you drill the soldier beams into the rock a few feet AND have at least a 3' rock bench before continuing with the rock excavation, you should be able to use one level of tieback anchors. If you don't drill the beams into good rock, you will need two levels of tiebacks or one level with toe ties. For the rock cut, you should line drill to subgrade to prevent or at least minimize overbreak and loss of the 3' minimum rock bench. The unknow is the rock bedding and inclination. Bad rock may need rock bolting and wire mesh or chain link fencing mesh. Good, hard rock may also need rock bolts if the rock is sloping such that it could slide down into the excavation on one side while possibly toppling over into th3 excavation on the opposite side. The other two opposing sides may no need any rock bolting.

Unfortunately, soil borings and geotech reports for building, highway, and bridge projects rarely give information on the bedrock other than recovery and RQD. Also, rock mechanics and soil mechanics are not the same science. So, there are no simple and clearcut recommendations that can be given for rock stability or rock pressures.

Photo shows soldier beams driven to rock with two levels of tiebacks, rock bolts, and chain link mesh for a 63' deep cut.

http://www.PeirceEngineering.com

 

[miningman]

With all due respect to PEinc's abilities and experiences, the wording of his post will likely lead you into trouble. I dont know much about shoring etc but I do have extensive experience with rock excavation. IMO , his post start with the word "If.", In my experience there is almost zero liklihood of the first 10 metres or so of the rock excavation proving to be "good rock" Normal weathering inevitably leads to detereration of the initial 10 metres of rock to produce what I term "rotten rock" conditions. Preliminary diamond drilling might provide an initial indication as to the severity of this , but ultimately excavation must take place to provide visual evidence. And then I must ask, how to you propose to excavate this rock.? Are you going to use explosives??? If the general rock quality is good , then explosives will likely be required. You will be drilling below your known soil conditions and you appear to have an existing building very close. A bad combination when using explosives.

 

[PEinc]

Drilling and blasting limestone rock.

http://www.PeirceEngineering.com

 

[ACtrafficengr]

the piles will be drilled in city sidewalk

Just an aside: you'll need a sidewalk detour This is often overlooked until someone complains. They could actually make a federal case of it in the US.

My glass has a v/c ratio of 0.5

Maybe the tyranny of Murphy is the penalty for hubris. -

http://xkcd.com/319/

 

[PEinc]

miningman, what your last response mentioned shows up in the 1st of the last three photos I posted. You can see the highly fractured, low RQD, upper rock zone. It was hard enough to prevent pile driving but not trusted to stand vertically stable, hence two levels of tieback anchors and the rock bolts below the sheeting wall.

http://www.PeirceEngineering.com

 

[EngMan40]

PEinc: the photos are really nice. The toe tie is good iddea since the rock is half decent in upper 5ft closely fractured schist with sporadic gneiss bands. I recorded the relative dip of the rock in the boring (30-45 deg) but unfortunately I won’t be able to know the true strike to figure out the rock bolts so I will have to specify it on all sides.

Any thoughts on underpinning side, have you heard excavated rock below underpinning?

 

[PEinc]

You do not want to excavate competent bedrock and replace it with underpinning concrete. If the rock is fractured enough to economically remove with small pneumatic (about 35#) or electric breakers, do it. Otherwise, drill in and grout some vertical dowels in the bottom of the excavated pit before placing the underpinning concrete. Then line drill to subgrade, excavate the rock in lifts, installing rock support as required. Monitor the building before starting, during work, and during rock excavation. Monitor the rock face too.

http://www.PeirceEngineering.com