Temporary shoring penetrations in sheeting and ground water

[RFreund]

We are looking at a project where the contractor needs to instal temporary excavation bracing adjacent to light rail tracks. There has been some concerned expressed regarding ground water. There are (3) 24" diameter pipes that will need to penetrate the excavation. These pipes are running below the tracks (perpendicular to tracks) and will connect into the new structure. While I am waiting to hear from the geotechnical engineer, I have a few questions.

One of the concerns is that if excessive ground water is lost then this could induce settlement of the tracks. How is this usually handled. Meaning a situation where you will have a hole in your sheeting, so you will get leakage, but you do not want to excessively dewater the adjacent soil?
- Maybe there is not that much leakage?
- Grouting?

Another not related to water question. Usually for holes in sheeting I see either a strip of wall designed using wood lagging between soldier piles. Or they just torch a hole in the sheathing. How would you analyze the latter situation if you are dealing with piles? I have seen this many times, but I'm not sure how to 'calc it out', especially for sheet piles. If you have cut your pile then you are relying on the pile to span horizontally between the adjacent piles. Do they have some sort of interlock capacity usually?

EIT

http://www.HowToEngineer.com

 

[oldestguy]

Your first question is a logical one. Assuming the excavation is dewatered from within, the rule I like to use is that the hydraulic gradient of water entering the excavation has to be much less than 1.0, preferably under 0.5 (0.5 feet of head loss in one foot). If the gradient is near 1.0 a quick condition results in the excavation bottom and migration of soil will occur into the excavation. If the excavation continues under that situation the migration will extend to outside the excavation and, in your case, from under the tracks. The total head is the difference in elevation between the level of groundwater away from the excavation with that water in the excavation. The length of flow is the path that water follows. Thus shallow penetration of sheets below the excavation likely results in the critical gradient.

At the side penetration a well point outside that area or deep well may resolve the gradient issue there.

Lowering the water table under tracks may cause settlement, but I suspect vibration of rail traffic in the past may have taken up that settlement already, depending on soil type.

Legally, sometimes what goes on outside the PL may not be the concern of the project owner. However, with water, that may not be the case. In some ares, the depth of excavation decides liability, with respect to protection against lateral movement.

 

[FixedEarth]

RF;

The dewatering time has to be long term and ongoing to induce significant soil settlement. I would be concerned with stability of the bottom of the excavation against piping & heave. Your geotechnical engineer can check for bottom stability and also do flow nets to determine volume, direction and seepage forces. Once you have this information, you can continue with the structural design of the braced excavation. Don't forget slope stability.

If your site water is above the bottom of the excavation, the contractor can dewater the site temporarily. You can then design for the uplift pressure on the mat for the long term condition.

http://www.soilstructure.com/