Soil Pressure

[Geotechnical76]

I have been asked to look at the soil structure interaction of the following structure.
We have an existing 38-inch concrete water pipe lie which is encased in concrete at some point along the alignment with a butterfly valve in a chamber connected to the concrete encased section. The pressure in the valve is 100 psi when the valve is open while the pressure on the bulkhead when the valve closed is 75,000 lb. The concrete encased section is proposed to provide some resistance to the pressure. The top of the encased section will be 5 feet below the ground surface. The encasement will be 12 inches above the crown of the pipe and along both sides of the pipe. Since it is an existing pipe, encasement will be difficult to be below the pipe.

In addition, wing walls are proposed along the concrete encased section (on both sides, left and right). These wing walls will provide additional resistance to the 75kip load. What I looking for at this time are (1) the spacing between the wing walls and (2) the soil pressure (reaction) on the wing walls due to the valve load. The walls and the encasement will be constructed using an open cut and the trench will backfilled with granular fill. The existing soil profile indicates 3 feet of silt sand over a deep layer of medium stiff to stiff silty clay.

I am thinking that the pressure on the walls can be modeled assuming a passive earth pressure. The passive earth pressure parameters will be phi=30 degrees, gamma=120 pcf. My other concern is the interaction between the passive pressure zones between the wing walls since soil will be between them. I am using the 2:1 method of stress distribution behind the walls to determine the minimum spacing to avoid stress concentration. Any thoughts are highly appreciated.

 

[civilperson]

"The pressure on the bulkhead is 75000 pounds", how was this number derived? The valve is not a dead end is it? The pipe continues after the valve into the soil where it is restrained by the backfill or what is the condition causes this force?

 

[dgillette]

1. Can the structure tolerate enough movement to mobilize passive pressure before it fails?

2. At a depth of 5 feet, is the contribution from the soil really significant?

I'd be hesitant to count on the soil for much. If you do, make doggone sure that the compaction of the soil against the bulkhead is very, very good, with careful density control, fanatical inspection, Wackers, etc.

 

[Geotechnical76]

civilperson
The pressure was given by the civil group. The valve will act as a dead end at some point when it is completely closed. My understanding that this force can be understood as a surge force.

dgillette
1-Half of the passive pressure can be mobilized when the horizontal compression(strain)is 0.5% while it is completely mobilized when the horizontal strain is 2%. Assuming that the wing wall is 5 ft high this means 0.3 inch (0.5% horizontal strain) to 1.2 inch(2% horizontal).The compaction of the granular fill around the walls will not be perfect and hence the movement of the walls,chamber, and encased concrete will be between 0.3 inch and 1.2(just and assumption).
2-The top soil will be 5 feet above the concrete. Adding the depth of the concrete wall, the total depth to the bottom of the wing walls will be 10 feet. Considering the confinement of the soil above the pipe,the surrounding soil, and the H/D=1.6 ratio(H:soil above pipe; D: pipe diameter),the soil will have contribute to the pressure envelope around the pipe.

I am also looking into the lateral subgrade modulus assuming loose granular fill (ks=500 kcf).

 

[dgillette]

I guess I don't understand the geometry very well.

Are the concrete and soil pressure being used to resist movement of the valve relative to the pipe in the pipe-axial direction? (i.e., to keep the pressure on the butterfly from pushing the valve off the end of the pipe?) If so, I would expect that the tolerable movements are very small.

I also don't understand your strain calculations. If I grasp correctly what you are saying, you are calculating horizontal strain as horizontal movement divided by the height of the wall?? Maybe you could describe the geometry a little more.

 

[civilperson]

The 75 kip load will be acting on the valve body which is connected both upstream and downstream with concrete pipe. The stiffer parts will take the most load. The compression acting on the down stream pipe will be transfered to the soil at some rate. If sufficient distance of pipe exists down stream the entire force can be resisted. A rigid connection of the valve body to the concrete chamber may be stiff enough to take the load as an alternate to the downstream pipe. The upstream pipe needs restrained or welded joints to contribute much in the restraint force.

 

[Geotechnical76]

dgillete
1-You are correct (the concrete and soil pressures are being used to resist movement of the valve relative to the pipe in the pipe-axial direction). I am concerned about the movement around the chamber area, and that's why the wing walls are proposed.
2-Normally the mobilizing horizontal strain is given (normalized)as a function of the height of the wall;i.e strain=0.0005H for approximation since it is easy to use.
Thanks

 

[dgillette]

OK, now I see. The wing walls are perpendicular to the pipe and are intended to make the concrete encasement wider so it presents more "sail area."

The movement of 0.3 to 1.2 inches is tolerable? Even if it is, you should still require special compaction (select materials, small equipment like Wackers or vibratory plate on the end of a backhoe arm, etc.) against the concrete and send your most finicky inspector to monitor the compaction.