Of course it depends on competent soils, friction, size of block, pressure, degree of bend, pipe type, welded or push in, etc etc, All of which you say nothing. I don't read minds, or have a crystal ball.
Give us a plan and profile of the setup.
--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."
Without a thrust face in undisturbed soil, the volume of material to hold the block in place needs to rely on gravity
and friction, a complex calculation.
Vertical thrust blocks use gravity for the vertical down bends and enough rebar is added to hold the block together, and there is enough concrete to hold the bend down. Soil does not behave as nicely.
I came across a job site where this situation was occurring a couple of decades ago, the laborer digging the hole
for a new sewer manhole asked what is this concrete doing in my way, it was on a 90° bend on a 16 inch pipe at 130 psi 5 ft away. The soil next to the concrete was 'raining' is the best way I could describe it, tiny bits of sand and gravel
were falling to the bottom of the ditch without anything causing it (other than the thrust block trying to move.)
I asked them to backfill the hole (right now) and figure out a better location for the sewer line, or to install
gravity blocks on our waterline with tie rods to the fitting. Luckily, we did not make the news that evening.
I think they bored past the bend and put the manhole on the other side of the block.
I'm not very good with posting pictures, but here goes:
The block isn't vertical 45deg out from the bend. The height of the block goes from the top of the bend to the bottom of the trench 6" below the pipe.
How do i calculate x such that the reaction block isn't compromised?
Assume thrust force = 10k, soil bearing pressure is 2ksf, soil density is 100 pcf, soil friction coefficient is 0.4. burial depth is 3 to top of block.
area of block in contact with soil meas 1.25'x4'
In this case , the new trench depth is an important factor.
Assuming the new pipe trench depth Ht ,
The resistance will be based on base and side friction of the soil block.
The size of the soil block resisting with sliding,
V= 4.25*4* X = 17*X
Weight of the block W=100 *17*X =1700X lbs.
Base friction resistance = 1700X*0.4= 680X lb
Side friction resistance assuming at rest pressure cof.. 0.5
Side friction resistance = 4.25*100*0.5*0.5*4.25*X*2=903 X lb 903X+680X=10000lb ⇒ X=6.3 ft apply a F.S . for temporary case , say X=8 ft.
Another solution, you may provide trench shoring at new trench to get smaller X value.
..
He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..
We don't know how big your pipe is but if your reaction force is 5 tons, that some size pipe.
From the outside of the block I wouldn't go much closer than 4 to 5 feet. Get too close and once the ground starts moving it's too late....
So find out how far the block goes back from the pipe and measure from there.
Or if you need to get closer hammer in some sheet piles about 10 ft long to contain the movement before digging the next trench. Then after you Bury and compact the soil around the new pipe closer then you can pull the sheet piles back out.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
Apparently the existing thrust block designed for passive thrust . However , you can add weight for the base friction . I think would be negligible.
Your soil bearing pressure 2ksf is overestimated. Assume friction angle 30 degr. Kp =1.5/0.5=3.0
Lateral soil bearing pressure would be the passive thrust at average depth =3.0*5*100=1500 psf apply a FS 2.0 gives 750 psf.
He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..
Sorry, I’m a little thrown off with the side friction. Is that the same as skin friction?
The terms look like (soil depth)^2 * (coef)^2 * (soil density) * 2 * X.
Did I get that right?
