Stress Zones around Deadman Anchor

[ONENGINEER]

For the dead man anchor construction the backfill around the anchor, above the underside of the deadman anchor, should be compacted. Is it only the passive wedge in front of the anchor which should be compacted or other peripheral areas need also be compacted. I am looking for a reference which shows the influence zone, which should be compacted and would appreciate your feedback. Thanks.

 

[GeoPaveTraffic]

Really all fill or backfill on a construction site should be compacted.

Your deadman anchor will load the area in front of the anchor at an angle of about 45+phi/2 from vertical. The loaded wedge will also spread horizontally away from the anchor at about the same angle.

Don't have a reference for you at this time since I'm not in my office, however, it should be in any soils mechancis book.

Mike Lambert

 

[ONENGINEER]

Thank you for the reply GeoPav. I could not understand "The loaded wedge will also spread horizontally away from the anchor at about the same angle". Is this 45+phi/2, a line from the underside corner of the deadman that it intersects the ground level?

 

[ONENGINEER]

Mike, Thank you for the reply.I could not understand "The loaded wedge will also spread horizontally away from the anchor at about the same angle". Is this 45+phi/2, a line from the underside corner of the deadman that it intersects the ground level?

Also may I explain further that the anchor is located within debris wastes with no buildings or essentially traffic around it. So it would be desirable to compact just the area that is affected by the loads.

As the area below the wedge is also debris wastes, could I design the deadman block heavy enough to resist all the uplift and horizontal forces and thus eliminate the passive resistance and hence the need to compact in front of the anchor?

 

[GeoPaveTraffic]

I assume that the load on the anchor is horizontal or near horizontal. If it is verical, we would normally just call that an anchor.

Hard to expain without a sketch. Very simple one below. Not that this sketch will be the same if you cut a vertical section through the anchor or a horizontal section through the anchor. Hope it helps some.
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deadman anchor ->

As for making the deadman heavy enough to resist the forces without passive pressure, I guess you could. But you would need to check sliding resistance of the block on the soil/debris that the block is resting on.

Mike Lambert

 

[ONENGINEER]

Thanks Mike about you explanation about horizontal stresses, Something often forgotten.

The deadman anchor is to resist a guy line about 45 degrees.
The deadman block is designed to be located 2 m below the existing ground surface.
The ground is made up of 8-9 m of random construction debris wastes overlying very stiff clay.

 

[GeoPaveTraffic]

Sorry my "sketch" didn't post correctly, but sounds like you got the idea.

As for your situation, I would design that as an anchor not a deadman. In that case a pull out cone is most often used. You need to check the weight of the soil within the cone versus the allowable shear along the edges of the cone. The lower of the two values is then added to the weight of the anchor (normally assumed to be zero) to determine the capacity of the anchor. Depending on the type of anchor the capacity of the anchor to develop the full load of the cone must also be checked.

With only 2 m of embedment, I would just use the weight of the deadman plus the weight of the soil directly above the deadman. A little conservative, but probably the best idea given that you have debris instead of soil.

Mike Lambert

 

[ONENGINEER]

Thank you for the feedback again and yes your figure was sufficient to convey the message.

My apology if it is seems a bit misunderstanding between the "deadman and "anchor" terms, as I called the whole assembly as a "deadman anchor". I use the daedman block instead of deadman anchor to avoid confusion. The dedman block is preliminary designed as a concrete block of about 10 m long (and about 0.6 deep).

I may have understood correctly that you agreed to assume that passive resistance would be zero and it would be only the weight of the deadman + weight of 2 m overburden to resist the inclined forces in the connected cable.

One of the questions is putting the deadman block within the random fill. The underside of the deadman would thus be random fill to a depth of 5-6 m. Could you advise if this is something that you could recommend or any other alternatives. My other concerns were the dimensions and type of materials in the zone shown as blank around the deadman block in the attached sketch.

 

[GeoPaveTraffic]

Yes I would use zero passive pressure.

The only issue I see witht the random fill is the potential for your anchor to settle. You will need to evaluate this potential.

Mike Lambert

 

[ONENGINEER]

Thanks again.

I have been worrying on the materials under the deadman. Seems we should put the deadman on a structural fill. Which makes it an expensive anchor as the structural fill should be sloped down with an angle of 45 degrees.

 

[oldestguy]

Also, consider if the site will ever have saturation. If so, the submerged unit weights of any concrete block as well as all the arth should be used, not the moist unit weights. Subtracting the unit weight of water is close, but not fully correct.

 

[ONENGINEER]

Oldestguy,
Thanks for the notification on saturation.

Having a 5 m high structural fill beneath the deadman anchor does not seem very smart to me. I am still looking for an alternative solution.