Soil nails and soil anchors

[DougHole]

I have a few questions on soil nails and anchors...

1. when are soil nails used instead of anchors?

2. apart from the free stress length on an anchor, what is the difference between a soil nail and soil anchor?

3. for modelling an anchor in a slope reinforcement application, we would usually use slope/w and apply a bond resistance in the slope/w run e.g. about say 50 kPa for an anchor bond in stiff clay. say we had a 1 m bond lengtn on a 100mm diameter anchor - the pull out resistance would be F = pi.dia.L x 50 kPa = 3.14 X 0.100 m X 1m x 50kPa = 15 kN/anchor. If i had a 12 m long anchor, why couldn't i increase the bond length to say 10 m to get 150 kN/anchor (with a 2 m free length)? i.e. why do we have to have such a long free length on anchors?..is it something to do with keeping loads from going into weaker materials and that's why we "free stress" it over the start?




Doug Hole
Junior Geotechnical Engineer

 

[Doctormo]

Soil nail walls are essentially an in-situ MSE Wall without the excavation and granular backfill. Therefore, the capacity of each nail is low and nail lengths are generally similar to MSE walls, ie; 70-80% of height. Soil anchors are high capacity anchors that typically anchor back pile walls with a much smaller number of anchors and are spaced much further apart. They are much longer as well for pullout at high loads.

The selection has a lot to do with soil conditions and what one is trying to accomplish. Soil nails are passive (not tensioned) whereas soil anchors are post tensioned to fairly high load levels. It is not reasonable to expect a soil nail wall not to move under load (would not use to hold up a 5 story building from an adjacent excavation) but a soil anchor wall is post tensioned to restrain the soil from normal movement (typically designed for restrained soil conditions) which can limit movement.

I am not an expert in either method so I am not going to get into the numbers but each has its place and functional consideration, water table and slope stability being a couple than come to mind. I am sure others will comment on all the other differences.

 

[Sawbux]

My guesses on free length:
1. Needed to allow length of slanted hole to maintain the grout pump into it.
2. Needed so that the jack can stretch the tendon enough to develop the load needed. A very short length = minimal tendon elongation. This makes it very hard to lock off and maintain tension. Most chucks/collets will have some slip in them. A short tensioned length will allow all the tension to "escape" before the tendon is locked off.
3. You want the anchorage back beyond the influence zone - the active pressure triangle. Otherwise the tendon just pushes against the back of the wall instead of bracing it.
4. The tendon with a long bond length will just "worm" it's way out of the hole and never develop the expected capacity. Think of a Robin pulling a worm out the ground - the part near the surface elongates first and pulls out easily, and this progresses until the entire worm is out.