Long Tie-rod Bond Length 2

[HSNIC]

Looking at the PTI for Rock and Soil Anchors Commentary section C6.7.1-7.2, I read that bond lengths for rock should be limited to 35 ft. While in soil, the limit is 50 ft. The reason is "inefficiencies".

The inefficiencies I thought up were in the construction process as result of economic factors. More powerful machinery. Longer tierods, more grout. Heavier testing equipment.

It mentioned in the case that if such bond lengths are used, "...special provisions are taken to to transfer the load throughout the bond zone". What provisions? Is having a longer bond zone the only design inefficiency?

 

[born2drill]

It's not economic ineffeciency, it's inefficiency in the load transfer that this suggestion is referring to. This is discussed in PTI as well as FHWA Geotechnical Engineering Circular #4. I quote Circular #4:

"For anchor bond zones that function in tension, initial load increments transferred to the anchor bond zone are resisted by the soil near the top of the anchor bond zone as strains occur in the upper grout body (figure 36). As additional increments of load are transferred to the anchor bond zone, the strains in the top of the anchor bond zone may exceed the peak strain for strain sensitive soils. In that case, the bond stress begins to decrease at the top and the peak strain shifts down the anchor body."

Basically you have diminishing returns because the relationship between bond length and anchor capacity is not strictly linear. AS far as special provisions, one might be to use an SBMA (single borehole multiple anchor) which staggers the free lengths of a multistrand tendon and changes the bond stress characteristics of an anchor in tension.

 

[PEinc]

Very good, born2drill.

http://www.PeirceEngineering.com

 

[HSNIC]

Born2drill,

You have two usernames on here?