Dear all,
I am designing the rock tiedown anchors to take uplift load of the footing due to temperature load. At the site boring, 2 rock corings were conducted, both had a 2' of void out of 5' run, that implied a Rec 60%. The RQD is 47%. Rock is Gneiss. Unfortunately, due to the technical difficulty the boring had to end there. So I don't know what's below, and where the sound rock is. My questions are:
1. How to explain the void length? Is that because the rock is so poor that the core barrel can't pick up some of the rock, or it's because the rock sample does not snap at the end of the core? or both could happen?
2. I put my bond length within this rock (Can I? or I should go depper to teh sound rock). I don't have an unbonded length (do I have to?). I used the bond stress between grout and rock 100 psi (Is that an approperate value to use?). I have total of 10 anchors with 2.5" dia rods from Williams, 6" holes. Using this big diameter of rod is because that the rods have to take the shear force at the bottom of the footing. The anchor is not prestressed. My design load is 135 kips, FS = 2.5, the length of the anchors is therefore calculated as 15'.
3. I checked the FHWA "Geotechnical Engineering Circular No. 4, Ground Anchors and Anchor Systems", and US Army Corps of Engineers Manual on Rock Foundations (EM 1110-1-2908). Since I have a 5 ft high concrete footing and the anchors connect teh footing to the ground, I didn't check the cone breakout failure (should I check?)
Thank you very much for your help!
I am designing the rock tiedown anchors to take uplift load of the footing due to temperature load. At the site boring, 2 rock corings were conducted, both had a 2' of void out of 5' run, that implied a Rec 60%. The RQD is 47%. Rock is Gneiss. Unfortunately, due to the technical difficulty the boring had to end there. So I don't know what's below, and where the sound rock is. My questions are:
1. How to explain the void length? Is that because the rock is so poor that the core barrel can't pick up some of the rock, or it's because the rock sample does not snap at the end of the core? or both could happen?
2. I put my bond length within this rock (Can I? or I should go depper to teh sound rock). I don't have an unbonded length (do I have to?). I used the bond stress between grout and rock 100 psi (Is that an approperate value to use?). I have total of 10 anchors with 2.5" dia rods from Williams, 6" holes. Using this big diameter of rod is because that the rods have to take the shear force at the bottom of the footing. The anchor is not prestressed. My design load is 135 kips, FS = 2.5, the length of the anchors is therefore calculated as 15'.
3. I checked the FHWA "Geotechnical Engineering Circular No. 4, Ground Anchors and Anchor Systems", and US Army Corps of Engineers Manual on Rock Foundations (EM 1110-1-2908). Since I have a 5 ft high concrete footing and the anchors connect teh footing to the ground, I didn't check the cone breakout failure (should I check?)
Thank you very much for your help!