sandusk
Civil/Environmental
- Apr 8, 2009
- 1
First, let me tell you that I am not a civil engineer. I am a construction management graduate in my first year of employment.
The plans show an eco block wall with geogrid extending behind the wall a distance yet to be determined. The material directly behind the proposed wall is hard rock (will determine what variety here pretty soon). The wall is said to be 8-14ft high, with a 2:1 from the original grade behind the wall to the proposed backfill.
The original slope has now been surveyed and it appears that a 2:1 from OG to the backfill will not be possible. In order to achieve a 2:1, the wall will have to be about 20ft tall in certain areas due to OG being higher then expected. A change order will probably result.
With the added wall height, the geogrid is going to have to extend significantly further back, which is going to increase the blasting and excavation items exponentially. I propose we use mechanical head rock bolts as a substitution for the grid. Drilling some holes is a lot less work then blasting thousands of CY of hard rock, not to mention cleaning up the mess.
I was thinking that we could have each eco block cast with a piece of 1" ( or whatever dia. rock bolt we wind up using) PVC so we could through bolt the rock bolts with a steel bearing plate on the outer face of the block wall. As it stands right now, I could probably get the design tensile strength numbers for various heights of the wall from the engineer, which I could use to size the rock bolts (dia. size I am referring to) based on their advertised yield strength.
The parts I do not understand are:
1. How do I determine the required embedment of the mechanical head rock bolts into the solid rock behind the wall? I will know the tensile strength requirements as they should be the same for geogrid or rock bolts. There should be some kind of shear cone formula that factors in the rock's shear characteristics, etc.
2. The spacing/pattern of the rock bolts? The overturn moment forces on the wall should increase as you get farther up the wall's face. I am sure that the weight of the backfill will also generate significant force on the lower courses of the wall, making it want to "kick out"
The sum of the tensile strength capacity of all the rock bolts used should be equal to or greater then the sum of the tensile strength the geogrid was designed to carry. Surely we can space rock bolts further apart then plastic geogrid. If I know (when the engineer tells me) what tensile cpacity the geogrid was designed for at given wall heights, I will be able to size and space the rockbolts accordingly.
The question is, how do I know what the adequete spacing of the rock bolts is to prevent the wall from wanting to bow out between each anchorage point?
The plans show an eco block wall with geogrid extending behind the wall a distance yet to be determined. The material directly behind the proposed wall is hard rock (will determine what variety here pretty soon). The wall is said to be 8-14ft high, with a 2:1 from the original grade behind the wall to the proposed backfill.
The original slope has now been surveyed and it appears that a 2:1 from OG to the backfill will not be possible. In order to achieve a 2:1, the wall will have to be about 20ft tall in certain areas due to OG being higher then expected. A change order will probably result.
With the added wall height, the geogrid is going to have to extend significantly further back, which is going to increase the blasting and excavation items exponentially. I propose we use mechanical head rock bolts as a substitution for the grid. Drilling some holes is a lot less work then blasting thousands of CY of hard rock, not to mention cleaning up the mess.
I was thinking that we could have each eco block cast with a piece of 1" ( or whatever dia. rock bolt we wind up using) PVC so we could through bolt the rock bolts with a steel bearing plate on the outer face of the block wall. As it stands right now, I could probably get the design tensile strength numbers for various heights of the wall from the engineer, which I could use to size the rock bolts (dia. size I am referring to) based on their advertised yield strength.
The parts I do not understand are:
1. How do I determine the required embedment of the mechanical head rock bolts into the solid rock behind the wall? I will know the tensile strength requirements as they should be the same for geogrid or rock bolts. There should be some kind of shear cone formula that factors in the rock's shear characteristics, etc.
2. The spacing/pattern of the rock bolts? The overturn moment forces on the wall should increase as you get farther up the wall's face. I am sure that the weight of the backfill will also generate significant force on the lower courses of the wall, making it want to "kick out"
The sum of the tensile strength capacity of all the rock bolts used should be equal to or greater then the sum of the tensile strength the geogrid was designed to carry. Surely we can space rock bolts further apart then plastic geogrid. If I know (when the engineer tells me) what tensile cpacity the geogrid was designed for at given wall heights, I will be able to size and space the rockbolts accordingly.
The question is, how do I know what the adequete spacing of the rock bolts is to prevent the wall from wanting to bow out between each anchorage point?
