LibraTech
Geotechnical
- May 28, 2008
- 5
We need to design and construct large diameter piles to take relatively high loads.
The friction Piles are constructed using a reverse circulation drill rig, to drill into a Shale type rock.
In an attempt to understand the properties of the weaker rock materials, various 110mm vertical core-holes were sunk and logged. Specific rock types to be tested were identified from this. The base of the hole (up to the level of the rock to be tested) were infilled with weak grout mix. A bentonite pellet isolation layer was put at the bottom before a 2m fixed length stranded anchor was installed. Negligable overbreak occurred, as this was measure by the qty of rock fragments expelled from the hole. The calculated apparent free length vs the actual free length was vitually 100% the same for all, so no over-grouting or excessively long fixed lengths were installed.
The anchors were stressed to 80% Fpu of the strands and in most cases resulted in failure adhesions double of that expected using the Horvath and Kenney design method adhesion predictions.
The underprediction of the Horvath and Kenney method, interestingly also to a factor of 2.0, have been well established in the Thesis titled "Prediction of Ultimate side shear for drilled Shafts in Missouri Shales". The shale material in this Thesis has very similair properties than that of the Shale where the pull out tests were done.
Seeing that this is very much in line with what the anchor pull out tests results showed, should any type of empirical / reduction / scaling factor be applied to small diamater pull out tests before using results in the refinement of a design for a large diameter pile, or can pull out test results be used as actual ultimate adhesion in the specific rock type and the conventional factors of safety applied ?
The friction Piles are constructed using a reverse circulation drill rig, to drill into a Shale type rock.
In an attempt to understand the properties of the weaker rock materials, various 110mm vertical core-holes were sunk and logged. Specific rock types to be tested were identified from this. The base of the hole (up to the level of the rock to be tested) were infilled with weak grout mix. A bentonite pellet isolation layer was put at the bottom before a 2m fixed length stranded anchor was installed. Negligable overbreak occurred, as this was measure by the qty of rock fragments expelled from the hole. The calculated apparent free length vs the actual free length was vitually 100% the same for all, so no over-grouting or excessively long fixed lengths were installed.
The anchors were stressed to 80% Fpu of the strands and in most cases resulted in failure adhesions double of that expected using the Horvath and Kenney design method adhesion predictions.
The underprediction of the Horvath and Kenney method, interestingly also to a factor of 2.0, have been well established in the Thesis titled "Prediction of Ultimate side shear for drilled Shafts in Missouri Shales". The shale material in this Thesis has very similair properties than that of the Shale where the pull out tests were done.
Seeing that this is very much in line with what the anchor pull out tests results showed, should any type of empirical / reduction / scaling factor be applied to small diamater pull out tests before using results in the refinement of a design for a large diameter pile, or can pull out test results be used as actual ultimate adhesion in the specific rock type and the conventional factors of safety applied ?
