Drilled Shafts socketed into bedrock
Drilled Shafts socketed into bedrock
(OP)
Anyone has any experience with resistances to uplift forces of drilled shafts socketed into bedrock. How much critical is to consider rock mass uplift failure for a 3 feet diameter shaft with 8 to 10 feet of overburden soils? Typically I analyze for both rock/concrete socket length bond, and rock mass uplift failure (i.e. like analyzing for a rock anchor). Any comment? Input?
Thanks for any help on this.
Thanks for any help on this.





RE: Drilled Shafts socketed into bedrock
RE: Drilled Shafts socketed into bedrock
Dik
RE: Drilled Shafts socketed into bedrock
Is the uplift force generated by expansive forces in the rock?
Uplift from expansive soils in the overburden?
Or, an uplift force applied at the top of the shaft?
RE: Drilled Shafts socketed into bedrock
RE: Drilled Shafts socketed into bedrock
- 3 feet diameter drilled shaft socketed 5 to 10 feet into bedrock
- shaft has to resist an uplift force of approximately 200 kips
- subsurface conditions consist of approximately 8 to 13 feet overburden loose soil over bedrock. Groundwater very shallow.
- I analyze the concrete/rock bond strength for the lateral surface of the rock socket.
The question is: should I also consider a possible failure within the rock surface (designated as Rock Mass Uplift Failure), like happen when analyzing a rock anchor. Typically for rock anchors I assume an failure surface within the rock in the shape of an upward cone (60 to 90 degree angle at the cone tip). I was wondering if I should consider also this failure when designing drilled shafts socketed into rock.
Thank you all for your comments.
RE: Drilled Shafts socketed into bedrock
Dik
RE: Drilled Shafts socketed into bedrock
Regards,

Qshake
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
RE: Drilled Shafts socketed into bedrock
Regards,

Qshake
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
RE: Drilled Shafts socketed into bedrock
I don't have any advice about the cone type failure you are contemplating, other than to say that intuitively it wouldn't happen. The confining effect of the overburden would extend the cone well above the rock. With conservative assumptions about side wall friction, withdrawal of the pile by failure at the concrete/soil interface would be the controlling case, I think.
The nature of the load would determine whether I would depend on tension piles or instead use rock anchors as others have suggested. If the load is transient, then the tension pile would be appropriate. If the load is constant, especially if with a vibratory component due to some type of equipment, then I would lean toward rock anchors.
RE: Drilled Shafts socketed into bedrock
First I see nothing incorrect with your approach using a cone of rock to evaluate uplift resistance. In design, this ia plausible. The use of rock anchors below the drilled pile is also geasible and can perhaps be cost effective. You may wish to review the texts "Foundations in Tension" by Hanna and "Pile Design and Foundation Practice" by Tomlinson. I have noted that with embeddment depths greater than 1.5 m the failure due to pullout is governed by the bond between the pile and rock. One requires also to assess the characteristics of the rock re the various indicators of quality and use judgment to make a final decision especially where uplift resistance will govern the design. The references provided in these texts would be worthwhile to review.
RE: Drilled Shafts socketed into bedrock
RE: Drilled Shafts socketed into bedrock
Dik
RE: Drilled Shafts socketed into bedrock
Regards,

Qshake
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
RE: Drilled Shafts socketed into bedrock
I do not think you need any rock anchors as suggested above by others. I am sure they would work fine. You can use the allowable side friction in the socket to resist the uplift. There is not requirement for analyzing uplift resistance of a drilled shaft by rock mass uplift failure like a rock anchor.
The available side friction calculated for compression is reduced for uplift resistance for lateral shortening. I worked on a similar project recently for case where for 270' diameter tanks may be submerged and buoyant when empty. You can add some additional uplift resistance due to weight of the shaft and side friction in the overburden.
RE: Drilled Shafts socketed into bedrock
Rey Villa, MS, PE
http://geotech-apps.com
RE: Drilled Shafts socketed into bedrock
www.PeirceEngineering.com
RE: Drilled Shafts socketed into bedrock