rock dowels, anchorage, cohesion intercept

[soiset]

For the wall mentioned in previous post:
What is or where could I find a safe value for c (rock mass cohesion intercept) for moderately to highly fractured basalt? I want to use the rock dowels to help in resisting overturning, but I want to be very conservative in estimating the tensile strength of the rock dowels/anchors. I'd like to use the equation D=(FS)(F)/cs, where D=depth, and s=anchor spacing.

 

[derubertis]

A safe value for cohesion is zero. Analyse the problem and determine how much cohesion is required to keep the amount of reinforcement within reason. If you are using dowels, they are untensioned and do not develop strength until movement occurs. Is this what you intend? Is water present?

Sorry, I didn't see the previous post.

 

[geomo]

Do you want a cohesion, c-value for designing rock dowels/bolts/anchors? If you want a value to determine the pullout capacity of the dowels, then you want the bond strength, not cohesion. This is a great reference paper:

Littlejohn, G.S., "Design Estimation and Load Holding Capacity of Ground Anchors"

Sorry, but I don't know where it was printed. You might be able to find it by searching technical journals, including international journals.

In that paper there is a table of values for design. In the table the closest rock type to fractured basalt is just plain basalt with a working bond of 1.21-1.38 N/mm^2 and an ultimate bond of 3.86 N/mm^2. You might use a working bond of 1.2 N/mm^2 to determine the allowable capacity of a dowel grouted into the rock.

For example, you have #6 bars so you might want a 1" drilled hole with a perimeter of about 80mm (sorry for mixing units). A one foot deep (300mm) hole gives 24,000 mm^2. At 1.2 N/mm^2, the allowable bond between grout and rock is 28.8 kN, or about 6.5 kips. That is less than the tensile capacity of the rebar (est'd 0.6 fy, or about 36ksi), so you might want to make the dowels deeper. The reason is that it is good practice to make steel yield be the controlling factor in the design. Remember, this is a tension calculation that helps with uplift of the footing, for overturning or otherwise.

If you are putting the dowels in the footing to resist shear, then that calculation doesn't do much for you. In the case of shear, you have #6 bars with 0.44 in^2 cross section. Multiply that area by the steel strength with appropriate shear reduction factor (maybe check ACI 318 building code).

Is your wall in a rock cut? If so, then you can check earth pressure using the following paper:

Frydman, Sam and Israel Keissar, "Earth Pressure on Retaining Walls Near Rock Faces," Journal of Geotechnical Engineering, Vol. 113, No. 6, June 1987, ASCE.

If the retained soil is not rock, earth pressure will be based on some anticipated movement of the wall. If the wall structure is stiff and prevented from rotating at the top (cantilevered concrete will bend even if the foundation doesn't move a bit), then you have at-rest (Ko) conditions. If you have some movement, the earth pressure will be less, down to a lower bound of active (Ka) conditions which requires about an inch of tilt per 20 feet of wall height.

Good luck.

 

[ishvaaag]

Only a comment respect shear or shear at the base.

If you use the dowels "shear-lug-like" then section and steel shear strength itself are paramount... but only in such case.

For all others, shear will be normally passed through bottom encasement in the rock, and you normally will have then your dowel reinforcement laid mainly to through splices resist tensile forces caused by bending at the base.

If for whatever the reason some relative displacement of the base is estimated feasible to happen, then any shear needs be taken in shear-friction. Assuming 45 deg friction angles at the interface is common.

Shear friction is NOT independent of the bending action at the base, and summing a requirement derived from tension from bending plus tension from shear friction is overly conservative, yet it may be a practical approach.

Looking at the dowel thing in strut-and-tie mindset helps to see that, but then friction (shear-friction is the name) from the compressive strut is what would be thougt be passing the load...so better always provide some wall keying.

 

[Sean2]

As I indicated in my reply to your "previous post", I strongly recommend that you consult a specialist geotechnical engineer in identifying, analysing, and solving this problem.

 

[soilover]

Dear Geomo,

Hi, I also encounter a case: to build a wall to retain the soil near rock. The wall acts as a basement wall for a building as well.
I found the following paper according to your recommendation.
Frydman, Sam and Israel Keissar, "Earth Pressure on Retaining Walls Near Rock Faces," Journal of Geotechnical Engineering, Vol. 113, No. 6, June 1987, ASCE.
May I know if there is any newer development of this paper? It seems a little bit old now, 15 years ago!
BTW, the rock for my case doesn't start from ground, it starts from about 10-20 m depth to 60 m depth. My basement wall will start from ground all the way to 30 m depth. Therefore, my wall will retain full soil at the top 10-20 m, and the soil near rock at the bottom.
Thank you.
Soilover

 

[DRC1]

If the excavation is dry and accessable, you can allways test the capacity with a center hole jack. This way you will know if each anchor is good.