Construction & Contractors/rock foundations

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QUESTION: when you are planning to build on a rock outcrop,do you need to trench onto the rock for the strip footing or the isolated pads.how is the use of fill a possible option ?

ANSWER: Buried strip footings are constructed in soils for reasons that may not hold in rock.  In soil, strip footings distribute the structure load so the soil can bear it, transfer the load to an elevation below the frost depth and at which the soils bearing the load are confined by the surrounding soils which enhances their shear resistance, provides some additional structure weight and shear resistance against uplift loads that can result from wind and seismic, and provides passive resistance against overturning and translational movement that can result from non-vertical structure loads such as live loads, wind, and seismic.

Rock, on the other hand, may be strong enough to resist the structure loads without the benefit of burial, and if non-porous, will not hold enough water to make frost heave a problem.

The reality is, though, that all rock is fractured and weathered to some degree, so it is common to remove scale, weathered and clayey rock, and fractured material to promote adhesion of the concrete foundation to sound, fresh, solid strong rock.  This may result in a foundation "trench".

It is also standard practice to secure the foundation to the rock with anchors (grouted dowels), because practically all foundation loads have nonvertical components and the adhesion between the concrete and the rock cannot be relied upon indefintely.  The depth, spacing, and size of the dowels has to do with the rock mass condition -- strength, fracture intensity, and condition of fractures -- and the rock's propensity for swelling, moisture absorption, and corrosion of the dowel material.  The anchor system should be designed by a geological engineering professional on the basis of a thorough study of the rock mass and all its variations, preferably incorporating a personal site visit.  The dowels will resist shifting due to nonvertical loads, ensure that the rock and the concrete behave as an integral unit, and as necessary, knit together the fractured rock under the foundation so that shifting would have to mobilize a greater volume of rock and involve stronger fractures.  In some cases the anchors need to be post-tensioned to resist uplift forces or provide sufficient friction betweenthe rock and the foundation.

If your question about fill refers to the placement of compacted fill on the rock just so a conventional strip footing can be constructed in the fill, that doesn't make a whole lot of sense once you realize how much stronger the rock is than the fill.  You sacrifice the much greater bearing capacity of the rock in favor of a spread footing in the fill, and then introduce the additional complexity of having to account for the stability of the fill itself which would have to be sloped or retained so as to prevent its movement or settlement under its own self weight and the weight of the imposed foundation loads.  It only makes sense if the grade is such that it is cheaper to place and compact the fill than it is to construct piers anchored to the rock and then span the structure load across the piers.

---------- FOLLOW-UP ----------

QUESTION: Thanks Rob for the helpful response. I would then love to know also about my concern for the 'sandwich effect' created by the rock-gravel fill interphase. how stable is the interphase and is there any preparation of the bedding rock needed prior to placement & compaction? also what amount of fill can be used as the upper limit ?

Answer
I have no idea what you mean by the "sandwich effect".

What gravel fill?

Assuming you are talking about a clean gravel fill or at least SGC mix with little clay/silt, the shear strength of gravel fill sitting atop a solid bedrock surface depends on the undulations (roughness) of the bedrock surface in comparison to the size of the gravel in the fill.  If the bedrock surface is very smooth then shear movement would first occur along the rock-fill interface, so it governs the shear strength, and could be approximated by the residual friction angle -- no cohesion -- of the sand-size fraction of the gravel.  If the rock surface has roughness whose amplitudes are equal to or greater than the mean size of the gravel and if those surface asperities have sides whose inclinations from the average bedrock surface angle are equal to or greater than the friction angle of the gravel, then the friction angle of the gravel will govern -- in other words, shear would have to take place mostly or entirely through the gravel.  Other conditions would lie between those two extremes.  Because in most cases the friction angle (fully drained) of the former would be in the 28-30 degree range and the latter in the 36-42 degree range (depending on grain size distribution and the degree of compaction), you are pretty safe in assuming the interface friction angle is 30 degrees.  Then you have to do a shear stability analysis.  The likelihood of shear depends on the angle of the bedrock surface from the horizontal in relation to the resultant applied loads -- the more the bedrock slopes in the direction of the applied loads, the lower the shear resistance.  It also depends on whether any impounded water can drain or would accumulate pore pressures.

Your preparation of the surface would be in accordance with whatever you assumed in the stability analysis.

As to whether there is an upper limit to the amount of fill, no, not really, but keep in mind that gravel fill would have to be retained, or sloped at an angle flatter than 3H:1V to avoid loss of material from the edges.  The point of application of the load should be at least 4 times the fill thickness away from the edge to be sure all loads stay are borne by the fill and bedrock.  

I am getting the feeling that you are addressing a fairly complicated issue and you should be working with a design professional with suitable expertise.  This is particularly the case if you are designing a foundation within a compacted gravel fill sitting on a sloping bedrock surface.  

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Robert Cummings, P.E.

Expertise

I can answer questions related to rock blasting, rock and soil excavation (such as tunnels and highway cuts), stability of such excavations, and foundations in rock and soil. I can also answer questions related to geology and mining.

Experience

30+ years as a geotechnical engineer and minerals engineer. Active consulting practice in rock blasting, geotechnical engineering, and rock mechanics for mining and heavy construction.

Organizations
Society of Mining Engineers, Deep Foundations Institute, Association of Engineering Geologists, and International Society of Explosives Engineers.

Publications
Mining Engineering, AEG Bulletin.

Education/Credentials
BS and MS Geological Engineering

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