foundation system of burj al arab

Settlement of Foundation Supported on Rock

Building foundations are supported on different types of soils encountered under construction site. We are always suspicious about loose sands, clay deposits, collapse potential soils etc. and always look for firm ground surface to avoid deep foundation. Even when there have no alternative to avoid deep foundation, we will try to support tip of such foundation over a firm deposit. Such firm deposit is rock; now should we fill safe when building is founded on rock? 

Rock deposit is not always safe, rock may be weathered out, may be disturbed by construction, some may be of expansive nature and various causes should be investigated before placing foundation. But extent of investigation should be based on type of structure;here we will differentiate structure by loading i.e. for lightly loaded and heavily loaded building.

Lightly loaded foundation:

In case of light buildings, foundation performance on sound and hard rock is very good. The total settlement and maximum value of differential settlement should be essentially zero. Sometimes, rocks supporting concrete foundation have more strength than foundation materials (i.e. concrete). When such light buildings are supported on sound and hard rock, comprehensive investigation is not considered important in usual condition and the reference is used from building code; the allowable bearing capacity recommended by international building code (TABLE 1806.2 in IBC) can be taken as design reference.

Now does this allowable bearing capacity have adequate factor of safety? For sound and hard rock, this allowable bearing capacity was found enough except following conditions: 

Fractured or weathered rock:

Rock may be subjected to such weathering that it may behave like soil rather than parent rock. Thus bearing capacity specified by building code will be considered over estimated for weakly cemented, friable, foliated, highly jointed or for other cases that make rock weak. 

Expansive rock:

Some rocks such as shale and claystone may often be expansive. In such situation the problems with heaving appeared instead of settlement. These expand when moisture are penetrated to the rock formation. For lightly loaded building this rocks are considered dangerous as they can lift upward. Again for a heavily loaded building during construction when foundation load is increased gradually, upward lift may occur with resulting differential heave.

Secondary influences:

Secondary influences may result significant settlement to lightly loaded building supported on sound rock. This sound rock even not capable to support foundation when sink holes are opened or settled resulted from collapse of tunnel and mines constructed or being constructed below building foundation.

Cutting-filling operation:

It is expected to rest foundation of building a level surface; to achieve this some portion of the earth surface may need to be cut and some portion may require to be filled. Cut-fill alteration for foundation is the case where some rocks are removed with a building pad that is created with soil filling. If soil fillings are considered well compacted and cut portions of building pad have no expansion potential rock (taken as sound and hard) supporting a lightly loaded building, no settlement will occur. The cut portions remain safe at this condition but the filling portions have possibility to settle and may result damage. Again in fill portion vertical movement of foundation (I.e. settlement of fill) may be associated with horizontal movement which may become appear as crack in slab and have drag effect.

Following figure shows an example of slab crack, where typically crack open at the points of transition of cut-fill foundation. In such cases, to strengthen foundation by introducing deepened footings or underpinning, the fill section is considered suitable for strengthening or retrofitting operation.As filled portion is underpinned, entire foundation can be taken as rock anchored. For a heavily loaded building, this option is also preferred.

Heavily loaded foundation:

In case of heavily loaded buildings, allowable bearing capacity specified by above table may often be too conservative for sound and hard rock; thus the foundation will be uneconomical. Let’s take an example, IBC recommended to take maximum allowable bearing capacity 570 Kpa (12,000 psf) for underlying crystalline bedrock whereas other sources like NAVFAC DM-7.2 recommended much higher values of allowable bearing capacity, such as 7.7 Mpa (160,000 psf) for large crystalline bedrocks.

Sometimes it is reported even higher allowable bearing capacities, but it is recommended not to consider in design allowable bearing capacity more than compressive strength of concrete used in foundation. Now we have no reliable method to predict the deformation characteristics and overall strength of particular rock mass based on result obtained laboratory tests like unconfined compression test of small rock sample; this is due to significant influence of in-situ properties of rock on settlement behavior.

Properties of rocks like fractures, joints, faults, weakness in plane, inhomogeneity and miscellaneous factors are often omitted in small rock samples. Thus specimens collected by core cut to set under laboratory testing procedures, often yield high deviation from actual results; the predicted settlement behaviors will also be erroneous as RQD (Rock Quality Designation) value decreases. For this reason, it is preferred to conduct tests in field