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 having some option to determine settlement of this type foundation as follows:

1. Foundation load test:

When foundation preferred for a building is pile or pier supported on rock, then test piles or piers need to be constructed to conduct load test; conducting load test of deep foundations are considered most accurate way to determine the load-settlement characteristics of the foundations founded on rock. But this type of test is very expensive and consume significant amount of time; generally it is chosen when foundation loads calculated are very high or while constructing any essential facilities. ASTM D 1143-94, “Standard Test Method for Piles Under Static Axial Compressive Load”.

2. Plate load test

To determine load-deformation behavior of rock under foundation loadings ASTM suggested two types of load test; ASTM D 4394-17 and ASTM D4395-17.

ASTM 4394-17 “Standard Test Method for Determining In Situ Modulus of Deformation of Rock Mass Using Rigid Plate Loading Method”

ASTM D 4395-17 “Standard Test Method for Determining the In Situ Modulus of Deformation of Rock Mass Using Flexible Plate Loading Method”

Rock Mass Strength:

Allowable bearing capacity of rock q_all can be determined based on rock mass strength. In this approach, unconfined compressive strength value qc is determined and then use appropriate factor of safety safety i.e. q_all=q_c/F. the objective of introducing this term is to account the influence of discontinuities exist within the sample block in addition to behavior of intact materials.

Rock mass strength can be determined by conducting in situ q_c tests as recommended in ASTM D 4555-10(Standard Test Method for Determining Deformability and Strength of Weak Rock by an In Situ Uniaxial Compressive Test). The procedure includes in situ test of compressive strength q_c of rock sample that is large enough to encounter any flaws or weakness. This test procedure involves significant cost and consumption of valuable time. 

Laboratory testing:

In case of soft rock like weak cementation in sedimentary rock, undisturbed sample of rock can be taken by Shelby tube samplers. The sample can be set under oedometer apparatus, by which we can produce vertical stress stain curve. Several samples of rock materials are collected from various depths and settlement would be calculated with consolidation theory.

There have other methods to determine settlement behavior of foundation supporting heavy structures resting on rocks. They are

• Elastic method
• Finite element method

Both of these method, unfortunately, require poisson’s ratio µ and modulus of elasticity E of rock mass. The limitation is determination of reliable E value; in most cases, it is obtained from unconfined compression test of a small rock sample. ASTM provided a procedure to determine these, ASTM D3148-02 “Standard Test Method for Elastic Moduli of Intact Rock Core Specimens in Uniaxial Compression” which was withdrawn later considering reliability of result in 2005.

This approach obviously will overestimate the E value which results a reduction in RQD. A greater accuracy can be achieved when E is determined from in situ large rock sample as recommended by ASTM D4555-10. In this case, E value is evaluated from large scale test on rock from vertical stress-strain curve obtained while testing and actually tangential modulus at 50% of maximum strength will furnish E_t value.

In conclusion, when we are ascertained that rock has high allowable bearing capacity, the final excavation level is carefully inspected and thoroughly cleaned to remove any loose particles or debris