Engineering and Construction

For heavily loaded foundations, using the allowable bearing capacity from Table 18.4 may be too conservative for hard and sound rock, which will result in an uneconomical foundation. For example, Table 18.4 specifies a maximum allowable bearing capacity of 12,000 psf (570 kPa) for crystalline bedrock. But other sources have recommended much higher allowable bearing values, such as 160,000 psf (7.7 MPa) for massive crystalline bedrock (NAVFAC DM-7.2, 1982). Local experience may even dictate higher allowable bearing values, but the allowable bearing capacity should never exceed the compressive strength of the foundation concrete.

It has been stated that there is no reliable method of predicting the overall strength and deformation behavior of a rock mass from the results of laboratory tests, such as the unconfined compression test on small rock specimens. This is because the settlement behavior of rock is strongly influenced by large-scale in situ properties, such as joints, fractures, faults, inhomogeneities, weakness planes, and other factors. Hence using small core specimens to predict settlement behavior will often produce highly inaccurate results, with the inaccuracy increasing as the RQD decreases. Because of this limitation, field tests are preferable, with some options for determining the settlement of heavily loaded foundations on rock as follows:

1. Foundation load test. If the proposed foundation were to consist of piers founded on rock, then test piers could be constructed and load tested. Performing load tests would be the most accu- rate method of determining the load-settlement behavior of a foundation element supported on rock (i.e., see ASTM D 1143-94, 2004, “Standard Test Method for Piles Under Static Axial Compressive Load”). Unfortunately, such a test would be very expensive and time consuming and would normally only be justified for essential facilities or very high foundation loads.

2. Plate loading method. Similar to the plate load test described in Sec. 7.3.2, the rock could be subjected to a plate loading test in order to determine the load-deformation behavior of the rock. For further details, see ASTM D 4394-98 (2004), “Standard Test Method for Determining the In Situ Modulus of Deformation of Rock Mass Using the Rigid Plate Loading Method” and ASTM D 4395- 98 (2004), “Standard Test Method for Determining the In Situ Modulus of Deformation of Rock Mass Using the Flexible Plate Loading Method.”

3. Rock mass strength. The allowable bearing value  qall could be based on the rock mass strength. The approach would be to determine the unconfined compressive strength qc and then apply a factor or safety, or qall = qc /F. The goal is to take into account the effect of both intact material behavior and the behavior of discontinuities contained within the specimen block. The rock mass strength could be determined by performing in situ unconfined compressive tests as described in ASTM D 4555-01 (2004), “Standard Test Method for Determining Deformability and Strength of Weak Rock by an In Situ Uniaxial Compressive Test.” The procedure is to perform in situ compression tests on specimens of rock that are large enough so that the rock mass unconfined compressive strength qc is obtained, such as illustrated in Fig. 7.23. Similar to the load tests described above, this approach is very expensive and time consuming.

4. Laboratory testing. Especially for soft rock, such as weakly cemented sedimentary rock, undisturbed specimens of the material can be obtained from Shelby tube samplers. The specimens could then be set-up in an oedometer apparatus and the vertical stress versus vertical stain curve can be obtained (i.e., similar to Fig. 7.12). Several specimens of rock would be obtained at various depths and the settlement calculations would be identical to the procedure as outlined previously.

Two other approaches for determining the settlement of heavily loaded foundations on rock are the elastic method (i.e., Eq. 7.9) and the finite element method. Unfortunately, both of these methods require that the modulus of elasticity E and Poisson’s ratio μ be determined for the rock mass.

A major limitation of these two methods is that the value of E is often obtained from an unconfined compression test on a small rock specimen (i.e., ASTM D 3148-02, 2004, “Standard Test Method for Elastic Moduli of Intact Rock Core Specimens in Uniaxial Compression.”). Using this approach will undoubtedly overestimate the value of E, with the overestimation increasing as the RQD decreases.

The methods will have much greater accuracy if the value of E is obtained from in situ tests on large rock specimens (i.e., ASTM D 4555-01, 2004). For large scale in situ tests on rock, the value of E is obtained from the vertical stress versus vertical strain curve and is often defined as the tangent modulus at 50 percent of the maximum strength and is then referred to as Et (see item a, Fig. 7.9).

When using high allowable bearing values for rock, it is essential that the footing or deep foundation excavation be inspected and cleaned of all loose debris so that the foundation bears directly on intact rock.

Even a thin layer of disturbed and loose material left at the bottom of the foundation excavation can lead to settlement that is greatly in excess of calculated values.