Non-homogeneous bedrock

[Mccoy]

This situation occurred to me a coupla times in a region of quaternary deposits, laterally pretty dishomogeneous: the excavated area on which the foundations were to be cast was made -up partly of rock (sandstone or fractured limestone), partly of soil (OC or NC clays, sandy/silty). In one instance the interface designed almost perfectly a diagonal w/ respect to the plane view of the building site.
Input data are as follows:
- seismic area w/ expected horizontal seismic stress in the order of 0.15 g as per regulations, but larger as per recent research.
- interface between soil and rock may be a fault plane, a dipping layer or a buried channel.
- usually the structures are of the residential type, concrete-frame or masonry, pressure at the base of foundations in the range of 120-200 KPa
-Routine configuration of foundations is a grid of rigidly connected inverted beams.
The problem here is obviously due to high expected structural stresses caused by differential settlements and different dinamic behaviour of rock and soil in the case of a seismic event.
As a solution I ruled out deep foundations beneath the soft soil, due to the above differences in dinamic behaviour and indetermination in the dinamic response of piles.
Is reclamation of soft soil (substitution of soil w/ compacted granular material) acceptable, and what should be the minimum depth of reclaimed soil?
So far it looks like an optimum solution in these conditions can be barely reached, if else.
I wonder if there are low-cost solutions either on the side of soil improvement or in the structural configuration of the foundatios.
Costly soil improvement methods, such as jet-grouting are not acceptable in this small-size residential type of buildings, nor switching location is possible, what you get is what you have!
In the instances of non homogeneous soil which occurred to me I simply suggested an interconnected grid or a foundation slab, with the substitution of a 1 thick m layer of clayey soil with well-compacted granular soil, + warned the structural engineer of the problems related to the geological situations. Buildings are of recent construction hence untested so far.

 

[ishvaaag]

We have built terraced houses in Marbella mountains that partly could be what you describe. Mansonries are customary there. Seismicity maybe lower than 0.15 g. We used inverted T plus floor rigidity for foundations, with the help of consultant. Giant cut and fill to soften the slopes was made (15 to 10 years ago). Compaction was quite stringent, upper layer we asked over 95% modified Proctor. Surely some houses stand half on rock, half on infill. Infill was made for land availability and to diminish slopes. No problem has developed but not EQ test yet (fortunately). Loads are almost despicable (equivalent to 1 m of earth atop an enbankment 20 m tall (1:2 or steeper slopes).

Maybe there's a partial solution to your question: include a friable (maybe rolling stone plus sand or even something more) layer in horizontal shear under the hard rock foundation part of the building, and give lateral offset for movement to avoid slab on the ground hammering. This way we uncouple a bit the response of the soft plus house part from the rock part. This should make some good.