Failed Roadway Slope

The height of this excavated hill slope is more than 15m. There were two benches with cut off drains. Average slope was about 1: 1.5, which was quite a standard design for cut slope. Typical soil profile is shallow firm to hard residual soils at the top and sedimentary rocks (Hard black Shale) at the bottom. Why the slope failed?

The answer is the exposure of the underlying very finely laminated black Shale to the rain after excavation plus the dipping direction of the bedrock, towards the road.
The laminated Shale at this site is very thin, some as thin as paper, which allowed easy entry of water along the laminated surfaces to the whole rock, the Exposed Shale rock crumpled to fragments and small particles, slipped along the dipping planes which happened to be towards the road. Just look carefully on the failed rock conditions, you will see how these thinly bedded Shale rocks disintegrated.  
Can we detect the problematic Shale during soil investigation? Yes, rock coring requires water and if you find substantial core loss or crumbled samples, then the rock has potential problem. Dipping plane can also be detected if there are at least two boreholes across the proposed slope. Design engineers should inspect the rock samples to have a visual and feel of the soil and rock conditions.
With this kind of rock, gentler slope has to be provided and has to reduce exposure of rock to the rain/water immediately. Construction is recommended during dry season.