Engineering Behaviour of Sedimentary Rocks

Sandstones may vary from thiniy iaminated micaceous types to very thickly bedded varieties. Moreover, they may be cross-bedded and are jointed. With the exception of shaley sandstone, sandstone is not subject to rapid surface deterioration on exposure. 

The dry density and, especially, the porosity of sandstone are influenced by the amount of cement and/or matrix material occupying the pores. Usually, the density of sandstone tends to increase with increasing depth below the surface, the porosity decreasing. 

The natural moisture content of shales varies from less than 5%, increasing to as high as 35% for some clayey shales (Table 5.31). When the natural moisture content of shales exceeds 20%, they frequently are suspect as they tend to develop potentially high pore water pressures. Usually, the moisture content in the weathered zone is higher than in the unweathered shale beneath. Depending on the relative humidity, many shales slake almost immediately when exposed to air. Desiccation of shale following exposure leads to the creation of 

Free swell of carbonaceous mudrock (after Jermy and Bell, 1991). 264 

Severe settlements may take place in low-grade compaction shales. Conversely, uplift frequently occurs in excavations in shales and is attributable to swelling and heave. Rebound on unloading of shales during excavation is attributed to heave due to the release of stored strain energy. The greatest amount of rebound occurs in heavily overconsolidated compaction shales. Sulphur compounds frequently are present in shales and mudstones. An expansion 

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