ENGINEERING GEOLOGY/Problematic Soils 565 preserved under conditions of incomplete aeration and high water content It accumulates in areas where there is an excess of rainfall and the ground is poorly drained Nonetheless, peat deposits tend to be most common in those regions with a comparatively cold wet climate The high water-holding capacity of peat maintains a surplus of water, which ensures continued plant growth and consequent peat accumulation Drying out, groundwater fluctuations, and snow loading bring about compression in the upper layers of a peat deposit As the water table in peat normally is near the surface, the effective overburden pressure is negligible The void ratio of peat is very large, ranging from up to 25 It usually tends to decrease with depth within a peat deposit Such high void ratios give rise to phenomenally high water content, varying from a few hundreds per cent dry weight to over 3000% in some coarse fibrous varieties Put another way, the water content may range from 75 to 98% by volume of peat Peat, therefore, undergoes significant shrinkage on drying out The magnitude of pore-water pressure is particularly significant in determining the stability of peat With the exception of those peats with low water contents (less than 500%) and high mineral contents, the average bulk density of peat is slightly lower than that of water Gas is formed in peat as plant material decomposes, the volume of gas varying from around to 7.5% Most of the gas is free and so has a significant influence on the rate of consolidation, pore pressure under load and permeability Differential and excessive settlement are the principal problems confronting the engineer working on a peat soil Serious shearing stresses are induced, even by moderate loads Worse still, should the loads exceed a given minimum, then settlement may be accompanied by creep, lateral spread or, in extreme cases, by rotational slip and upheaval of adjacent ground At any given time, the total settlement in peat, due to loading, involves settlement with and without volume change Settlement without volume change is the more serious for it can give rise to the types of failure mentioned above What is more, it does not enhance the strength of peat Because of the potential problem of settlement arising from loading peat, especially in the construction of embankments carrying roads, some method of dealing with this problem has to be employed Bulk excavation of peat frequently is undertaken if the deposit is less than m in thickness When a deposit exceeds m or peat occurs as layers within soft sediments, precompression, involving surcharge loading, commonly is used With few exceptions, improved drainage has no beneficial effect on the rate of consolidation When peatlands are drained artificially for reclamation purposes, the ground level can experience significant subsidence The subsidence is not simply due to the consolidation that occurs as a result of the loss of the buoyant force of groundwater but also is attributable to desiccation and shrinkage associated with drying out in the zone of aeration and oxidation For instance, in some parts of the Fenlands of eastern England, the thickness of peat has been almost halved as a result of drainage (e.g., Holme Post was installed in 1848 and by 1932 the thickness of the peat had been reduced from 6.7 m to 3.4 m; Figure 9) See Also Engineering Geology: Problematic Rocks; Rock Properties and Their Assessment; Site and Ground Investigation; Subsidence Further Reading Andersland OB and Ladanyi B (1994) An Introduction to Frozen Ground Engineering New York: Chapman and Hall Bell FG (1993) Engineering Treatment of Soils London: E and FN Spon Bell FG (2000) Engineering Properties of Soils and Rocks, Fourth Edition Oxford: Blackwell Scientific Publications Blight GE (ed.) (1997) Mechanics of Residual Soils Rotterdam: AA Balkema Chen FH (1988) Foundations on Expansive Soils Amsterdam: Elsevier Charman JM (1988) Laterite in Road Pavements Special Publication 47, London: Construction Industry Research and Information Association (CIRIA) Fookes PG (ed.) (1997) Tropical Residual Soils Engineering Group Working Party Revised Report, London: Geological Society Fookes PG and Parry RHG (eds.) (1994) Characteristics of Arid Soils Rotterdam: AA Balkema Jefferson I, Murray EJ, Faragher E, and Fleming PR (eds.) (2001) Problematic Soils London: Thomas Telford Press Jefferson IF, Rosenbaum MS, and Smalley IJ (eds.) (2004) Silt and Siltation, Problems and Engineering Solutions Berlin: Springer Verlag