7/28/2019 Case Summary PEAT (FINAL) http://slidepdf.com/reader/full/case-summary-peat-final 1/2 CASE HISTORIES: PEAT KOSTAS SYMEONIDIS MSc Engineering geology for ground models 2011 6/3/2012 1. Peat Peat is an accumulation of plant remains that has undergone some degree of decomposition. Inorganic soil material may occur as secondary constituents in peat. 2. Classifications Principal types of peat forming ecosystems: Fens, Bogs, Swamps, Marshes. Main peat formations: Raised bog peat, Fen peat Peat flow, Blanket bog peat, Hill peat, Basin peat 3. Accumulation and development Accumulation depends on factors like: climate, topography, superfluity of water, geological conditions in terms of nutrient source availability. Peat accumulation takes place when the rate of addition of matter exceeds the rate of decay 4. Engineering geology of peat Multiple dep. environments and sensitivity to changes (like climatic changes, sea level changes, glacial advance or retreat) leads to complexity and variability in nature and extend of peat formations. Peat and organic soils can be hazardous to engineering works, due to their nature: highly compressible and subject to severe long term creep, low unit weight, methane gas may be present, cause very large settlements, slope stability problems. Can also be hazardous to engineering works, also due to their complexity as geological formations Field identification: Fibrous peat, Pseudo-fibrous/hemic peat, Amorphous/sapric peat. 5. Engineering properties of peat Difficult to obtain undisturbed samples: Presence of gas, Loss of moisture during extrusion and preparation, Disturbance of sample when forces in the retainer Cone penetration testing (standard CPT and piezocone CPTu) may be used to obtain detailed site stratigraphy information. Physical properties The shear strength of a peat deposit depends on its moisture content, degree of humification and mineral content. Relatively lightweight (i.e., low dry density), saturated, no significant stress history, thus their strengths are usually very low. Primary settlement will occur over a relatively short time while the majority of the total settlements will result from the long-term secondary compression. 6. Engineering in peat Construction methods include Load adjustment, Soil replacement, Stage construction/Soil improvement, geotextiles and embankment piling. 7. Case: Wilnis peat dyke breach, Netherland Location: Wilnis is a village in the Dutch province of Utrecht, about 30km south of Amsterdam. Failure: At 26-Aug-2003 1:30am a peat dyke failed along the ring canal near the village center. About 60m of dyke translated horizontally ~15m towards the north, leaving two breaches at the sides. Consequences: 600 houses under 0.5m of water, 2000 residents evacuated but returned to their house at the evening. Site investigation: No prior available data about the dyke, or the soil profile at the site. Investigation conducted after the failure and included borings, CPTs and piezometer, near and in the dyke. Lab tests including triaxial tests (CU) and Direct shear test (more propriate). Failure mechanism: The prolonged dry and hot season caused the drying of the crest peat, thus further reducing the unit weight of the already lightweight peat and consequently the passive shear resistance enough to be: SF < 1. Possible precaution: Pressure berm calculation Pressure berm: 0.9m height and 3.75m length, plus