Geognostic Surveys PDF

Geognostic surveys Surveys aimed at identifying and characterising the ground. Geological and geological-technical characterisation ⇒ geomaterials with their physical and mechanical properties. Geognostic surveys can be:   DIRECT: actual inspection of the ground ⇒ stratigraphy, by means of tr...

Geognostic surveys Surveys aimed at identifying and characterising the ground. Geological and geological-technical characterisation ⇒ geomaterials with their physical and mechanical properties. Geognostic surveys can be:   DIRECT: actual inspection of the ground ⇒ stratigraphy, by means of trenches or drillings, in situ tests, samples for lab tests INDIRECT: ground parameters based on which stratigraphy can be inferred ⇒ geophysical methods (i.e., electric or seismic) DIRECT ONSITE SURVEYS   Trenches ⇒ few meters depth (2-4m, max 8m), actual inspection of the ground along the chosen alignment Drillings: boreholes having diameter ranging from some cm to few dm Number, location and depth    The number depends on the extent of the study area as well as on its geological complexity. Anyhow, you always need at least 2 drillings in order to correlate data The location depends on the number and it must be chosen considering the critical situations Depth must be enough to cross the whole bearing volume of ground (affected by the project) FOUNDATIONS WALLS TRENCHES EMBANKMENT Drilling techniques Excavation Diameter tool Depth Kind of ground Percussion Cutting or hammering 150mm 60m Not Very cohesive disturbed soils and weak rocks Rotary Core bit 75÷150mm 50÷150m Any Quite good Auger 60÷300mm Any No sample Any Sample quality Percussion method By moving the rope or cable (2) up and down, the cutting or hammering (1) bit loosens the soil in the borehole, which is then extracted later by using a bailer Only for not cohesive soils. Speed: about 10m/d. Max diameter: 1.5 m (decreasing in depth). Max depth: 150m. A casing in steel or plastic may be used to prevent the hole from collapsing (5), during the ongoing drilling. Rotary method Any kind of ground. It involves a powered rotary cutting head on the end of a shaft, driven into the ground as it rotates. There are two types of rotary boring: open-hole drilling (auger): recovered material is mixed with the drilling lubricant and unsuitable for effective sampling. It is suitable for rapid drilling to enable core recovery at a greater depth or for the installation of monitoring wells. core drilling (core bit): it is carried out using simple or double-tube core barrels, with diamond or tungsten-tipped core bits, allowing the recovery of the whole drilled core. The system requires lubrication (air, water or drilling mud) to keep the cutting head cool. Speed: 10-15 m/h. Diameter: 75-300mm. Depth: over 8000m. d) auger, e,f) tipped core bits, g,h) diamond core bit. Example of core samples Results from drillings Aim Results To identify stratigraphy Identification and correlation of layers To assess the fracturing Quantification of the RQD (only for rotary degree core drillings in rock) Sampling Sample collection for lab tests In bore-hole tests Water research Landslide monitoring E.g., penetrometric tests Wells or piezometers for groundwater monitoring Inclinometric probes Description of geomaterials in the ground ⇒ stratigraphy. well It should contains:  name,  location (and date),  altitude,  depths and description of layers,  possible presence of water or gas  filters, if any. Gravel and sand Coarse gravel Sand and gravel Clay Cemented gravel Gravel and sand Cemented gravel Gravel and sand Example of geological cross-section drawn by correlation of stratigraphies RQD (Rock Quality Designation) Total lenght Poor or very poor Good or very good Moderate Rock quality Very poor Poor Moderate Good Very good Bore-hole tests and sample collection for lab tests ⇒ characterising physical and mechanical properties of geomaterials.   In situ tests ⇒ give results at the same scale of the studied problem, as they refer to geomaterials as they are in their natural undisturbed condition Lab tests ⇒ on small samples collected during drilling, often disturbed and out of their geological context INTACT CORE SAMPLE is a sample which has the same properties it has within the ground (i.e., density and water content). Example of core samples Semidisturbed sample  shaft shaft External tube External tube Inner barrel Cutting tip Sampling tube: in soils, for limiting cross-contamination and maintaining core integrity. It is a thin steel tube 60-200 cm long, having a cutting tip and an inner barrel for sampling. Piston Caratteristiche di un buon campionatore: • D < Di • thin tube • 8 < L/D < 12 Cp = De − D ≤ 15% D