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Mine Workings - These can cause subsidence at the surface when the collapse of the workings will transfer through the rock to the surface. As a rule of thumb this is when the distance between the top of the mine working and the level of the rock nearest the surface (rock head) is less than 10—15 times the depth of the workings. Mine workings are normally at least the same thickness as the seam in which they are worked.
Mine Shaft - The route from the surface to the mine workings is the mine shaft. Shafts can suffer deterioration and partial collapse after they have been built on. Shafts should be assumed to be an extreme hazard, in particular on sites intended for construction, unless proved otherwise. The Coal Authority or its agents have jurisdiction over most shafts and do not generally permit new residential development on them or nearby. Shafts may be unrecorded and the positions of shafts on geological maps are not always accurate.
Solution Features - These are holes/weak points formed by water running across and dissolving the rock surface at a defect such as a fissure. These rocks contain calcium carbonate (calcareous rock, e.g. chalk or limestone). Solution features can be long thin ‘pipes’ flaring out at the surface, often known as swallow or swallet holes.
After formation of a solution feature, a further sedimentary deposit often fills in the solution feature with a different material, commonly sand. Whilst sand can be an acceptable material to build on, when it appears in solution features it can move if it is subjected to groundwater movement. This may occur if a soakaway is positioned on or adjacent to a solution feature.
A geological map is unlikely to show solution features on a site, but it can show calcareous rock.
Landslip - Active landslip areas are shown on the key of geological maps as ‘landslip’ or, if they are known to have slipped in the past, as ‘foundered strata’. Construction on foundered strata can be carried out in the right circumstances, but instability can result if substantial reprofiling of the area of foundered strata is undertaken. Not all areas of foundered strata are mapped on all geological maps.
Clay Shrinkage and Heave - Probably one of the most common potential problems that give rise to foundation defects is clay. Geological maps will indicate that particular strata are ‘mainly clay’. The ‘high shrinkage’ potential category has a significant risk and, unless known, high-volume change should be assumed, as defined in the National House Building Council Standards 2014 Part 4.
Soft/Weak Soils - Soft ground such as alluvium can often be shown on geological maps. Alluvium is usually a drift deposit and can be soft. When overlain with a crust of firm or even stiff clay, careful examination of the map and the slope of strata is important to understand the potential risk of subsidence.
What is a "Disturbed Sample"
A disturbed soil sample is one which has the same moisture content and particle size distribution as the in situ soil, but one in which the in situ structure has been disrupted or completely destroyed.
An auger or pile sample is this
What is an undisturbed sample?
An undisturbed soil sample is one in which the in situ structure of the soil has experienced the least possible disruption. If this is the case then, needless to say, the particle size distribution and moisture content will be the same as the in situ soil.
For example a block cut carefully from the side of a pit of a tube sample
This hand auger will produce a disturbed soil sample.
This tube sample will produce an undisturbed sample
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