Ground Profiles Flashcards
What are UCS tests used to measure and what must an engineer do with the data
They are used to measure rock strength at specific locations and depths. It is then up to the engineer to use this data to create a meaningful profile of the ground as a whole
What does the strength of a region depend on
strength of intact rock (so this depends on the type of rock, igneous, sedimentary, metamorphic), weathering of the rock, joint properties (joints are fractures in the rock due to movement of the rock causes tension) such as orientation, spacing, width, continuity and also water saturation
Using a roof analogy, what is the dip and strike
The strike would be the ridges of the roof and the dip would be the direction the rain flows in after hitting the roof. The strike is the horizontal line across the plane, (the dip direction is the line perpendicular to the strike) and the dip angle is the measured angle of inclination downward from the horizontal
What are unconformities
Give brief explanation of each type
A boundary between rocks that is caused by either a period of erosion or a pause in sediment accumulation
Disconformity - a pause in sediment accumulation and subsequent weathering causes the boundary between rocks to become jagged and uneven
Nonconformity - same as disconformity but the rock boundary also marks a change in rock types from igneous and metamorphic to sedimentary
Paraconformity-there was a pause in sediment accumulation but no/ little subsequent erosion which means it is almost impossible to tell there was unconformity unless compared with other regions or there is other context
Angular conformity - During the pause in sediment accumulation, shifts in the rocks such as folds causes the angular orientation of the layers to change. Then maybe after subsequent erosion more sediment accumulates, resulting in a layering angle change between the boundary.
What does it mean when a fault line is not straight on a geological map
This means that the fault is not perfectly vertical in the cross section and is instead slightly angled. However it will still be straight in the cross section.
What does it mean when the boundaries of rocks are:
vshaped but blunter than contours
vshaped but follow contours
vshaped but sharper than contours
vshaped but opposite to contours
vshaped but blunter than contours: bends gently upstream
vshaped but follow contours: horizontal beds
vshaped but sharper than contours: bends gently downstream
vshaped but opposite to contours: bends steeply downstream
Upstream means as the depth increases, the rock boundary moves towards the higher topology and downstream vice versa
When can an apparent dip appear in a cross section
When the strike is not perpendicular to the cut of the cross section, an apparent dip will arise. The true dip is the actual dip of the formation while the apparent dip is the one that is seen in the cut cross section
What do geological maps provide and not provide information about
They give information about the cross section of the site and can give clues about any geological formations. However they do not show information specific to a geographic location such as strength of the ground.
What is the point of ground investigation
To gather information about the suitability of the site for a project, potential geological issues and ground properties
What are the three characteristics in the description of rocks in a borehole log
Material Characteristics (free from discontinuity)
-strength, bedding/layering, colour
Discontinuity characteristics (bedding, jointing and shear)
-orientation, spacing, roughness
Mass characteristics (rock material + rock discontinuity)
-overall structure, fracture state
What is RQD
Rock quality designation. The percentage of the rock that has a joint spacing of 100mm or more (the intact rock lasts at least 100mm). Higher RQD is often an indicator of stronger rock. Mechanical joints, those that are made by humans are not included in this calculation.
