Week 12: Flashcards
(25 cards)
What is the most expensive problem soil? Why?
Expansive clays. Whole buildings break down.
Most disruptive rock? Only one to kill people.
Dolomite.
How does the expansive clay work?
Volume change of the soil due to addition or subtraction of water to the clay.
How do you diagnose expansive soils?
Shattered and slickensides structure. Surface ground cracks. Clay mineralogy and montmorillonite (affinity of water to adhere to the plate surfaces).
Worse two expansive clays?
Non-kaolin and illite.
What does PI, the lab test stand for?
Plasticity Index. Van der Merwe’s chart. An indicator.
What does shattered mean? What does slickensided mean?
Shattered - Break it up into blocks even to sand sized.
Slickensided - Shiny, smooth (polished even) surface.
How does these diagnostic features happen? For expansive clays.
As it dries, cracks form in the ground. Filled with sand/soil and other particles. During wetter season moisture content increases. Clay wants to absorb it and has nowhere to go so shears upwards. This process is repeated over every summer and winter.
Clay turns itself over so there is no pebble marker.
In what groups are expansive clays found?
Residual souls above basic igneous rocks.
Finegrained metamorphic rocks and sedimentary rocks.
Poorly drained areas.
Transported soils from these areas.
Associated stratigraphic units with expansive soils?
Humid, where N>5.
Bushveld Complex - Norites and gabbros. (Black clay)
Andesite units (like in Southern Joburg).
Karoo Supergroup: Mud rocks, siltstones and some sandstones.
Basalt.
What happens when a house is placed on these expansive soils?
Moisture will migrate to cooler soil. (Under a building where it is shaded).
Soil heaves in the middle of the structure.
Constant heaving over the seasons.
Solutions for expansive soils?
Remove and replace with a gravel mattress.
Reinforced concrete rafts, that floats on the soil. (For the floor).
Transfer loads to lower competent horizons (pilling). But isolate the piles from the clay. Otherwise the clay will grab the pile.
Keep the moisture content stable (used especially in roads, widen shoulder).
Keep hydrophilic (water loving) trees away. (Especially bluegum trees).
Solutions for existing structures?
Stabilise the moisture content. (Build aprons and geo-membranes/vertical impermeable barriers)
Stop water getting under the foundations.
Remove trees.
What is a rare problem that is also very costly?
Dispersive clays. They are incredibly difficult to identify. Normally in dams.
How do dispersive clays happen?
Only in clays. More than 12% clay.
Cations are absorbed onto clay surfaces. (Like charges repel.)
The clay deflocculates and is carried out in the water flow.
Piping failure then occurs (because it happens in a pipe).
What are some diagnostic features of dispersive clays?
Milky water flowing after rain.
Piping/erosion failures present in nature.
Lab tests: Crumb test (good test), double hydrometer test (with a dispersant or not). Also more unreliable - pinhole test and chemical tests.
What kind of rock is dolomite?
Sedimentary rock. Incredibly hard. It precipitates in shallow seas. It’s basic/alkaline. Quartz is held in solution then when fresh water goes in the quartz precipitates. Dolomite is a carbonate. (So soluble in any acid)
Explain how dolomite is weathered. The process.
Dolomite (hard) + Carbonic acid = Calcium and Magnesium bicarbonate (soluble and can be carried away).
Very slow process. (Because rainwater is very weakly acidic).
It develops caves over a long period of time, giving an opportunity for a sinkhole to develop.
Formation of cave systems?
Blowhole - entrance to a cave system.
Water percolates down a fissure then into the water table.
Cast terrain developed. A lot of South Africa’s cast is buried below younger rock.
Water table is the base level of erosion.
Process of mine formation.
When water table above mine system water loses its kinetic energy above mine and doesn’t carry soil.
When water table is below the mine system the water doesn’t lose its kinetic energy and carries soil into mine.
It formes arches/domes above.
When it is wet is can collapse forming sinkholes.
Soil called WAD.
The bulk of solid rock is carried away in solution. The remainder consists of fine clay sized particles of quartz coated with Mg or Fe oxides forming a dark brown/black soil.
Lowest density soil in SA. Fairly strong but incredibly compressible.
Def of sinkhole?
Def of subsidence (doline)?
Steep sided hole that develops rapidly, deep in the ground.
Shallow wide depression which may develop slowly.
Dolomite is regulated.
Need to do a dolomite risk assessment. (SANS 1936) The outcome of the investigation can limit the new development.
How is dolomitic land investigated?
Desk study. Gravity survey measures density contrast. Drilling, normally percussion. Create hazard maps.
