Residual soil formed by weathering of bedrock with no lateral movement, glacial moraine/outwash moved laterally, and soil/air containing elements that may have percolated upward from deep in vapour form.
Residual soils typically retain the chemical memory of the parent rock’s composition. Anomaly will always exist in residual soil formed from mineralized bedrock.
In this article you’ll learn:
- Intro of residual soil.
- Types of residual soil.
- Merits and Demerits of residual soil.
- Lots more.
So, if you’re ready to go with it, this article is for you.
Let’s dive right in.
Introduction to Residual Soil:
Although residual soils are as abundant as silt soils on the earth’s surface, their occurrence and characteristics are rarely emphasized in soil mechanics.
As a result, many of the theories established from the sedimentary properties of soils are generally applied to residual soils, resulting in a misunderstanding of their behaviour.
Temperate climate— The residual soil anomaly appears as a thin layer of loose material atop the bedrock.
In tropical environments (as in Africa and South America), it can be 20-40 m thick, entirely degraded, and the identification of the parent rock is unknown.
Formation of Residual Soil:
Residual soils are formed directly from their bedrock by physical and chemical weathering.
Sedimentary soils are subjected to additional processes like movement by streams and rivers, sedimentation in ponds or the sea, and consolidation.
Sedimentary soils undergo sorting during erosion and re-deposition, which gives them a degree of homogeneity not found in residual soils.
Since residual soils do not consolidate, their qualities cannot be linked to stress past.
The phrases typically and over-consolidated are meaningless in the context of residual soils. Cc and Cs are not strictly relevant to residual soils.
The variable Cc is described as the (log) slope of the virgin consolidation line.
There is no such object as a virgin consolidation line for a residual soil, as evidenced by their creation process.
Some residual soils, particularly those produced from volcanic parent material, include clay minerals that are not present in sedimentary soils.
Residual soils have substantially higher permeability than sedimentary soils, which has crucial consequences for oedometer testing and predictions of short and long-term stability of cut slopes.
What are the Benefits of using Residual Soil in Construction?
Residual soils are ones that have not been considerably moved, and their upper surfaces often show the most weathering.
With increasing depth, the soils become less worn, stronger, and lack broad differences in strength/consistency.
Footings are less likely to need changing (size depth, permitted pressures, etc.) on-site, reducing delays and costs (no re-doing/additional reinforcement, no extra concrete charges, inspections, etc.).
Because machinery operates best on the uniform ground, productivity is normally better in residual soils that are relatively consistent.
Instability is less likely because of the necessity for shoring/formwork/protecting excavations, among other things.
Residual soil is derived from a parent rock that might be geographically extensive and weather to soils with comparable properties over a vast area.
Local earthworks contractors are more likely to be knowledgeable about the soil and how to get the best end result possible during earthworks.
Organics and oversized materials are often not difficult to remove.
Residual soils have lengthier sedimentological histories than surface soils. Two clays of equal strength, but one superficial and one residual, will settle in distinct ways.
Aside from making design easier and more cost-effective, the smaller the footing, the less work necessary on-site with footing preparation and steel work—less to do, fewer to go incorrect.
In general, residual soil forms include recognized chemical concentrations.
Site testing/validation testing is rarely necessary, as opposed to alluvial soil, which may include sulphates and must be treated prior to use/removal.
Advantages of Residual Soil:
- It is best suited for places where new outcrops are few and all surface materials appear to be the same.
- Utilized efficiently in initial “blue sky” reconnaissance of huge areas when surface drainage pathways are little established (central and west Africa).
- Utilized widely in all geographic and climatic circumstances for follow-up or validating anomalies discovered by geochemical sediment or geophysical survey
- In conjunction with other technologies, it is effective in examining the near neighbourhood of known deposits.
Disadvantages of Residual Soil:
- Not suitable in mountainous terrains where outcrops abound and conventional prospecting methods are less expensive and more effective.
Also read: Soil Classification | Soil Nailing | Soil Liquefaction
Despite the fact that residual soils cover over half of the world’s surface.
Residual soils predominate in the former counties, although degree courses in the latter exclusively address sedimentary soil behavior.