RESIDUAL SOIL

Residual soils form from weathered rocks and can be found in many parts of the world with different characteristics and appearance based on the type of parent rock they have formed of. This large range of difference in residual soils leads to variety of studies that focus on different aspects of this type of soil and therefore in the

following sections summary of these studies will be presented based on the area they are focusing on. This paper attempts to summarize the theoretical basis of residual soil formation and its application in engineering projects.

 INTRODUCTION

Residual soils are those that remain at the place of their formation as a result of the weathering of the parent rocks. They are products of the in-situ physical and chemical weathering of bedrock and are commonly situated above the groundwater table. They are often saturated in nature and possess negative pore-water pressures or matric solutions relative to the atmospheric conditions that contribute to the shear strength of soils.

Residual soils are formed directly from the physical and chemical weathering of the parent material, normally rock of some sort. The diagram above shows diagrammatically the physical processes that lead to the formation of sedimentary and residual soils.

The depth of residual soils depends primarily on climatic conditions and the time of exposure. In some areas, this depth might be considerable. In temperate zones, residual soils are commonly stiff and stable. An important characteristic of residual soil is that the sizes of grains are indefinite. For example, when a residual sample is sieved, the amount passing any given sieve size depends greatly on the time and energy expended in shaking, because of the greatly disintegrated condition. They tend to be characterized by angular to subangular particles, mineralogy similar to parent rock, and the presence of large angular the nature of the products of weathering are climate (temperature and rainfall), time and type of parent rock, vegetation, drainage and bacterial activity.

The diagram above is an attempt to summarise the factors involved in the formation processes that influence the properties of the two soil types. Sedimentary soils are seen to undergo a various additional processes beyond the initial physical and chemical weathering of the parent rock. It might appear from this diagram that the factors involved in the formation of sedimentary soils are more complex than those involved in forming residual soils. There is some truth in this, but in practice two important factors lead to a degree of homogeneity and predictability with sedimentary soils that is absent from residual soils. These factors are:

• The sorting process which take place during erosion, transportation and deposition of sedimentary soils tend to produce homogeneous deposits.
• Stress history is a prominent factor in determining the behavioural characteristics of sedimentary soils, and leads to the convenient division of these soils into normally and over consolidated materials.

The absence of these factors with residual soils means that they are generally more complex and less capable of being divided into tidy categories or groups. It is perhaps helpful to consider that the behaviour of a soil, whether residual or sedimentary, is dependent on two factors, or two groups of factors. These are: firstly the nature of the soil particles themselves (i.e. their size, shape, and mineralogical composition) and secondly, the particular state in which these particles exist in the ground. For convenience, these factors can be referred to respectively as composition and structure. With sedimentary clays, the influence of composition is well known — kaolinite group clays are relatively "inert" with consequent low shrinkage/swell characteristics and relatively low compressibility, while montmorillinite clays are highly active and of opposite characteristics to the kaolinite group. Notwithstanding the influence of mineralogy, by far the most important "attribute" of sedimentary clays in their undisturbed state (at least according to conventional soil mechanics) is their stress history i.e. whether they are normally consolidated or over-consolidated. This is generally given greater importance in the literature than either mineralogy or structure.

The Characteristics of Residual Soils are:

1.  Empirical correlations/relationships valid for sedimentary soils may not apply;

2.  They have no stress history influence;

3.  They have unusual Clay minerals (composition);

4.  Often strong structural influence (macro and/or micro);

5.  Often relatively high permeability and high c_v values;

6.  Often strongly influenced by heterogeneity;

APPLICATION OF RESIDUAL SOIL IN ENGINEERING PROJECTS

A foundation is an integral part of a structure and its stability depends on the stability of the supporting soil. The foundation must be stable against shear failure of the supporting soil and must not settle beyond a tolerable limit to avoid damage to the structure. All engineering structures are expectedly founded on residual soils and all construction works involve the use of residual soils but not all residual soils however can be assumed to be suitable for construction purposes. Foundation studies usually provide subsurface information that normally assists civil engineers in the design of foundation of civil engineering structures.

 CONCLUSION

A review of residual soils and their modes of formation have been discussed in this paper. They are formed directly from the physical and chemical weathering of the parent material.  Residual soils are key materials for construction as all engineering structures are founded upon them. Not all residual soils can be assumed to be suitable for construction purposes hence we should avoid preconceived ideas about how soils should behave. Also, the behaviour of residual soils on site  should be checked before looking at test results. Residual soils should not be mistaken for transported soil or sedimentary soils as empirical studies derived from the study of sedimentary soils may not be valid and there usually are difficulties during construction with the use of transported soils.

 REFERENCES

1.  Townsend, F., Geotechnical Characteristics of Residual Soils. Journal of Geotechnical Engineering, 1985. 111(1): p. 77-94.

2.  Chin, I.T.Y. and I.D.G.S. Sew, THE DETERMINATION OF SHEAR STRENGTH IN RESIDUAL SOILS FOR SLOPE STABILITY ANALYSIS, in Seminar Cerun  ebangsaan2001: Cameron Highlands, Malaysia.

3.  Wesley, L.D., Geotechnical Engineering in Residual Soils. 2012: Wiley.

4.  Loganathan, N., S. de Silva, and A. Thurairajah, Strength Correlation Factor for Residual Soils. Journal of Geotechnical Engineering, 1992. 118(4): p. 593-610.

5.  Coutinho, R.Q., J.B.S. Neto, and F.Q. Costa, Design Strength Parameters of a Slope on Unsaturated Gneissic Residual Soil, in Advances in Unsaturated Geotechnics. 2000. p. 247-261.

6.  Azevedo, R., A. Parreira, and J. Zornberg, Numerical Analysis of a Tunnel in Residual Soils. Journal of Geotechnical and Geoenvironmental Engineering, 2002. 128(3): p. 227-236.