Cohesive (Clay) Soils
Earlier, we made mention of clay soils, but at that time we made no effort to say much about it, so now lets cover some important aspects about this topic, and see what we can glean or derive from these interesting facts.
Like granular soils; clayey soils (Cohesive) are also, from combinations of various soil particles that range in size and shape from fine-grained to very-fine-grained, some of these clay particles cannot even be seen under a low-powered microscope.Cohesive soils are held together by a binding force called cohesion.
Under the classification system devised by the American Society for Testing and Materials (ASTM) describe this second group, Cohesive soils as follows;
Cohesive: (Fine-grained soils 50% or more passes the 0.075 mm #200 sieve.)
The cohesive group symbols Silts and Clays Liquid Limit 50% or less are:
- "ML"; Inorganic silts, very fine sands, rock flour, silty or clayey fine sands
- "CL"; Inorganic clays of low to medium plasticity, gravely/sandy/silty/lean clays
- "OL"; Organic clays of medium to high plasticity
The cohesive group symbols Silts and Clays Liquid Limit greater than 50%:
- "MH";Inorganic silts, micaceous or diatomaceous fine sands or silts, elastic silts
- "CH"; Inorganic clays or high plasticity, fat clays
- "OH"; Organic clays of medium to high plasticity
- Peat: Highly Organic Soils; "PT"; Peat, muck, and other highly organic soils
I felt it necessary to go the extra mile, and also decided that we had to
cover clay soil at some length to get some since of the need to understand why they are termed " Problem Soils or Expansive Soils",and why stress is placed so heavily concerning this topic. We have to better understand the natural tendencies of clay soil due to the fact that they directly affect stability. This stability, with respect to foundation support, is of utmost importance and what precautionary steps we should take to alleviate or counter the reactions that are associated with this soil type.
Clayey soil, termed problem soils, are said to be not suitable for
engineering needs. What's the reason for all the controversy? The reasons are, these soils are the main culprit to so much foundation trouble, seeing that the stability of these clays is directly affected by natural occurrence (climate). They become plastic and elongate under stress, impervious, poor drainage, high susceptibility to frost action, and low bearing capacity. And the major problem with clay soils, is a characteristic behavior that is common to clay soils and that is; these clays contain mineral constituents that as variations in natural ambient
conditions (heat, rain, wind, and conditions that result from water leaks in the sub-soils) cause these minerals to swell with higher moisture content and contract in areas of widely variant moisture, and this problem is compounded by an interaction of these climatic changes.
This swelling is directly attributed to a mineral,montmorillonite or
smectite present in the soil and changes or swells with moisture content. Montmorillonite when taken from a dehydrated state to a saturated state has the capacity to free swell up to 20 fold. So a confined clay for example under a foundation or concrete pad; these clays could conceiveably exert pressures up to several tons per square foot, and that; my friend, is
the reason that foundation repairs exceed or equal to the cost of damages due to natural disasters. The last I read was 2 billion.
Let me know what you think.
Have a good day!!
