Collapsible Soils Definitions Collapsible soils are also known as metastable soils. They are unsaturated soils that undergo a large volume change upon saturation. The sudden and usually large volume change could cause considerable structural damage. The volume change may or may not occur due to an additional load. Behavior under load: 1
ab: is determined from the consolidation test on a specimen at its in-situ moisture content. e1: is the equilibrium voids ratio before saturation. e2: is the equilibrium voids ratio after saturation. cd: is the curve that ensues from additional loading after saturation. 1) Aeolian soils Types of collapsible soils They are the most common examples of this type of collapsible soil. Typical aeolian soils are wind-deposited sands and or silts, such as loess, aeolic beaches, and volcanic dust deposits. They are characterized by showing in-situ high void ratios and low unit weights. 2
They are typically granular or only slightly cohesive. The Dust Bowl winds of the late 1920 s in the USA stripped large areas of Oklahoma and Texas of their top soils and carried them to Missouri, Kentucky and Tennessee. Other areas around the world that have large aeolian deposits are: South Africa, Rhodesia, Europe and China. 3
2) Residual soils Residual soils, which are a product of the in-situ weathering of local parent rocks may also have a potential for collapse. The weathering process produces soils with a large range of particle size distribution. The soluble and colloidal materials are leached out by weathering, resulting in large void ratios and thus unstable structures. Identification of the critical parameters. 4
The degree of volume change is expressed as the collapse potential Cp, where a sample is flooded during a 24-hour period under a pressure p w = 200 kn/m2 (30 psi). Values of Cp greater than 5% are indications of trouble for a foundation. This test provides void ratios before and after flooding and the collapse potential Cp can be calculated with equation: 5
where, eo = the natural (in-situ) voids ratio of the soil, and ε = the vertical strain Foundation Design The choice of a foundation system depends on whether the foundation will be exposed to wetting or not. Soils susceptible to wetting: If the expected depth of wetting is about 150 to 200 cm below the ground surface, then, The soil may be moistened and recompacted with heavy rollers, or Compacted with heavy tamping such as deep dynamic compaction, wherein a heavy weight repeatedly dropped upon the ground. 6
Alternatively, if conditions are favorable, the foundation trenches could be flooded with solutions of : sodium silicate and calcium chloride to stabilize the soil chemically. The soil will behave like a soft sandstone and resist collapse upon saturation. This method is successful only if the solutions can penetrate to the desired depth. Another method used for dry collapsible soils and for wet collapsible soils that are likely to compress under the added weight of the structure to be built is to: 1) Inject carbon dioxide into the treated stratum to remove any water present. 2) Inject sodium silicate grout. 7
When a soil stratum is susceptible to wetting to a depth of about 10.0 m, there are several techniques that may be used to cause the collapse of the soil before foundations are constructed: a) Vibroflotation: used successfully in free-draining soils (granular). b) Ponding, flooding an area by building low dikes around the site. However, some additional settlement of the soil may occur after the foundations are built. If precollapsing the soil is not practical, the foundations must be extended below the zone of possible wetting, which may require drilled shafts or other type of deep piles. However, their design must take into consideration the effect of negative skin friction resulting from the collapse of the soil structure around them, and the associated settlement of the critical soil zone when wetted. 8
Sensitivity (S t ) Sensitive Soils A measure of the change in strength of clays upon disturbance: For ordinary clays S t = 1 to 4, sensitive clays 4 to 8, quick clays 16-100. 9
The sensitivity ratio S t of most clays range from 1 to 8. However, highly flocculent marine clay deposits may show sensitivity ratios ranging from about 10 to 80! There are some clays that turn into viscous fluids upon remolding. These are found mostly in the previously glaciated areas of North America and Scandinavia. These clays are referred to a quick clays. Quick clays 10
Table 8: Undrained strength and sensitivity description for clays. Undrained strength classification for clays BS 8004:1986 Consistency Undrained strength, c u (kpa) Very stiff or hard Stiff Firm to stiff Firm Soft to firm Soft Very soft > 150 100 150 75 100 50-75 40 50 20 40 <20 Clay Sensitivity Description Non-sensitive Slightly sensitive Medium sensitive Very sensitive Slightly quick Medium quick Very quick Extra quick Sensitivity < 1 1-2 2 4 4 8 8 16 16 32 32 64 > 64 even 100 11