SWELL AND SWELLING POTENTAL PRESSURE TESTS

Transcription

1 SWELL AND SWELLING POTENTAL PRESSURE TESTS

2 When geotechnical engineers refer to expansive soils, we usually are thinking about clays, and the volume changes that occur as a result of changes in moisture content. Clays are fundamentally very different from gravels, sands, and silts. All the later consist of relatively inert bulky particles and their engineering properties depend primarily on the size, shape, and texture of these particles. In contrast, clays are made up of very small particles that are usually plate-shaped. The engineering properties of clays are strongly influenced by the very small size and large surface area of these particles and their inherent electrical charges.

3 What cause a clay to Expand? Several different clay minerals occur in nature, the differences being defined by their chemical makeup and structural configuration. Three of the most common clay minerals are Kaolinite Illite Montmorillonite The different chemical compositions and crystalline structures of these minerals give each a different susceptibility to swelling. Swelling occurs when water infiltrates between and within the clay particles, causing them to separate.

4 Kaolinite Mineral Its basic structural unit consists of aluminum sheet (gibbsite) (G) combined with a silica sheet (S).Tips of the silica sheet and one base of the aluminum sheet form a common interface. The total thickness of the structural unit is about 7 Angstrom (A o ), where one Angstrom is equal to m or 10-7 mm. The kaolinite mineral is formed by stacking, one over the other, several such basic structural units. The structural units join together by hydrogen bond, which develops between the oxygen of silica sheet and hydroxyls of alumina sheet. As the bond is fairly strong, the mineral is stable. Moreover, water cannot easily enter between the structural units and cause expansion. 7A o 7A o G S G S HODROGEN BOND

5 Montmorillonite Mineral The basic structural unit consists of an aluminum sheet sandwiched between two silica sheets. Successive structural units are stacked one over another, like leaves of a book. The thickness of each structural unit is about 10A o. The two successive structural units are joined together by a link between oxygen ions of the two silica sheets. The negatively charged surfaces of the silica sheet attract water in the space between two structural units. This results in an expansion of the mineral 10A o 10A o S G S S G S WATER MOLECULES

6 Illite Mineral The basic structural unit is similar to that of the mineral montmorillonite. However, the link between different structural unit is through non-exchangeable potassium (K + ) and not through water. This bonds the units more firmly than in montmorillonite. Illite swells less than montmorillonite. However, swelling is more than in kaolinite. 10A o 10A o S G S K K K S G S POTASSIUM IONS

7 Identification of expansive soils When working in an area where expansive soil can cause problems, geotechnical engineers must have a systematic method of identifying, testing, and evaluating the swelling potential of troublesome soil. An experienced geotechnical engineer is usually able to visually identify potentially expansive soils. To be expansive, a soil must have a significant clay content, probably falling within the unified symbols CL or CH. A dry expansive soil will often have fissures, slickensides, or shattering, all of which are sign of previous swelling and shrinkage. When dry, these soils usually have cracks at the ground surface. However, any such visual classification is only a first step ; we must obtain more information before we can develop specific design recommendations.

8 X-ray diffraction, Differential thermal analysis and Electronmicroscope resolution These methods are usually employed for mineralogical examination. Indirect Methods In these methods, the swelling potential of expansive soils is estimated based on the Atterberg limits, sometimes in conjunction with grain size analysis. Plasticity chart Developed by Casagrande. Employing this plasticity chart, many authors found that the A-line in the chart generally defines the expansive soils, since the great majority of plots, representing soils known to be expansive, fall above the A-line.

9 Plasticity chart Plasticity Index inorganic clays of low plasticity inorganic silts of low compressibility inorganic clays of medium plasticity inorganicclays of high plasticity inorganic silts of high compressibility and organic clays inorganic silts of medium compressibility and organic silts Liquid limit

10 Activity of soils On the basis of laboratory tests, Skempton observed that for a given soil the plasticity is directly proportional to the percent of clay size fraction. He introduced the concept of activity, by relating the plasticity to the quantity of clay-size particles, and define the activity as the ratio of plasticity index to the percent by weight of soil particles of diameter smaller than two microns present in the soil. Degree of Activity Inactive clay Activity A<0.75 Normal clay A = 0.75 to 1.25 Active clay A>1.25

11 Activity chart, A= Ip/clay fraction plasticity index ACTIVE NORMAL INACTIVE Clay fraction

12 Free swell of clay particles This test tries to give a fair approximation of the degree of expansiveness of a given soil sample. Test Procedure Pouring very slowly 10cc of dry soil passing No. 40 sieve into 100cc of distilled water Letting the content stand for about 24 hrs until all the sample completely settles on the bottom of the cylinder. Record the final volume of the sample. Free Swell Final Volume Initial Volume = x100 Initial Volume

13 Free swell< 50%,Not Expansive Free swell between %, Marginal Free swell > 100,Expansive Relation between swelling potential and plasticity index of clays. 1. According to Chen Swelling potential Plasticity index Low 0-15 Medium High Very high >55 2. According to Anderson et al S=0.23I p -3.12, where S= swelling potential I p = plasticity index

14 Degree of expansion Plasticity index Swelling potential Low Medium High Very high Correlation with common soil tests (Adapted from Holtz and Gibbs, 1969) Percent colloids Plasticity index Shrinkage limit Liquid limit <15 <18 <15 <39 Low Swelling potential Medium High >28 >35 >11 >63 Very high

15 Swelling Potential and Swelling Pressure The term swelling potential is generally used to indicate the amount of vertical swell (expressed as percent of initial sample thickness ) obtained under a particular surcharge (usually 1psi). δh S p = x100 h Where Sp = swelling potential in percent δh = amount of vertical swell h = initial height The swelling pressure is defined as the vertical pressure required to prevent volume change of laterally confined sample when it is allowed to take in water.

16 The magnitude of swelling and/or swelling pressure is governed by the following factors Amount and type of clay in the soil Placement conditions which involve initial water content, initial density and confining pressure Time allowed for swelling In general For a given dry density percent expansion decreases with increase in moisture content For a given initial moisture content the swelling pressure increases with increase in dry density Percent expansion also increases with increase in dry density for a given initial moisture content

17 Percent expansion decreases with increase in confining pressure for a given initial moisture content and density For expansion time is necessary. The same thing is true for swelling pressure. Laboratory Testing Methods for Determining Swelling Potential Swell Consolidation Method (Free swell method) Place an undisturbed soil sample in a consolidometer and apply a normal load equal to 1psi (7kN/m 2 ) Inundate the sample and allow it to swell. After the swelling is complete, load the sample in increments until the soil returns to its original volume. The pressure that corresponds to the original volume is the swelling pressure.

18 Loading Plate P Dial gauge for measuring vertical deformations Water Porous stone Metal Ring Soil Sample Porous stone Consolidometer

19 e Wetting Setting load Log P Ps ( swelling pressure)

20 Constant volume swell test Place an undisturbed soil sample in a consolidometer and apply a normal load equal to 1psi (7kN/m 2 ) or in-situ overburden pressure Inundate the sample and begun increasing the normal load in increments as necessary to restrain any swelling. Continue until the swelling pressure is fully developed. Unload the soil in increments to obtain the swell curve.

21 e Setting load Log P Ps ( swelling pressure)