Indicadores microbianos de la calidad de agua

The density of a soil is simply its weight per volume, and is typically expressed in terms of pounds per cubic foot (kilograms per cubic meter). Density is a function of the relative amounts of soil, water, and air in the sample, and therefore to eliminate the variability introduced by a variable water content, it is usual to express the density in terms of the dry density, i.e., the oven-dry density for which no water content is present. Since the weight of air is negligible, the dry density γd is:

_V W_s

d =

γ (4.11)

where Ws is the weight of the soil solids, and V is the total volume of solids and the air in the

voids between the grains.

Compaction is the process by which soils are made more dense, by either reorienting the particles to achieve closer packing, or by bending or distortion of the particles. In either case, the net result is an expulsion of air, such that there is a greater proportion of solid particles occupying a given volume of soil. Achieving a more dense state requires energy, which is known as compactive effort.

A standard test method developed by soil scientist R. R. Proctor uses a calibrated compactive effort to determine the compaction characteristics of a given soil, as detailed in AASHTO test method T 99, "Moisture-Density Relations of Soils Using a 2.5-kg (5.5-lb) Rammer and 305-mm (12-in.) Drop." The test is conducted using a 4-inch diameter cylindrical mold, and a 2-inch rammer weighing 5.5 pounds. The rammer is dropped 25 times from a height of 12 inches above the soil layer, with 3 layers being successively compacted in this manner (Figure 4.14).

For any soil, there is a unique relationship between the dry density that can be achieved and the water content of the soil during compaction. This relationship can be graphically expressed by a moisture-density curve, as shown in Figure 4.15. Note that there is a specific water content for which compaction is maximized, known as the optimum water content. The optimum water content will be different for different compactive efforts, as shown in the figure. The optimum moisture represents a compromise whereby there is enough water to permit the grains to distort and reposition themselves, but not so much water that the voids are filled. Providing more compactive energy will result in a more dense soil. Many current highway projects use a higher compactive effort than test method T 99, which is called Modified effort (AASHTO test method T 180, "Moisture-Density Relations of Soils Using a 4.54-kg (10-lb) Rammer and a 457-mm (18-in.) Drop") and Figure 4.15 shows an example of how these two efforts typically compare.

For purely cohesionless soils, the most effective compactive effort is achieved through vibration, which reduces the friction between grains and allows repositioning. With all other soils, compaction is best achieved with a combination of static and kneading pressures that bend and reorient the grain structure. Theoretically, given enough compactive effort, the maximum dry density that can be achieved at any given water content is described by the zero air voids curve, as shown in Figure 4.15.

For purely cohesionless soils, relative density, rather than relative compaction, is used. Relative density is based on void ratio. The highest void ratio for a given soil is denoted the maximum void ratio emax, and the smallest void ratio, upon combined tamping and vibration

until no further densification is possible, is the minimum void ratio emin. The relative density

Dr of a soil is found by its actual void ratio compared to the maximum and minimum values:

_{(e e )} ) e e ( D min max max r − − = (4.12)

Figure 4.14. Standard laboratory compaction test (from the University of British Columbia web site).

Figure 4.15. Moisture-density curves for different compactive efforts. 90 100 110 120 130 140 150 160 170 0 5 10 15 20 25 30 35

Moisture Content Ww/Ws, percent

D ry D e n s it y , lb /f t 3 High effort Low effort

Optimum moisture content

where e is the actual void ratio of the soil, defined as the ratio of the volume of voids Vv to the volume of solids Vs: s v V V e= (4.13)

Qualitative descriptions of relative density are provided in Table 4.4. Table 4.4. Soil Density (Sowers and Sowers 1970). Term Relative Density, percent Field test

Loose 0 – 50 Easily penetrated by ½-inch rebar pushed by hand

Firm 50 - 70 Easily penetrated by ½-inch rebar driven with a 5-lb hammer

Dense 70 - 90 Penetrated 12 inches by ½-inch rebar driven with a 5-lb hammer

Very dense 90 - 100 Penetrated only a few inches by ½-inch rebar driven with a 5-lb hammer

A few things to remember with respect to density and compaction are as follows:

1. All soils exhibit a range of density. The range for coarse grained soils tends to be larger when soils have multiple grain sizes and smaller when soils have more uniform grain size.

2. Uniformly graded soils get about as dense as they are going to get with little effort and well graded soils take considerable effort to densify.

3. Relative density and relative compaction are not the same. A soil with a relative density of 90% is about as dense as can practically be achieved in the field. A soil with relative compaction of 90% (with respect to Standard or Modified effort) can be made much denser and is generally less dense than acceptable in transportation related construction. 4. Increasing the density improves strength and stiffness, and reduces hydraulic

conductivity. 4.4.4 Shear Strength

Shear strength is a term used in soil mechanics to describe the magnitude of the shear stress that a soil can sustain. The shear resistance of a saturated soil is a result of friction and interlocking of particles, and possibly cementation or bonding at particle contacts. Due to interlocking, particulate material may attempt to expand or contract in volume as it is subject to shear strains. If soil expands its volume, the density of particles will decrease and the strength will decrease; in this case, the peak strength would be followed by a reduction of shear stress. The stress-strain relationship levels off when the material stops expanding or contracting, and when interparticle bonds are broken. The theoretical state at which the shear stress and density remain constant while the shear strain increases is often referred to as the residual strength. However, soils with high clay content will continue to lose strength with even larger strains because the clay particles, which are platy in shape, become aligned with one another and form shear surfaces.

The shear strength of soil is always a function of the effective stress acting to confine the soil, not the total stress. Effective stress represents the intergranular forces between particles that contributes to the frictional strength and it is calculated by subtracting water pressure from the total confining stress.

The stress-strain relationship of soils, and therefore the shear strength, is affected by:

1. Soil composition (basic soil material): Mineralogy, grain size and grain size distribution, shape of particles, pore fluid type and content, ions on grain and in pore fluid.

2. State (initial): Defined by the initial void ratio, effective normal stress and shear stress (stress history). State can be described by terms such as: loose, dense, overconsolidated, normally consolidated, stiff, soft, contractive, dilative, etc.

3. Structure: Refers to the arrangement of particles within the soil mass; the manner the particles are packed or distributed. Features such as layers, joints, fissures, slickensides, voids, pockets, cementation, etc., are part of the structure. Structure of soils is described by terms such as: undisturbed, disturbed, remolded, compacted, cemented; flocculent, honey-combed, single-grained; flocculated, deflocculated; stratified, layered, laminated; isotropic and anisotropic.

4. Loading conditions: Effective stress path, i.e., drained vs. undrained; and type of loading, i.e., magnitude, rate (static, dynamic), and time history (monotonic, cyclic).

If water is not allowed to flow in or out of the soil, the stress path is called an undrained

stress path. During undrained shear, if the particles are surrounded by a nearly

incompressible fluid such as water, then the density of the particles cannot change without drainage, but the water pressure and effective stress will change. On the other hand, if the fluids are allowed to freely drain out of the pores, then the pore pressures will remain constant and the stress path is called a drained stress path. The soil is free to dilate or contract during shear if the soil is drained. In reality, soil is partially drained, somewhere between the perfectly undrained and drained idealized conditions. Exactly where is a function of the rate of load application and the hydraulic conductivity of the soil. For example, sandy and gravelly soils require the rapid loading of an earthquake to behave as an undrained material, whereas clayey soils can behave as undrained material at common excavation or embankment construction rates.

Probably the two most common tests for determining the shear strength of soils are (1) the direct shear test (AASHTO standard test T 236), and (2) the triaxial shear test (AASHTO standard tests T 296 and T 297). In the direct shear test, a sample of soil is placed in a rectangular box, the top half of which is free to slide over the bottom half. The lid of the box is free to move vertically, and a normal stress σn is applied to the lid. A horizontal shearing

stress τ is applied to the top half of the box, gradually increasing in strength until the soil begins to shear.

In the triaxial shear test, a cylindrical soil sample is encased in a rubber membrane with rigid caps on top and bottom. The sample is then placed in a closed chamber and subjected to a confining pressure σ3 on all sides using air or water as the confining medium. An axial stress

σ1 is applied to the ends of the cylinder. The axial stress is either increased, or the confining

stress decreased, until the sample fails in shear, which happens along a diagonal plane or number of planes. A special case of the triaxial test is when the confining stress σ3 is zero,

4.5 provides a qualitative description of soils as defined by the unconfined compressive strength.

Unconfined compressive strength is a special case and is primarily useful for classifying soils. More generally, shear strength is a function of confining stress and the direct shear and triaxial tests, and other tests, are used to develop shear strength parameters of c and Φ (with respect to total stress) or c' and Φ' (with respect to effective stress). These parameters represent the coefficients of a straight line plotted through through the results of similar tests on the same soil, with the only variable being changes in confining stress. The tangent of Φ (or Φ') is the coefficient of friction and represents the frictional component of soil strength and c (or c') is the value of the intercept of the line, representing strength with no confining pressure (no friction). Under certain conditions soils with significant fines content (especially clay) exhibit a significant c intercept and this is the source of their label as 'cohesive' soils.

Table 4.5. Soil Strength (Sowers and Sowers 1970). Term Unconfined compressive strength

(kips per square foot) Field test

Very soft 0 – 0.5 Squeezes between fingers

when fist is closed

Soft 0.5 – 1.0 Easily molded by fingers

Firm 1.0 – 2.0 Molded by strong pressure of

fingers

Stiff 2.0 – 3.0 Dented by strong pressure of

fingers

Very stiff 3.0 – 4.0 Dented only slightly by finger

pressure

Hard 4.0 or greater Dented only slightly by pencil

point

A few important things to remember with respect to soil strength are as follows: 1. Soils do not have intrinsic strength properties.

2. Soil strength is proportional to the effective stress and it is possible to develop strength parameters c or c' and Φ or Φ' relative to a range of effective stress and other variables as listed in bullets 1 through 4 above.

3. Some strength parameters such as unconfined compressive strength and the parameters c and Φ do not refer to the effective stress. This only because assumptions have been made with respect to effective stress and this observation is useful to emphasize that strength parameters should only be used for engineering problems for which they are applicable, or as indices for empirical approaches, such as classification.

4. When a soil is partially saturated the water pressure in the pore spaces is actually negative and this causes an increase in effective stress. This is difficult to measure and is not often measured. Instead, the increased strength caused by the increase in effective stress is recognized as an apparent cohesion and it can be quite significant in fine grained soils. Partially saturated soil strength is higher than saturated soil strength and is not often relied upon in practice because of the likelihood that saturation will occur at some point and that stability needs to be ensured at that time.