Frequently asked Exam Questions with Answers on Soil Mechanics [Geotechnical Engineering]
Ans. Soil is a complex body composed of five major components:
a. Mineral matter obtained by the disintegration and decomposition of rocks;
b. Organic matter, obtained by the decay of plant residues, animal remains and microbial tissues;
c. Water, obtained from the atmosphere and the reactions in soil (chemical, physical and microbial);
d. Air or gases, from atmosphere, reactions of roots, microbes and chemicals in the soil, and
e. Organisms, both big (worms, insects) and small (microbes).
Q. 2. What are the main purpose of soil testing?
Ans. Soil testing program is an analysis of the soil physical (chemical or biological, if applicable) properties. Soil sample testing would also evaluate the soil nutrient-supplying capacity at the time of sampling.
In general, there are 4 activities involved in soil testing for construction project:
i. The soil sampling, as part of soil investigation process.
ii. The soil sample testing which normally analyze at the soil testing laboratory.
iii. The outcome of the analysis results obtained through soil testing. Then, interpreting the results before taking it as the safe guidelines – based on the technical standard.
iv. Making recommendations for the geotechnical engineers, and also the construction staffs.
For every construction site, the designated location of soil is where the soil samples would be taken into the lab for testing. In order to determine the soil physical properties, the process of soil testing would be carried out in the laboratory.
The soil sample testing can be divided into 2 main categories:
i. The soil classification tests whereby indicating soil types in general form and finding the right engineering category to which it fits in.
ii. The engineering properties assessment of soil, for examples the compressibility, shear strength, and permeability.
Soil testing in the laboratory would produce all the required parameters, and recorded together with descriptive data relating to the soil. Therefore, the data are required by the soil engineers (geotechnical) for many construction purposes.
The more usual applications are as follows:
i. The soil strata could be identified through the data acquired from classification tests. The process known as site investigation would start exploring the subsurface conditions of the project site.
ii. Apart from that, the other test data would facilitate the soils engineering properties to be qualified in numerical terms. The properties of soils can then be used as the basis of analysis that normally based on the recommendations of the site investigation report.
iii. For ensuring the quality control assurance, the laboratory soil testing should be declared as part of the construction control measures. The design criteria should be met if comply with the lab results, especially during the construction of earthworks or excavations.
iv. The soil criteria of the acceptance that was used in civil construction can be drawn up in the light of available test results – this is possibly after the processing operation.
v. Whenever a new ground is being opened up for construction, the findings of the site investigation can be supplemented by further testing as construction work proceeds.
vi. The soil sample testing data can be used for the conformation of assumptions which normally based on the previous construction experience and engineering judgement.
Q. 3. What do you mean by infiltration?
Ans. Infiltration is the process of downward water entry into the soil. The values are usually sensitive to near surface conditions as well as to the antecedent water state. Hence, they are subject to significant change with soil use and management and time.
Three stages of infiltration may be recognized— preponded, transient ponded, and steady ponded. Preponded infiltration pertains to downward water entry into the soil under conditions that free water is absent on the land surface. The rate of water addition determines the rate of water entry. If rainfall intensity increases twofold, then the infiltration increases twofold. In this stage, surface- connected macropores are relatively ineffective in transporting water downward. No runoff occurs during this stage.
As water addition continues, the point may be reached where free water occurs on the ground surface. This condition is called ponding. The term in this context is less restrictive than its use in inundation. The free water may be restricted to depressions and be absent from the majority of the ground surface.
Once ponding has taken place, the control over the infiltration shifts from the rate of water addition to characteristics of the soil. Surface-connected non-matrix and subsurface- initiated cracks then become effective in transporting water downward.
Infiltration under conditions where free water is present on the ground surface is referred to as ponded infiltration. In the initial stages of ponded infiltration, the rate of water entry usually decreases appreciably with time because of the deeper wetting of the soil, which results in a reduced suction gradient, and the closing of cracks and other surface-connected macropores.
Transient ponded infiltration is the stage at which the ponded infiltration decreases markedly with time. After long continued wetting under ponded conditions, the rate of infiltration becomes steady. This stage is referred to as steady ponded infiltration. Surface-connected cracks would be closed, if reversible. The suction gradient would be small and the driving force reduced to near that of the gravitational gradient.
Assuming the absence of ice and of zones of free water within moderate depths and that surface or near surface features (crust, for example) do not control infiltration, the minimum saturated hydraulic conductivity within a depth of 1/2 to 1 meter should be a useful predictor of steady ponded infiltration rate.
Minimum Annual Steady Ponded Infiltration:
The steady ponded infiltration rate while the soil is in the wettest state that regularly occurs while not frozen is called the minimum annual steady ponded infiltration rate. The quantity is subject to reduction because of the presence of free water at shallow depths if this is a predictable feature of the soil.
Allowance for the effect of free water differentiates the quantity from minimum saturated hydraulic conductivity for the upper meter of the soil. The minimum annual steady ponded infiltration rate has application for prediction of runoff at the wettest times of the year when the runoff potential should be the highest.
Q. 4. How does Land Use Alter the Habitat of Soil Organisms?
Ans. Soil disturbance has major effects on soil habitat diversity. Disturbance changes the physical uniformity of soil and in natural ecosystems it changes the diversity of plant species growing in the soil. Disturbances are not the same and they can have different impacts in different soil types.
Two examples of the effects of disturbance in different soil environments are:
i. Severe soil disturbance due to cultivation tends to create a more homogeneous soil environment than does disturbance of a forest during a process such as logging.
ii. The higher diversity of plant species in a natural forest creates more habitats in soil than does a more uniform plant community (such as a crop grown in monoculture).
When soil conditions are altered in any way, the number of organism changes and the relative abundance of different types of organisms in the soil changes as well. A change in environment may favour one group of organisms so that its abundance increases, while the number of other organisms decreases either because the conditions are less favourable or because of the increased influence the organisms that have become dominant.
An example of this is the change in soil organisms following the addition of nitrogen and straw to soil. Adding nitrogen increased the length of hyphae, the number of bacteria and the number of amoebae. The thickness of the hyphae also increased when nitrogen was added. The change in thickness of hyphae is likely to have occurred because the increased nitrogen favours species of fungi with thicker hyphae.
Liming a soil by the addition of calcium carbonate can also change the number of bacteria and fungi present, although the response is not always predictable. In this case, the outcome depended on the soil characteristics and the types of organisms present.
In one soil, the addition of calcium carbonate increased the number of bacteria but decreased the abundance of fungi. In contrast, the addition of calcium carbonate to a second soil decreased the abundance of bacteria and had little effect on the abundance of fungi.
Q. 5. Do Organisms Live Deep in the Regolith?
Ans. Living organisms are not restricted to the surface layers of soil. Bacteria are the most common organisms at depth, and they even occur at more than 1 kilometre below the earth’s surface. These bacteria cannot rely on sources of plant organic matter to meet their need for carbon and energy.
One possible source of carbon for organisms deep in the regolith in some regions is oil. Microbial activity in oil wells, including their role in the corrosion of drilling rigs, has been well documented indicating that they are able to live and thrive using oil as an energy source.
Bacteria living at a great depth in the regolith have to be able to survive in environments with low levels of all nutrients. But they appear to be uniquely adapted: they are small (most are less than 1 µm in size), generally have low rates of respiration and can exist in a physiologically stressed state.
We know relatively little about the biology of organisms that occur deep in the soil. It is difficult to study deep soil organisms. This is because their surroundings are often greatly altered during the investigation from what they normally experience and few deep soil bacteria will grow on artificial media. For example, deep soil organisms may experience increased or even toxic levels of oxygen when they are brought into the laboratory.
To gain accurate information about the biology of organisms that occur deep down in the soil the experimental conditions need to resemble as closely as possible the conditions normally experienced by such organisms. If these requirements are not met, the experimental results may not represent what actually occurs at depth in the regolith.
This is important for studies of all soil organisms and explains why investigations of organisms in artificial media need to be scrutinised carefully before conclusions can be drawn about the function of the same organisms in field soil.
Ans. Concerns and Limitations:
Salts exert both general and specific effects on plants which directly influence crop yield. Additionally, salts affect certain soil physicochemical properties which, in turn, may affect the suitability of the soil as a medium for plant growth.
The development of appropriate criteria and standards for judging the suitability of saline water for irrigation and for selecting appropriate salinity control practices requires relevant knowledge of how salts affect soils and plants. This section presents a brief summary of the principal salinity effects that should be thoroughly understood in this regard.
The suitability of soils for cropping depends heavily on the readiness with which they conduct water and air (permeability) and on aggregate properties which control the friability of the seedbed (tilth). Poor permeability and tilth are often major problems in irrigated lands.
Contrary to saline soils, sodic soils may have greatly reduced permeability and poorer tilth. This comes about because of certain physico-chemical reactions associated, in large part, with the colloidal fraction of soils which are primarily manifested in the slaking of aggregates and in the swelling and dispersion of clay minerals.
To understand how the poor physical properties of sodic soils are developed, one must look to the binding mechanisms involving the negatively charged colloidal clays and organic matter of the soil and the associated envelope of electrostatically adsorbed cations around the colloids, and to the means by which exchangeable sodium, electrolyte concentration and pH affect this association.
The cations in the “envelope” are subject to two opposing processes:
i. They are attracted to the negatively-charged clay and organic matter surfaces by electrostatic forces, and
ii. They tend to diffuse away from these surfaces, where their concentration is higher, into the bulk of the solution, where their concentration is generally lower.
The two opposing processes result in an approximately exponential decrease in cation concentration with distance from the clay surfaces into the bulk solution. Divalent cations, like calcium and magnesium, are attracted by the negatively-charged surfaces with a force twice as great as monovalent cations like sodium.
Thus, the cation envelope in the divalent system is more compressed toward the particle surfaces. The envelope is also compressed by an increase in the electrolyte concentration of the bulk solution, since the tendency of the cations to diffuse away from the surfaces is reduced as the concentration gradient is reduced.
The associations of individual clay particles and organic matter micelles with themselves, each other and with other soil particles to form assemblages called aggregates are diminished when the cation “envelope” is expanded (with reference to the surface of the particle) and are enhanced when it is compressed. The like-electrostatic charges of the particles which repel one another and the opposite-electrostatic charges which attract one another are relatively long-range in effect.
On the other hand, the adhesive forces, called Vanderwaal forces, and chemical bonding reactions involved in the particle-to-particle associations which bind such units into assemblages, are relatively short-range forces. The greater the compression of the cation “envelope” toward the particle surface, the smaller the overlap of the “envelopes” and the repulsion between adjacent particles for a given distance between them. Consequently, the particles can approach one another closely enough to permit the adhesive forces to dominate and assemblages (aggregates) to form.
The phenomenon of repulsion between particles causes more soil solution to be imbibed between them (this is called swelling). Because clay particles are plate-like in shape and tend to be arranged in parallel orientation with respect to one another, swelling reduces the size of the inter-aggregate pore spaces in the soil and, hence, permeability. Swelling is primarily important in soils which contain substantial amounts of expanding-layer phyllosilicate clay minerals (smectites like montmorillonite) and which have ESP values in excess of about 15.
The reason for this is that, in such minerals, the sodium ions in the pore fluid are first, attracted to the external surfaces of the clay plate. Only after satisfying this do the sodium ions occupy the space between the parallel platelets of the oriented and associated clay particles of the sub-aggregates (called domains) where they create the repulsion forces between adjacent platelets which lead to swelling.
Dispersion (release of individual clay platelets from aggregates) and slaking (breakdown of aggregates into subaggregate assemblages) can occur at relatively low ESP values (<15), provided the electrolyte concentration is sufficiently low. The packing of aggregates is more porous than that of individual particles or sub-aggregates, hence permeability and tilth are better in aggregated conditions. Repulsed clay platelets or slaked subaggregate assembles can lodge in pore interstices, also reducing permeability.
Thus, soil solutions composed of high solute concentrations (salinity), or dominated by calcium and magnesium salts, are conducive to good soil physical properties. Conversely, low salt concentrations and relatively high proportions of sodium salts adversely affect permeability and tilth. High pH (> 8) also adversely affects permeability and tilth because it enhances the negative charge of soil clay and organic matter and, hence, the repulsive forces between them.
During an infiltration event, the soil solution of the topsoil is essentially that of the infiltrating water and the exchangeable sodium percentage is essentially that pre-existent in the soil (since ESP is buffered against rapid change by the soil cation exchange capacity). Because all water entering the soil must pass through the soil surface, which is most subject to loss of aggregation, topsoil properties largely control the water entry rate of the soil.
These observations taken together with knowledge of the effects of the processes discussed above explain why soil permeability and tilth problems must be assessed in terms of both the salinity of the infiltrating water and the exchangeable sodium percentage (or its equivalent SAR value) and the pH of the topsoil. Representative threshold values of SAR (- ESP) and the electrical conductivity of infiltrating water for maintenance of soil permeability.
Because there are significant differences among soils in their susceptibilities in this regard, this relation should only be used as a guideline. The data available on the effect of pH are not yet extensive enough to develop the third axis relation needed to refine this guideline.
Decreases in the infiltration rate (IR) of a soil generally occur over the irrigation season because of the gradual deterioration of the soil’s structure and the formation of a surface seal (horizontally layered arrangement of discrete soil particles) created during successive irrigations (sedimentation, wetting and drying events). IR is even more sensitive to exchangeable sodium, electrolyte concentration and pH than is hydraulic conductivity.
This is due to the increased vulnerability of the topsoil to mechanical forces, which enhance clay dispersion, aggregate slaking and the movement of clay in the “loose” near-surface soil, and to the lower electrolyte concentration that generally exists there, especially under conditions of rainfall.
Depositional crusts often form in the furrows of irrigated soils where soil particles suspended in water are deposited as the water flow rate slows or the water infiltrates. The hydraulic conductivity of such crusts is often two to three orders of magnitude lower than that of the underlying bulk soil, especially when the electrolyte concentration of the infiltrating water is low and exchangeable sodium is relatively high.
The addition of gypsum (either to the soil or water) can often help appreciably in avoiding or alleviating problems of reduced infiltration rate and hydraulic conductivity.
For more specific information on the effects of exchangeable sodium, electrolyte concentration and pH, as well as of exchangeable Mg and K, and use of amendments on the permeability and infiltration rate of soils reference should be made to the reviews of Keren and Shainberg (1984); Shainberg (1984); Emerson (1984); Shainberg and Letey (1984); Shainberg and Singer (1990).
Q. 7. What are the effects of salts on plants and crop quality:
Ans. Excess salinity within the plant rootzone has a general deleterious effect on plant growth which is manifested as nearly equivalent reductions in the transpiration and growth rates (including cell enlargement and the synthesis of metabolites and structural compounds).
This effect is primarily related to total electrolyte concentration and is largely independent of specific solute composition. The hypothesis that best seems to fit observations is that excessive salinity reduces plant growth primarily because it increases the energy that must be expended to acquire water from the soil of the rootzone and to make the biochemical adjustments necessary to survive under stress.
This energy is diverted from the processes which lead to growth and yield. Growth suppression is typically initiated at some threshold value of salinity, which varies with crop tolerance and external environmental factors which influence the need of the plant for water, especially the evaporative demand of the atmosphere (temperature, relative humidity, windspeed, etc.) and the water-supplying potential of the rootzone, and increases as salinity increases until the plant dies. The salt tolerances of various crops are conventionally expressed, in terms of relative yield (Yr), threshold salinity value (a), and percentage decrement value per unit increase of salinity in excess of the threshold (b); where soil salinity is expressed in terms of ECe, in dS/m), as follows-
Yr = 100-b (ECe-a)
Where Yr– is the percentage of the yield of the crop grown under saline conditions relative to that obtained under non-saline, but otherwise comparable, conditions. This use of ECe to express the effect of salinity on yield implies that crops respond primarily to the osmotic potential of the soil solution. Tolerances to specific ions or elements are considered separately, where appropriate.
It is important to recognize that such salt tolerance data cannot provide accurate, quantitative crop yield losses from salinity for every situation, since actual response to salinity varies with other conditions of growth including climatic and soil conditions, agronomic and irrigation management, crop variety, stage of growth, etc.
While the values are not exact, since they incorporate interactions between salinity and the other factors, they can be used to predict how one crop might fare relative to another under saline conditions.
Climate is a major factor affecting salt tolerance; most crops can tolerate greater salt stress if the weather is cool and humid than if it is hot and dry. Yield is reduced more by salinity when atmospheric humidity is low. Ozone decreases the yield of crops more under non- saline than saline conditions, thus the effects of ozone and humidity increase the apparent salt tolerance of certain crops.
Plants are generally relatively tolerant during germination but become more sensitive during emergence and early seedling stages of growth; hence it is imperative to keep salinity in the seedbed low at these times.
Significant differences in salt tolerance occur among varieties of some species though this issue is confused because of the different climatic or nutritional conditions under which the crops were tested and the possibility of better varietal adaption in this regard. Rootstocks affect the salt tolerances of tree and vine crops because they affect the ability of the plant to extract soil water and the uptake and translocation to the shoots of the potentially toxic sodium and chloride salts.
Salt tolerance also depends somewhat upon the type, method and frequency of irrigation. As the soil dries, plants experience matric stresses, as well as osmotic stresses, which also limit water uptake. The prevalent salt tolerance data apply most directly to crops irrigated by surface (furrow and flood) methods and conventional irrigation management.
Salt concentrations may differ several-fold within irrigated soil profiles and they change constantly. The plant is most responsive to salinity in that part of the rootzone where most of the water uptake occurs. Therefore, ideally, tolerance should be related to salinity weighted over time and measured where the roots absorb most of the water.
Sprinkler-irrigated crops are potentially subject to additional damage caused by foliar salt uptake and desiccation (burn) from spray contact of the foliage. For example, Bernstein and Francois (1973a) found that the yields of bell peppers were reduced by 59 percent more when 4.4 dS/m water was applied by sprinklers compared to a drip system. Meiri (1984) found similar results for potatoes.
Susceptibility of plants to foliar salt injury depends on leaf characteristics affecting rate of absorption and is not generally correlated with tolerance to soil salinity. The degree of spray injury varies with weather conditions, especially the water deficit of the atmosphere. Visible symptoms may appear suddenly following irrigations when the weather is hot and dry. Increased frequency of sprinkling, in addition to increased temperature and evaporation, leads to increases in salt concentration in the leaves and in foliar damage.
While the primary effect of soil salinity on herbaceous crops is one of retarding growth, certain salt constituents are specifically toxic to some crops. Boron is such a solute and, when present in the soil solution at concentrations of only a few mg/l, is highly toxic to susceptible crops. Boron toxicities may also be described in terms of a threshold value and yield-decrement slope parameters, as is salinity. Generally these plants are also salt-sensitive and the two effects are difficult to separate.
Sodic soil conditions may induce calcium, as well as other nutrient, deficiencies because the associated high pH and bicarbonate conditions repress the solubilities of many soil minerals, hence limiting nutrient concentrations in solution and, thus, availability to the plant.
These conditions can be improved through the use of certain amendments such as gypsum and sulphuric acid. Sodic soils are of less extent than saline soils in most irrigated lands. Crops grown on fertile soil may seem more salt tolerant than those grown with adequate fertility, because fertility is the primary factor limiting growth.
However, the addition of extra fertilizer will not alleviate growth inhibition by salinity. For a more thorough treatise on the effects of salinity on the physiology and biochemistry of plants, see the reviews of Maas and Nieman (1978), Maas (1990) and Lauchli and Epstein (1990).
Effects of Salts on Crop Quality:
Information on the effects of water salinity and/or soil salinity on crop quality is very scant although such effects are apparent and have been noticed under field conditions. In general, soil salinity, either caused by saline irrigation water or by a combination of water, soil and crop management factors, may result in- reduction in size of the produce; change in colour and appearance; and change in the composition of the produce. Shalhevet et al. (1969) reported a reduction of seed size in groundnuts beginning at soil salinity levels (ECe) of 3 dS/m.
However, there is an increase in seed oil content with increasing salinity up to a point. In the case of tomatoes, it was reported that for every increase in 1.5dS/m in mean ECe beyond 2 dS/m, there was a 10 percent reduction in yield. The yield reduction was due only to reduction in fruit size and weight and not to reduction in fruit number.
However, there was a marked increase in soluble solids in the extract, which may be an important criterion for tomato juice production. If ever tomato juice processors purchase tomatoes on the basis of total solids content, there would be no economic penalty for salinity in the range up to 6.0 dS/m in ECe.
The mean pH of the juice was 4.3 with no meaningful differences among treatments. Fruits from higher salinity treatments were less liable to damage and the number of spoiled fruits was less.
Meiri et al. (1981) reported that increased salinity reduced fruit size in muskmelons (Cucumis melo). However, ripening was accelerated by salinity.
Bielorai et al. (1978) reported that grapefruit yield decreased with increase in chloride ion concentration; the yield reduction was caused more by reduction in fruit size and weight. Salinity effects on fruit quality were similar to those caused by water stress.
Comparing the low and high salinity levels, there is an increase in soluble solids and tritratable acidity in the juice. There were no differences in juice content. Rhoades et al. (1989) obtained increases in the quality of wheat, melons and alfalfa from use of saline drainage water for irrigation.