Z. Szilvásszy
Research Centre for Water Resources Development
Budapest, Hungary
1. INTRODUCTION - THE PROBLEMS ENCOUNTERED
2. SITE EXPLORATIONS
3. LABORATORY TESTS
4. DESIGNING FISH-POND DIKES AND DAMS
5. CONSTRUCTION SUPERVISION
Ponds are essential components of most fish and aquaculture farms. Lowlands or valleys less suited to other agricultural development are usually selected as sites for these ponds and this is often the decisive consideration in selecting the site for the entire project. The ponds are normally shallow, cover relatively large areas and are surrounded or impounded in the majority of cases by low earth dikes or dams. The ponds are usually filled and drained through open canals; other methods, such as filling through a pipeline, being exceptional.
Potential seepage losses from the ponds and the canals connecting thereto, in particular the supply canal, and the long-term stability and performance of the low dikes surrounding the ponds and flanking the canals are the main problems encountered by the soils engineer.
In dealing subsequently with methods of exploration, laboratory and field testing, and with the interpretation of the results obtained and their use in dimensioning and analysis, familiarity with the conventional soil mechanics practice will be assumed, so that reference will merely be made thereto, as well as to some excellent handbooks in which the subject is treated exhaustively. Attention will be concentrated instead on problems due to the particular conditions to which the earth structures of fish farms are exposed, on the methods of testing which enable us to recognize any potentially adverse soil properties under these particular conditions and to indicate the remedial measures.
Seepage losses are liable to occur mainly by underseepage and infiltration from the ponds and the supply canal, since the embankments are built normally of cohesive soils, placed and compacted carefully so that they will be practically watertight. The soils to be used in the embankments must be checked for their susceptibility to long-term changes in permeability caused by atmospheric factors, such as development of 'stable density', or aggregation of particles. Leakage losses may be caused by burrowing animals, and after extended times of operation by the decaying roots of vegetation.
The losses due to underseepage and infiltration into the subsoil can be estimated by the usual methods, provided that the areal pattern and vertical position (stratification) of any previous soils, and the permeability coefficient, 'k', thereof are determined with the accuracy required.
When considering seepage losses over longer time ranges, the sediment content of the water feeding the ponds should also be determined. This will indicate whether in the course of time natural sealing (colmatation) can be anticipated. This may result also from decaying debris, fish feed, wastes, algae, etc.
Embankment stability in the 'as-built' condition can be checked by the conventional methods of soil mechanics. For appraising the long-term performance of the earth structures, the particular conditions under which these are expected to function must be remembered.
As mentioned before, these low embankments are built normally of cohesive soils, with a uniform, homogeneous cross section. Rising above the terrain, they receive little or no capillary moisture from the groundwater, and are, owing to their small cross sectional area and low mass, more exposed than large earth dams to atmospheric effects, such as repeated drying and wetting, and, in some areas to cycles of freezing and thawing. Under hot, arid climates, or during the dry season, virtually the entire embankment body may become desiccated and shrinkage cracks are liable to develop, which facilitate the entrance of water into the embankment at times of rain, or upon filling the pond. The soil will swell, but the extent of swelling at any particular point within the embankment will depend not only on the swelling potential of the soil, but also on the magnitude of the confining pressure exerted by the surrounding, mainly overlying soil masses. Repeated cycles of drying and rewetting, thus shrinking and swelling, will result in the development of a stable density distribution, with higher densities encountered normally in the interior of the cross section. At any particular point in the embankment, the actual density will fluctuate about this stable value.
These atmospheric effects trigger, however, other adverse changes as well in the silty-clayey fraction of cohesive soils. These changes will be evaluated by studying the cohesive components of the soil, the clay minerals, the clay-water system and the mechanisms which control their interactions. Because of their fundamental importance in controlling the properties and response to atmospheric effects of the earth structures involved in fish and aquaculture farms, the cohesive plastic soils will be dealt with first.
The soils displaying a certain degree of plasticity are called cohesive, e.g., the Hungarian Standard Specifications MSZ 4487-58 define a soil as cohesive if the index of plasticity surpasses the value 15.
Plasticity in cohesive soils is due to the interaction between water and the cohesive plastic components present in the soil. These plastic components may be classified into microscopic and submicroscopic clay minerals, with a definite crystal structure (kaolinite, illite, montmorillonite, etc.) and into the amorphous components. The latter are either originally vitreous volcanic ashes, or clay minerals weathered and decomposed by the lime present in the soil. Over the extremely large specific surface (up to 800 m2 /g) of the clay minerals, the unbonded interface atoms produce a strong negative electric field, the role of which surpasses by several orders of magnitude that of gravity related to the negligible mass of these particles.
The amorphous components are polisilicate and polialuminate hydrates, which have a gel structure. In the case of weathered clay minerals the amorphous gel may form a thin coat on the surface, or may be present as a separate phase, once the mineral is totally decomposed. In a dry, desiccated condition these policompounds form a hard xerogel of considerable strength, but are capable of absorbing extraordinary amounts of water, swelling and becoming spontaneously liquid in the process. The gel coat will act as a lubricant reducing the shear strength and causing even fairly coarse soils to flow.
Non-plastic components are invariably present in cohesive soils. These are mostly mineral particles with a low surface-to-mass ratio (e.g., sands and gravels), which have an influence on the plastic properties, such as plasticity, shrinkage, etc., but are not essential elements, in that their presence is not a prerequisite to the development of clay properties. Still, in desirable proportions they have a beneficial effect.
The other element involved in plastic soil is water. The fundamental feature is the dipolar nature of the water molecules, with a positive and a negative pole making them thus behave like minute magnets.
In the liquid phase these dipoles tend to aggregate into loose tetrahedron-like crystalline clusters, but owing to thermal movement these are unstable, and are rearranged continually. The state and size of these aggregations depend on temperature. Water in this state is called a Newtonian fluid.
Under high pressures (about 2000 atmospheres) water turns into a solid phase even at normal temperatures, but with a structure differing from that of normal ice. Calculated from the heat of hydration, some clay minerals were found to bond the first few layers of water molecules with energies corresponding to 20-25 thousand atmospheres pressure. Such water is necessarily solid even under the hottest climate and has a specific density of 1.2 to 1.4 g/cm3.
Clay-water interaction is due to the aforementioned negative charges prevailing on the surface of the clay minerals. These cause forces of attraction, which may be classified as direct, when the positively charged atoms of the dipolar water molecule are attracted, and indirect, when a cation is adsorbed and this relays the attractive force toward the water molecules. The cations control decisively the hydration properties of the clay minerals, since the magnitude of the attractive force between the clay particles and water, and the range over which this attraction is effective, depend on the kind and amount of the cations present. The water molecules are oriented with their positive pole toward the negative clay surface, their negative poles point outward and attract the positive poles of further water molecules, although with a force decreasing exponentially with distance. It has been demonstrated experimentally that in some clays the attractive force is still 4 atmospheres (around 40 m water column) at a distance of 1-micron (around 3000-4000 water-molecule 'diameters') from the clay surface.
In clay-water systems attractive and repulsive interparticle forces act simultaneously. The distance between the discrete clay particles, their relative orientation and thus the permeability and strength, etc., of the clay will depend on the equilibrium between attraction and repulsion, or the predominance of one or the other.
Repulsion is due to the negative poles of the water molecules oriented outward. This is termed Coulomb repulsion of the diffuse electric double layer, and varies with the square of distance.
Attraction between the clay particles is due to the van der Waals-London force of mass attraction, which varies with the fifth, or sixth power of distance. This becomes effective whenever the clay particles approach each other closer than 1-2 millimicrons (roughly 4-8 water molecules) and produces usually irreversible changes in the clay structure (e.g., burning brick or pottery).
These conditions may be visualized by the force versus distance diagram in Figure 1, showing the attraction, repulsion and the resultant potentials. If the resultant curve displays a pronounced minimum, then the clay is said to be in a stable condition, the particles are situated at the distance corresponding thereto and any change can be brought about only by introducing external energy. In the absence of a minimum the suspension is said to be dispersed, while a weak minimum is the case of thixotropic gels, where equilibrium is upset already by very low external energies.
Any change in the clay-water systems is due to a disturbance of the equilibrium, thus to a change in the attraction or repulsive forces prevailing between the particles. In such cases the distance between the particles is changed, and the clay shrinks or swells until a new equilibrium is established.
The attractive forces depend exclusively on the mass of the particles and at a given distance no spontaneous change is possible. The distance may, however, change under external load, capillary or seepage pressure, and thus the attractive force may increase suddenly (since it varies with the fifth, or even sixth power of distance), causing the clay to consolidate, coagulate and shrink.
The magnitude of the repulsive force is, on the other hand, influenced by the ions absorbed on the particles, and by the concentration of electrolytes in the water. Any change in these entails necessarily - even under otherwise identical conditions - a volume change, coagulation or dispersion of the clay.
The interstices between the particles (the pores) are however filled with water, so that any volume change due to a change in the external forces or electric charges is only possible if interstitial water is squeezed from or drawn into the pores. Since the pores are submicroscopic in size and the non-Newtonian water has, because of the surface forces, a very high viscosity, the transition into the new equilibrium state is a very slow one, tending asymptotically to the terminal state. The mechanism outlined in the foregoing is suited to explaining the changes brought about by atmospheric effects in the condition and properties of cohesive clay soils. Of these changes those due to a reduction in moisture content (drying) are most important. Using Kelvin's equation uw = 45 000 log H (uw is the pressure or tension of the pore water in psi, and H the relative vapour content, in decimals) it will be appreciated that at 30% relative humidity and 20 C temperature the tension in the pore water may be as high as 130 atmospheres, so that much of the water adsorbed on the particle surfaces will be removed.
Figure 1. Force versus distance diagram
Drying has the following consequences in clay soils:
(i) Air-water interfaces develop in the three-phase soil, and surface tension acts on these. Owing to the small pore sizes and, in turn, to the short radii of curvature, the resulting capillary pressure is a very high one and tends to bring the particles together.(ii) Evaporation removes some of the water enveloping the particles, which reduces the distance between them.
(iii) Water is evaporated, thus the electrolyte concentration of the remaining pore water is increased, which may neutralize the repulsive forces between the particles.
As a combined result of the foregoing three processes the clay particles may -depending on the degree of desiccation - be drawn together sufficiently for the van der Waals attractive forces to become predominant (see the potential versus distance diagram), resulting in flucculation and aggregation. As a consequence the grain size distribution becomes coarser (as illustrated in Figure 2), the cohesive soil becomes more pervious, the specific surface decreases and as a corrolary thereto the plastic properties are diminished. The resulting reduction of volume or shrinkage leads to cracking and the increased attractive forces give rise to considerable apparent strength.
If rewetting by pond filling or rain occurs, the cracks facilitate the entrance of water into the embankment body. The external parts of the individual blocks formed by the original slope and the crack faces are saturated, while the moisture penetrates toward their interior by capillary suction. The saturated parts become virtually impervious to air, so that the air enclosed in the pores is placed under pressure, the magnitude of which equals the sum of the highest capillary pressure and, over the underwater parts, the acting hydrostatic pressure. The pressure is increased also by the heat of hydration, which is prevented from dissipating and may raise considerably the pressure of the entrapped air. The pressure of the confined air leads to explosion-like soil failure and is the direct cause of spalling and subsequent liquefaction of unprotected slopes. If the cracks convey flowing water, then the suddenly liquefying soil will be scoured at a faster rate than if the cracks are closed by swelling, leading to the development of gully or tunnel erosion, which has often been identified as the cause of failure in small embankments.
The mechanism described in the foregoing applies to soils containing clay minerals as the cohesive component. The presence of amorphous components tends to aggravate the situation, since these are capable of absorbing considerable amounts of water and thus may induce extremely high swelling rates.
The susceptibility of soils to such adverse changes in properties, as well as the need and extent of control measures can be identified on the basis of simple tests conducted parallel to those commonly performed at soil mechanics laboratories.
Careful site explorations are essential to the soils engineer in the earliest possible stage of any fish farm project, because sometimes a proposed site will have to be abandoned and another one selected if early explorations indicate adverse soil conditions.
Site explorations enable the soils engineer to provide answers to the main problems of seepage and dike stability and to furnish information to the designer of the structures about the foundation conditions.
As mentioned in the introduction, the accuracy of seepage loss estimations will depend on how well we succeed in determining the areal pattern and stratification of any pervious soils in the pond area and along the trace of the filling canal, and the permeability coefficient 'k' of these soils.
Figure 2. Semi-logarithmic grain-size curve of soils
Efforts must therefore be made to check the surface layer in the area of the proposed pond(s) and along the trace of the canal for uniformity, to detect any singularities such as beds of rivulets and abandoned streams, which are always potentially present in alluvial lowland areas.
The plan of explorations should accordingly be prepared on the basis of a thorough field inspection including preliminary sampling with a manual sampling kit and classifying the samples visually.1/ Valuable information can be derived further from a careful study of detailed maps and aerial photographs, if such are, or can be made available at reasonable cost. The aim should be to locate any areas in which the soil consists of granular materials, such as loose silts, sands or even coarser gravels.
1/ U.S. Department of the Interior, Bureau of Reclamation: 'Earth Manual'. U.S. Government Printing Office, Washington
The points of exploration should be determined according to the results of this survey, rather than by adopting a fixed grid pattern, since a grid pattern will always involve the danger of two adjacent boreholes straddling a narrow strip of higher permeability.
The soils along the canal traces should be explored also for their hydraulic properties, to estimate slope inclinations and the allowable (non-scouring) velocity of flow in the canal.
The equipment and techniques of these explorations are those commonly used for shallow work in soil mechanics. It should be noted, however, that care should be exercised in determining as accurately as possible the sequence of soil strata down to the first impervious layer. If the first layer of topsoil is impervious and at least 0.6 to 1.0 m thick, no deeper exploration is needed. Disturbed samples taken with a hand auger are suited to determine the granulometry and index properties of the soil, but for permeability testing at the laboratory undisturbed samples are required, since any remoulding may cause drastic changes in the 'k' value and lead thus to false conclusions.
Since truly undisturbed samples are difficult to take, field permeability tests should be performed in the vicinity of each exploratory borehole or shaft by the infiltration method.2/3/
2/ U.S. Department of the Interior, Bureau of Reclamation: "Earth Manual". U.S. Government Printing Office, Washington, pp. 717-26.3/ Szilágyi Gyula, Vágás I.: Field determination of seepage losses from earth irrigation canals (Hungarian). Vizügyi Közlemények, 1962. No. 1
2.1 Pond Areas and Canal Traces
2.2 Borrow Sites
2.3 Structure Sites
Within economical hauling distance all cohesive soils should be checked for their suitability as construction material for the dikes. In these potential borrow sites the explorations should be organized so as to provide information on the extent, depth and occasional stratification of the deposit.
Where the cohesive material for the dikes can be excavated from the pond area, care should be taken not to cut into any pervious sublayer, which may cause seepage losses, or underseepage.
It should be remembered that samples of fairly large volume are needed for laboratory analyses, since each compaction test should be performed on fresh samples. Around 12 kg are needed for classification and compaction (Proctor) testing. The compaction test can be made also by reprocessing the same sample, and increasing its water content gradually, but the result will be slightly higher than with fresh samples.
The subsoil in the proposed structure sites should be explored to a depth equal to twice the larger dimension of the foundation slab in plan. The structures involved in fish-farm projects are normally small and so are the loads acting on them. For this reason the bearing capacity of the subsoil and if relevant, the position of the load bearing layer, can be determined with the accuracy required by a sounding method. The Swedish Weight Sounding Technique as described by the Swedish Geotechnical Society (1970), is simple and fast, and yields acceptable results for this purpose.
The Swedish weight penetrometer consists of a screw point, sounding rods, a rotating handle and weights of 5 kg, 10 kg (2), and 25 kg (3), making a total of 100 kg. During a test the penetrometer is gradually loaded until the point starts penetrating into the soil. This load is recorded, just as any further loads, required to continue penetration.
As soon as the rod refuses to penetrate under a load of 100 kg, the penetrometer is rotated by hand. The penetrometer should not be rotated if the load applied is less than 100 kg. The penetration resistance is recorded as the number of half-turns required for 20 cm penetration.
If a firm, or hard layer is situated close to the surface (from fill or a thick dry crust), the frictional resistance along the sounding rods may affect appreciably the penetration resistance measured. A small diameter borehole of 5 cm should then be drilled through this layer.
Penetration testing should be terminated when a certain penetration resistance or a certain depth has been reached. A firm bottom is reached when the penetrometer does not sink any more when rotated, or driven by at least 10 blows of a 3.0 kg sledge hammer. (The two top 25 kg weights can be used instead. These should be dropped from 20 cm height.). If this criterion has not been satisfied, the penetration test can be terminated when two adjacent 20 cm thick layers are encountered with increasing penetration resistance exceeding 40 half-turns per 20 cm, and penetration is less than 1 cm per blow after 5 blows with the top two 25 kg weights. If the penetration is larger than 5 cm per 5 blows, or if the penetration resistance decreases, the sounding rods should be rotated before driving is continued. The depth of penetration achieved by driving should be recorded on the plot. High resistance may be caused by a stone, or boulder. The noise and vibrations in the sounding rods during a test can also be observed to identify the soil penetrated as clay, sand or stone.
A cohesionless soil is considered to be dense, when the weight sounding resistance is equal to, or higher than 15 half-turns per 20 cm penetration, and to be loose when it is less. The average permissible soil pressure r m for gravel, sand and silt is given by the formula:
where
r m = average permissible soil pressure, MPa
B = width of footing, m
L = length of footing, m
N = bearing capacity factor, MPa/m
Some examples are shown as follows:
|
|
Determination of Permissible Soil Pressure | |||||
|
Type of Soil |
Half-turns/20 cm |
H (m) |
Factor D=0 m N MPa/m |
| ||
|
|
|
|
D=0 |
1 m |
2 m |
r m , max (MPa) m |
|
|
15 |
0 |
0.17 |
0.25 |
0.29 |
|
|
Gravel |
|
|
|
|
|
0.60 |
|
|
1-15 ³ 2B |
|
0.27 |
0.40 |
0.47 |
|
|
|
|
0 |
0.13 |
0.19 |
0.22 |
|
|
|
15 |
|
|
|
|
0.50 |
|
|
|
³ 2B |
0.20 |
0.30 |
0.35 |
|
|
Sand |
|
0 |
0.08 |
0.11 |
0.13 |
|
|
|
1-15 |
|
|
|
|
0.30 |
|
|
|
³ 2B |
0.12 |
0.18 |
0.21 |
|
|
|
|
0 |
0.10 |
0.15 |
0.17 |
|
|
Fine sand and coarse silt1/ |
15 |
|
|
|
|
0.40 |
|
|
|
³ 2B |
0.16 |
0.24 |
0.28 |
|
|
|
|
0 |
0.04 |
0.06 |
0.08 |
|
|
|
1-15 |
³ 2B |
0.07 |
0.10 |
0.12 |
0.20 |
1/ For coarse silt the permissible soil pressure is limited to the values given for a loose state of compactnessNote:
H is the distance between the footing and the highest ground water level. When the ground water level is located above the foundation level, H is equal to zero. For values of H between 0 and 2B rectilinear interpolation is recommended.D is the minimum depth of foundation. For values of D between 0, 1 and 2 m rectilinear interpolation is recommended.
For determining stratification, auger drilling and the retrieval of disturbed samples are acceptable and normally no undisturbed cores need be taken.
The samples taken during the foregoing explorations should be preserved, labelled as in standard practice and taken to the laboratory for testing.
3.1 Additional Tests for Pike and Dam Materials
3.2 Interpretation of the Results
Permeability tests on the samples collected from the pond area and the canal trace, index tests on the samples from the structure sites and borrow areas, and compaction (Proctor) tests on the samples from the borrow areas should be performed following standard soil mechanics laboratory practice. The standard Proctor test is preferred, since this will more closely approximate the conditions in low dams and dikes. The shear strength of each compacted specimen should be determined by measuring the penetration resistance of the Proctor needle.
Natural moisture content should be determined on every sample, since this value will be needed in the subsequent tests.
Granulometry should be determined by sieve and hydrometer analysis depending on soil type. With granular soils the conventional procedure should be adopted. Cohesive soils, which contain coarser particles as well, such as sandy silts and clays, should be oven dried to constant weight at 60°C, since the cohesive fractions may undergo irreversible changes at higher temperatures. The dried samples should be crushed carefully and the particles larger than 0.1 mm removed by sieving.
The granulometry of the fine fractions should be determined by 'wet', i.e., hydrometer, analysis. Two analyses should be run on each sample, one in the water to which the earth structure will be exposed, the other in water to which the optimal amount of dispersing agent is added. Sodium carbonate is recommended as the dispersing agent. Sodium hexa-metaphasphate may produce higher degrees of dispersion, but sodium carbonate more closely reproduces conditions likely to occur in practice. The optimal amount should be used, since smaller amounts produce insufficient dispersion, while overdosage will cause the clay particles to coagulate.
To find the optimal amount, prepare a 5 percent solution of sodium carbonate in distilled water. Weigh specimens of 1 g dry weight each into 6 test tubes and add 0.2, 0.4, 0.6, 0.8 and 1.0 ml of the dispersant solutions to the successive test tubes. No dispersant is added to the sixth test tube. Then add 4-5 ml of distilled water to each and work the sample to putty using a glass rod. Fill up to 10 ml, using the water to flush any material from the glass rod.
Boil the test tubes in a water bath for about 20 minutes and allow them to cool in a stand arranged in the sequence of increasing dispersant concentration. After 24 hours, the test tube showing the largest loose sediment volume will indicate the highest degree of dispersion attainable.
The amount of dispersing agent to be used for the hydrometer analysis is found from the following table:
|
Preliminary |
test |
Hydrometer analysis | |
|
5% solution of NA2CO3 |
to 25 g soil dry weight add from 25% sodium carbonate solution | ||
|
Test tube 1 |
0.2 ml |
1.0 ml | |
|
|
2 |
0.4 |
2.0 |
|
|
3 |
0.6 |
3.0 |
|
|
4 |
0.8 |
4.0 |
|
|
5 |
1.0 |
5.0 |
The hydrometer analyses should thereafter be performed in the conventional way and the two granulometry curves for each sample should be plotted on the same chart.
By comparing the two curves, conclusions can be reached on the actual condition of the soil, and on the condition which may result from repeated or extended exposure to water. The difference between the actual and dispersed granulometries will also indicate the width of the range within which changes in soil condition and thus in mechanical properties can be anticipated. If the actual granulometry is close to that in the dispersed condition, no further disaggregation is likely to occur and the soil is as watertight as can be expected. Conversely, a wide difference between the two curves implies coagulated, aggregated conditions. At a reduced specific surface larger void sizes predominate and, therefore, the soil will be pervious even if a highly cohesive soil could be inferred on the basis of the clay content or index of plasticity. Some coagulated clay soils may disperse spontaneously under prolonged exposure to water, which would mean a corresponding loss of stability of the soil.
The CaCO3 content will indicate the extent to which the individual particles are liable to be weathered, i.e., the crystalline structures are decomposed to amorphous, vitreous gels. This weathering results in the deterioration of the mechanical properties, such as stability and permeability, so it is one of the main causes of soil slumping and flowing after extended exposure to water.
The pH values indicate the water adsorption or hydrophyllic properties of the soil, and the occurrence and extent of the weathering process mentioned in the foregoing.
The specific surface, together with the surface index calculated therefrom, will provide information on the type of clay minerals predominating in the soil, or on the crystalline, or weathered surface of the discrete particles. A high specific surface area may be due to the presence of a high proportion of clay minerals (montmorillonite, illite), which resemble scales and have thus extremely large surfaces, or to the weathered condition of the individual particles. The extent of weathering is, however, known from the foregoing tests, so the high specific surface area can be attributed positively to one of the two causes mentioned.
Sediment volume is an indication of the volume stability of the soil. Boiling accelerates the soil-water interaction, so that changes in soil conditions can be determined in a shorter time.
Based on experiences gained with a number of low dikes, the critical void ratio has been introduced in Hungary, as an index of volume stability. This is determined by the following procedure:
First, the sample is air dried to constant weight and crushed for sieving on the 0.1 mm mesh sieve. From the material passing, a sample of 4 g mass is weighed to 0.1 g accuracy and filled loosely into a 25 cm3 measuring cylinder, which is then filled up to 20 cm3 with water of a composition similar to that to which the earth structure will be exposed. The sample is allowed to soak for 48 hours, shaking it vigorously several times during this period. Thereafter the sample is allowed to settle for approximately 24 hours and the volume of sediment is read on the scale of the cylinder.
The critical void ratio is found as:
where
V is the sediment volume read on the cylinder, in cm3 unitsW is the weight of the air-dry sample placed in the cylinder, in gram units
s is the specific gravity of the soil in g/cm3 units. (This may be entered with the value 2.75 g/cm3 for all practical purposes.)
Some soils will refuse to settle, so that no sediment volume can be determined and the critical void ratio will be very high.
Secondly, for earth structures, where leaching is liable to occur, the clear fluid is decanted carefully or syphoned off after the first reading and refilled to the original mark. The sample is shaken, allowed to rest and settle as before and the reading on the sediment volume is repeated. After three to four repetitions the growing or decreasing trend of the successive critical void ratios will indicate the final value, which will characterize the soil after leaching, or basic exchange processes, have taken place.
In terms of the critical void ratio, the soils are classified into the following groups:
Group A: ec =2.5-3.5 (V =5.7-8 cm3)Slightly cohesive, pervious under low heads, low swelling potential, high mineral and chemical stability under environmental exposure. The soil mechanics characteristics determined on the original sample are likely to remain constant over the service life of the structure.Group B: ec = 3.5 - 6 (V = 8 - 13.5 cm3)
Swelling soils, susceptible to weathering. Medium to highly cohesive, impervious under virtually all conditions, high swelling and shrinkage potential. Under atmospheric exposure liable to weathering, making them more cohesive.Group C: ec > 6 (including soils which refuse to settle)
Highly swelling soils tending to become liquid spontaneously, flowing even on very flat slopes. Conventional testing by soil mechanical methods will produce irrelevant results.Group D: ec < 2.5
Soils usually aggregated as a result of leaching or oxidation, or formed by weathering from acidic parent rock. Although cohesive, such soils display sand characteristics, high volume stability, and also permeability. Owing to aggregation, the original deposits are loose, and highly compressible. The surface roughness and strength of the aggregations lends them a high angle of internal friction, but cohesion is low. Conventional testing by soil mechanics methods will produce irrelevant results.
The properties of mixed fractions will be governed by the relative proportion and properties of the fines of less than 0.1 mm present.
In a sandy-silt soil, where the fraction less than 0.1 mm is over 15 percent by weight and belongs to Group A or B, the sand will display a rather low angle of internal friction and a susceptibility to liquefaction, but will be practically impervious. If however, the fines belong to Group D, then the sand will display higher strength characteristics, but will remain pervious regardless of the high fines content.
This simple test, together with the normal index tests, will often be found to provide sufficient information to the designer to enable him to predict the future performance of low dikes and to decide on the advisability of any protective measure.
The extent of mineral weathering is indicated by the chemical solubility. Of the crystalline silicate soil components the main SiO2 and Al2O3 compounds dissolve poorly (a few tenths of a percent) in acids of rather high concentration (e.g. HCl of 10% concentration) or in weak bases (e.g. NaOH of 27, concentration), whereas these main components of the weathered, vitreous-amorphous phase can be dissolved in quantities up to several percents, depending on the degree of weathering. This test will provide quantitative information on the amorphous content of the soil.
The surface index mentioned previously is defined as the ratio of the specific surface area, Fspec, to the percentage by weight of the minus 0.002 mm fraction, G0.002, as follows: and has the following values:
|
Soil Component |
I (m2/g) |
|
Quartz flour, glacial till |
0.02 |
|
Feldspar |
0.30 |
|
Aplite |
0.56 |
|
Kaolinite |
0.92 |
|
Illite |
1.20 |
|
Montmorillonite |
3.0-3.6 |
The index thus gives guidance as to the type of clay mineral present in the soil and, in combination with the result of the solubility analysis, to the extent to which the soil is weathered.
In terms of the foregoing tests and analyses a soil is considered susceptible to adverse changes under exposure to atmospheric factors and water if the following criteria are satisfied simultaneously:
(i) The two granulometric curves are situated close to each other and the minus 0.002 mm fraction is at least 15% by weight.(ii) The pH in the 1% NaF solution is higher than 10.5.
(iii) The pH in water is higher than 8.3. The susceptibility to loss of stability by liquefaction increasing with the pH.
(iv) The specific surface area determined by methylene-blue adsorption is higher than 15 m2/g. In this connection the results of the pH in NaF and the solubility analysis should also be taken into consideration, since the polialuminates, as products of weathering, carry a predominantly positive charge and do not adsorb the methylene blue, which is also positively charged.
(v) The solubility analysis indicates the presence of polisilicate and polisilicic acid gels.
The results of the tests described in the foregoing are entered in the granulometric chart shown in Figure 2.
The conclusions arrived at from these more elaborate tests will coincide in the vast majority of cases with those derived from the critical void ratio and will indicate whether and under what conditions a particular soil can be used as the material for dike construction
Designing earth structures involves a process of successive approximation requiring several trials. The starting data available to the soils engineer are normally the height of the structure (depth of water plus the safety freeboard), the foundation conditions, available construction materials and occasionally the minimum crest width, if movement of some equipment on the crest is required. The problem is to design a dike cross section which will produce a safe and sufficiently watertight structure, and which will result in a project of minimum cost. The first step consists of finding the steepest slope inclinations which will ensure stability over the service life of the structure, so that the embankment will be the cheapest to construct. The resulting base width must then be checked for underseepage and for the danger of piping failure. The foundations of structures must also be checked for settlement behaviour. In cases where cohesive, impervious materials are available in limited quantities only, the dike design providing the water tightness required must also be found.
The depth of water is given by the technological designer, the safety freeboard by the hydrologist and the designer of the spillway. Freeboard may also be governed by the wave action to which the dike is likely to be exposed. The wave height is found by taking the direction and force of the prevailing wind, the fetch length and the depth of water into consideration.
There are a number of formulae in use for estimating wave height. The following two are in the metric system:
where
hw is the wave height in metres
W is the wind speed in m/sec
B is the pond width in the wind direction (fetch length) in km
H is the water depth in metres
For comparison, a formula used in British units is:
where
hw is the wave height in feet
W is the wind speed in mph
B is the pond width in the wind direction (fetch length) in miles
The results of exploration and the soil properties determined by laboratory tests afford guidance to the soils engineer in design. It should, however, be emphasized that earth structures are already exposed to atmospheric influences in the course of construction, and also later during their whole service life. They must never be considered as static, but as dynamic structures, taking into account the changes in conditions and character of the factors to which they are exposed. This kind of philosophy will alone lead to a technically and economically sound solution, in which allowance can be made for the mutual interactions between the subsoil, the earth structure and the atmosphere. It should be remembered, moreover, that the number and variability of the various factors involved prevents us from predicting and estimating all possibilities and influences in advance.
Thus, in analysing the slopes for their stability, the shear strength determined by laboratory tests is used. Great care must, however, be exercised in choosing the design value. Reliable mean values can only be adopted if the soil and subsoil is uniform and, therefore, stability analysis will yield useful results in these cases alone. If this is not the case, then the results of the stability analyses should be accepted as rather rough estimates only. An evaluation of local experience gained with similar structures and a critical examination of the particular pedological and geological conditions are indispensable under such circumstances.
Another important consideration is that the low dikes and dams normally involved in aquaculture projects do not warrant expensive tests in a soils laboratory. It should be realized that design values of soil strength are costly to determine and yield information which must be accepted with circumspection by the engineer who must use sound engineering judgement in their application. For the foregoing reasons practical guidelines have been applied in Hungary for dimensioning dikes, levees and dams lower than 3.0 m in height and retaining less than 3 million m3 of water.
The dikes are classified into homogeneous (a) and zoned (b) cross sections, the latter including those with an impervious core, with or without an impervious upstream blanket (Figure 3). The individual structural elements (zones) of the embankment are classified again as narrow or wide, depending also on the material used as shown in Table 1.
The degree of compaction required for the various structural elements of the dike is indicated in Table 2.
Low earth structures designed and constructed according to these guidelines have proven satisfactory in Hungary.
In connection with these guidelines it should be noted, however, that no cohesive soils classified according to the critical void ratio as Group C must be used as construction material for the dikes. The soils belonging by the same criterion to Group B can be used as impervious core material, if the core is protected on both upstream and downstream sides by shells (Figure 3) belonging to the size category wide of Table 1 and constructed of sand.
Under arid climates, where dry spells of extended duration are liable to occur, no material classified as Groups B and C by the critical void ratio must be used for dikes of uniform cross section. Soils belonging to Group B may be used for the core, but must be protected by shells of coarse to medium sand. A transition layer at least 0.5 m wide should be provided of sand, if gravelly sand or gravel is used for the shells.
Slopes built with the inclinations and of the materials shown in Table 1 and observing the foregoing limitations will normally be stable. The most important single factor causing deterioration is erosion on the upstream side by wave action, and on the downstream side by heavy rains. The slopes must be protected against these contingencies. A sound grass cover will offer adequate protection on the downstream side and the crest against rain erosion and normally even on the upstream side above the line of wave run up. Grass cover should be established as an organic part of the construction work, as soon as the slopes are finished to grade. The kind of grass to be seeded and the method of seeding will vary from region to region and the advice of an agronomist should be sought on this matter.
Over sections exposed to wave attack, the slope must usually be protected. Reed belts of about 4 m width will attenuate waves effectively and should be planted on berms with a horizontal, or mildly sloping surface. Where reed belts are unacceptable, or where reed refuses to grow, other means of slope protection should be envisaged. The material commonly used for such protection is riprap, which may be dumped in a thickness of 0.5 m, or hand placed, where labour is cheap, in a thickness of 0.25-0.30 m. Fifty percent of the stone by weight should be of a size about equal to the thickness of the riprap, the remaining half should be graded uniformly down to the largest diameter of the underlying filter bed.
Other alternatives of slope protection that may be considered where rock is unavailable are blocks of concrete, ceramic blocks, continuous concrete mattress, or lime stabilization, but brushwork mattresses have also been used on minor projects. These latter, however, require frequent maintenance and must be replaced at rather short intervals of time.
The soils classified as unsuited can be improved in their properties by lime treatment, which results in greatly reduced swelling and shrinking and makes the soils much easier to compact. The soils stabilized by such treatment will also offer higher resistance to erosive action. There is ample literature available on the theory and methods of lime treatment, the economic desirability of which depends on local conditions and must be evaluated separately for each particular project.
Table 1 Classification of Component Elements of Dikes according to Size
Table 2 Densities related to Proctor Optimum in Embankments
The dike cross section determined in the foregoing manner must next be checked for underseepage and piping failure. Whereas underseepage is merely a problem of water loss from the pond, piping must be prevented since it presents a serious danger to the stability of the structure. Piping occurs when the force exerted by seeping water on the soil exceeds the resistive force offered by the soil. The potential energy represented as the differential head created by the dike is dissipated as frictional loss as the water flows past the soil particles. The seepage force S acting on the soil is the product of the unit weight of water g w and the hydraulic gradient i. The latter is found when the differential head is divided by the distance between the points of seepage entry and emergence. When upward flow exists in a cohesionless soil and the head loss per unit length of the flow path exceeds the submerged effective weight of the material, movement of the soil particles will take place. For example, if the weight of a cubic metre of saturated cohesionless soil is equal to 2 000 kg, the submerged weight will be 1 000 kg. Hence, if there is an upward hydraulic gradient of 1, the seepage force will be equal to 1 000 kg and the material will be in a condition of unstable equilibrium, termed a 'quick' condition.
Whenever water flows from a less pervious material to a more pervious one, or out onto the ground surface, the possibility of migration of fines, or piping should be considered. Even a very minor washing away of fines at the downstream side of the dike is serious. As soon as some fines are washed away, the resistance to erosion along the path of seepage is reduced and this results in an increase of flow. Owing to the increased flow, the rate of washing away is further increased, and so forth, until failure occurs.
To analyse the subsoil of the dikes for potential piping and to estimate the factor of safety relative to piping failure, two cases of interest in designing fish pond dikes will be considered (Figure 4).
1. Case of a single pervious layer under the dike base
No detailed analysis of piping failure is necessary, if the condition
is satisfied.
Here
B is the base width of the dike, in metres,H is the differential head, also in metres and
i0 is the 'inherent failure gradient' of the soil material forming the layer, and has the following values:
|
gravel to coarse sand |
io =0.9 |
|
fine sand to silt |
io = 0.6 |
|
cohesive soil |
io = 0.85 |
The limit failure gradient is found as
where
a is the irregularity index of the soil material, given in terms of the uniformity coefficient U = D60/D10
|
U |
a |
|
5 |
0.2 |
|
5 -15 |
0.5 |
|
>15 |
0.8 |
Figure 4. Cross sections of dikes
The limit failure gradient must be smaller than the effective gradient Ieff = H/B and the factor of safety is Ieff/If
2. Case of dike built on a single cohesive top layer
The subsoil may be considered impervious, so that no analysis of piping failure is necessary, if the thickness of the cohesive layer under the dike is
and on a sample taken from 1 m depth the liquid limit wL >45%, the index of plasticity Ip = 15% and the critical void ratio eM > 3.
If the layer is thinner than 3 m, then it may burst under the gradient
, that is > 0.85, which is the inherent failure gradient of the cohesive top layer.
Piping failure will occur, if the material of the pervious sublayer is eroded after the cohesive top cover has burst, thus if
It should be noted that to account for a concentration of flow lines in the vicinity of the break in the top layer, where the seepage flow emerges, a factor of 3 has been introduced. In the foregoing formula the inherent failure gradient of the pervious sublayer must be introduced. The factor of safety is again Ieff/If.
In order to prevent piping, movement of soil particles under the action of seepage forces must be prevented. Where the soil subject to possible piping is exposed at the downstream side of the dike, piping can be prevented either by reducing the seepage gradient at the exit point so that the seepage force is too small to cause movement of particles, or by stabilizing the soil at the exit area mechanically.
Mechanical stabilization consists of covering the exit area with a filter blanket of coarser material, so graded that the pores of the coarser material are small enough to prevent the larger particles of the finer soil from passing through. The two layers must obey Terzaghi's filter rule which is to be found in all handbooks on soil mechanics.
Further methods of piping control consist of the construction of drainage trenches and relief wells, or of extending the path of seepage by an upstream blanket. These methods are, however, normally too expensive to be justified in fish-pond projects. On the other hand, the possibilities and alternatives of controlling underseepage and thus seepage losses from the pond must be considered.
The problem of underseepage and the desirability of control measures should be examined by a dynamic approach, i.e., by examining the potential decrease of seepage losses over time. At any particular instant the magnitude of the seepage loss from a given pond area is given by the product of permeability and the hydraulic gradient, obtained as the ratio of the water head to the thickness of the soil layer above the ground-water table. However, in this product both the permeability coefficient and the position of the groundwater table may be variable with time.
The permeability coefficient can be expected to decrease with time, as the sediment particles carried in suspension by the entering flow are washed into the pervious bottom layer and fill in the voids of the latter. This process, known as clogging, or 'colmatation' in French papers depends thus on the relative granulometries of the pervious layer and the silt, and on the sediment content of the feeding water. The mineralogical composition of the sediment (the presence of clay minerals) is also an important factor. Both laboratory experiments and field experience indicate that the permeability coefficient of a 10 cm thick surface layer may decrease by as much as two orders of magnitude within 10 to 14 days, and practically complete sealing may develop within six months. The rate of clogging will become faster if the granulometry of the sediment is comparable to the fine fraction of the pervious top layer, and for perfect sealing to develop the presence of clay particles is essential.
Where the flow feeding the ponds carries no sediment in suspension, such as a supply from a reservoir, the process may be induced artificially. For this purpose a thin (2-3 cm thick) layer of fine, non-cohesive material, such as fine sand, is spread underwater. This can be done from a barge evenly on the pond bottom proceeding from the edges toward the centre. The largest size of this material should equal 0.6 to 0.75 times the effective diameter of the bottom material. The seepage loss should be observed by readings on a staff gauge during this treatment. The first, or at the most the second such treatment should already produce a marked decrease in the seepage losses. As the final step of treatment a clay (bentonite) slurry should be spread by similar methods on the bottom. The concentration of the slurry depends on the activity of the bentonite available. Thus if the bentonite displays a viscosity of 26 centipoise in a 6 percent suspension (60 g of bentonite suspended in 1 000 ml of water), an application rate of 2-3 litres of slurry per square metre of bottom area should suffice for complete sealing. The application rate should be increased if the bentonite is less active.
As pointed out in the foregoing, repeated cycles of desiccation and rewetting are most liable to cause adverse changes in the soil structure. The sealing just described should be maintained in a wet condition, therefore, and the pond should not be kept empty for extended periods of time, especially in the dry season.
Seepage losses from the feeder canal can also be controlled by this method, but to prevent the bentonite slurry from being washed away, it is usually necessary to impound the canal by reaches for a few days during which the slurry is placed and allowed to penetrate into the soil pores.
During construction work on the dikes and canals, soil mechanics supervision is necessary to check whether the soils envisaged are used for the various parts of the dikes, whether these are placed at the moisture content specified and whether they are compacted to the density specified.
The soils from the borrow areas previously explored are classified visually to identify them with those tested at the laboratory.
The moisture content of the borrow material can be determined by burning if a major departure from the average is observed.
The conventional method of determining the density and moisture of the material placed and compacted in the dike consists of taking core samples, which are then taken to the laboratory for moisture determination by oven drying.
Where the dike material is moved and placed mechanically, even moderate-capacity equipment will produce fast progress, so that the point of sampling will usually become buried by the time that the result becomes available, leaving the soils engineer with no possibility of any remedial measure. To remedy this, devices have been developed by which the moist unit weight and the volumetric moisture content of the soil can be found instantly, on the spot, indicating whether the density attained is acceptable, or not. The unit weight of the compacted layer up to about 30 cm thickness is found by a gamma probe, based on the attenuation of the intensity of gamma radiation by the soil between the source and the detector. Moisture is determined by detecting the slow neutrons reaching the detector through the soil layer from the source of fast neutrons. Devices such as these available on the market have proven reliable and effective.
