Using the calculations for the plastic limit and liquid limit; the Plasticity Index value can be calculated and therefore the soil can be catalogued.
Discussion
During the experiment several errors could occur. One of the basic errors is misreading the dial gauge scale for the Penetrometer test, fortunately we where briefed thoroughly about the scale from our tutor so that was avoided.
The Penetrometer cone is a crucial part of the apparatus when undertaking the penetration test to work out the liquid limit. These cones have to bee sharp so that they can adequately penetrate the soil sample. Before carrying out the test the penetrometer cone was checked and gauged to the British Standard (as stated in BS 1377). So the overall reduced penetration error, which the blunt penetrometer cone would have caused was avoided.
Also when carrying out the penetrometer test, when you fill up the 55mm diameter cup you have to make sure that creation of air holes are avoided while carrying out that required procedure. It is quite right to say that everything was done to avoid this incident but there are no guarantees that this was avoidable.
When emptying out the 55mm diameter cup and then refilling with a more penetrable soil sample (due to addition of distilled water), there could have been and error of a soil sample, which was not used in previous penetration and which was left on the glass plate, had not been covered to avoid evaporation. This then was added with the penetration soil sample to be used after sample had been distilled. This could then reduce the penetration of the penetrometer cone and affect our results.
Another error which could occur is when carrying out the plastic limit test. When rolling the sample to a 3mm diameter thread, if excessive force had been applied and a wrong rolling method undertaken the soil sample would then break but unknown to the roller who would assume that the soil plastic limit breaking conditions had been reached.
A major error of heating the sample could have occurred while conducting this experiment. Due to the time restrictions that we where under we decided to heat the samples by microwave. When referring to BS 1377 Classification test it says that microwaves should not be used to dry samples. BS 1377 states: - “A microwave oven should not be used for the determination of moisture content by the definitive method for soils containing clay or organic matter because of the difficulty of ensuring that the temperature of the soil does not exceed 110ºc before the water is removed. Higher temperatures than this can alter the properties of clay material”. Even though we obtained the results it is hard to say if they are accurate. Even though this has occurred I would like to carry out the rest to identify what our findings produce.
Experiments where done really carefully to avoid all the errors above and to produce the best results as possible to allow us to classify the soil sample.
Prior to designing any structural components (i.e. foundations for a building or structure) the properties of the subsoil(s) must be assessed. These processes can also be carried out to confirm the suitability of the proposed component (i.e. foundation). Soil assessment can include classification, grading tests to establish shear strength and consolidation. The full range of methods for testing soils is given in BS 1377.
It is necessary to provide a conventional classification of types of soil for the purpose of describing the various materials encountered in site exploration. The system adopted needs to be sufficiently comprehensive to include all but the rarest of natural deposits; while still being reasonable, systematic and concise. Such a system is required if useful conclusions are to be drawn from the knowledge of the type of material. Without the use of a classification system, published information or recommendations on design and construction based on the type of material are likely to be misleading, and it will be difficult to apply experience gained to future design. Furthermore, unless a system of conventional nomenclature is adopted, conflicting interpretations of the terms used may lead to confusion, rendering the process of communication ineffective. To be sufficiently adequate for this basic purpose, a classification system must satisfy a number of conditions:
- It must incorporate as descriptions definitive terms that are brief and yet meaningful to the user.
- Its classes and sub-classes must be defined by parameters that are reasonably easy to measure quantitatively.
- Its classes and sub-classes must group together soils having characteristics that will imply similar engineering properties.
There are several ways in which a soil may be classified: by geological origin, by mineral content, by grain size or by plasticity. The last two are most widely used by engineers.
Grain Size: - In this system, soils are split into coarse-grained non-cohesive, fine-grained cohesive and organic soils. They are then further subdivided into gravels, sands, slits etc. The division of the coarse-grained non-cohesive soils into gravels and sands is accordance to grain size, which is readily determined by sieving.
Most systems of soil classification depend to some extent upon the distribution of various-sized particles in the soil. For coarse-grained material this distribution may be determined by sieving, and for finer particles a method of measuring the rate of settlement in water is used. The determination of particle-sized distribution by these methods is known as mechanical analysis. The figure below shows the British Standard range of particle sizes. The particle-size analysis of a soil is carried out by determining the weight percentage falling within bands of size represented by these divisions and subdivisions.
Plasticity: - In the case of fine-grained (i.e. cohesive) soils, it is the shape rather than size of particles that has the greater influence on engineering properties. The combination of very flaky particles and circumstances, which may bring about changes in moisture content results in a material (soil) having properties, which are inherently variable. For example, the shear strength of cohesive soils will vary markedly with changes in moisture content. Also, soils with flaky particles behave as plastic material: an increase in applied stress usually brings about an irrecoverable deformation, while the volume remains constant or is reduced and without any signs of cracking or disruption.
Since the plasticity of fine-grained soils has an important effect on such engineering properties as shear strength and compressibility, plastic consistency is used as a basis for their classification. The consistency of a soil is its physical state characteristic at given moisture content. Four consistency states may be defined for cohesive soils: solid, semi-plastic solid, plastic and liquid. The change in volume of a saturated cohesive soil is approximately proportional to a change in moisture content (as illustrated in diagram bellow).
The transition transition from one state to the next in fact is gradual; however convenient to define limits corresponding to a changeover moisture content: -
LL= the Liquid Limit: the moisture content at which the soil ceases to be liquid and becomes plastic
PL = the Plastic Limit: the moisture content at which the soil ceases to be plastic and beomes a semi-plastic solid.
SL= the shrinkage limit: the moisture content at which drying-shrinkage at constant stress ceases.
The experiments that we have undertaken only proceed into the context of the liquid and plastic limit. These two values are the most important stages because these represent the upper and lower stage respectively.
The significance of classification is that the classification of a soil gives us an overall idea of how the soil is going to behave in different circumatnces. It provides us with knowledge of the characteristics of the soil and once classified it can give us information on consolidation and shear stress of the soil therefore determining what forces the soil is capable of withstanding before failure.
It is very important to figure out soil classifiction and therefore workout an overall charateristic of soil. When considering that all structures are initally on soil it is important for us to gather the stability of soil. If we talk in context of geotechnical engineering (e.g. earthworks, embankments, etc), there are forces which cause instability (i.e. sloping surface), which are mainly those associated with gravity and seepage, while resistance to failure is derived mainly from a combination of slope geometry and the shear strength of the soil mass itself. By using classification and grading the shear strength and the consolidation can be worked out and therefore correct soil masses can be implemented at the required locations or an alternative design to the geotechnical structure can be implemented.
Embankments are built by rolling or otherwise compacting layers of selected soil in succession.initally, the compaction process squeezes out air, but as the built-up height increases, the lower layers experience an increase in pore pressure. In coarse-grained soils, the excess pore pressures dissipate quickly. In fine-grained soils, the excess pore pressure is slow to dissipate and consolidation may continue for several years. In the course of time, the pore pressure decreases and the therefore the shear strength increases. Thus, the most critical stability condition for an embankment occurs at the end of construction, or sometimes during construction so it is vital that the soils classifictaion is known and therefore the stability can be calcultaed. Consolidation can lead to settlememt in the soillong after the structure has been built and therefore this settlement could cause extensive damage to the structure in consideration. Also if the shear capacity of the soil is not sufficient there could be a slope failure which could cause a landslide. In consideration to embankments where train loadage is concerned, settlement of embankments due to consolidation could cause track misalignment and therefore cause a risk to train derailment and a serious concern of loss to life. And also a land slide could block train routes having the same affect as mention previously. So therefore it is vital that soil charactersitics are determined and an overall idea of soil behaviour is determined. This all leads to Atterberg limits and classification so soils.
Results
Penetrometer Experiment Test Results (to gather Liquid limit)
Plastic Limit Experiment Test Results
What needs to be done next is to calculate the moisture contents (percentage) for all of the test results that we have gained.
In our case it is: -
M1: - is the mass of the sample bowl (in g)
M2: - is the mass of the sample bowl and wet soil sample (in g)
M3: - is the mass of the sample bowl and dry soil sample (in g)
New set of results: -
Liquid Limit Results: -
Plastic Limit Experiment Test Results
On the graph paper (following page) is the relationship between moisture content and penetration. The best straight line between these points is drawn and the moisture content corresponding to 20mm penetration has been taken as the liquid limit.
The results from the graph have given me the liquid limit (L.L.) of the soil as 56% moisture content.
Plastic limit: -
To calculate the Plastic Limit the average of the two-moisture content readings need to be divided by 2 to, find the average.
Average = 26.3% + 28.5% / 2 = 27.4%
BS standard 1377 states that values should be stated as: -
If lower than 10% to the nearest 0.1%
If greater than 10% reported to nearest 1%
So therefore the plastic limit is 27%
Natural Moisture Content
The natural moisture content is that m.c. which prevails or the material in an equilibrium state in the ground. A moisture content determination must be undertaken on a sample of soil, which has been sealed immediately after sampling. In this case the natural m.c. was found to be about 26%.
Plasticity Index (P.L./ I.p.)
The plasticity index is defined as P.I. or I.p. = L.L. – P.L. this defines the plastic range.
In our case it is: -
Plasticity Index (P.I. or I.p.) = 56 – 27 = 29%.
On the graph paper following is the Casagrande plasticity chart. This chart helps us classify the material.
From the graph that has our plotted values we can determine that this soil sample is a clay with high plasticity.
Liquidity Index
The liquidity index is defined as: -
Where m = natural moisture content
P.L. = plastic limit
P.I. = plasticity index
Therefore liquidity index (L.I.) =
L.I. = -0.034
Activity of to soil
Activity of the soil is defined as: =
For the moment we assume that % clay particles is equal to 40%. This will be explained in the conclusion later where 40% was gained.
Therefore activity of soil =
Activity = 0.725
These results are discussed further in the conclusion to this assignment.
Conclusion
The two most important limits are the liquid and plastic limits, which represent respectively the upper and lower bound of the plastic state; the range of the plastic is given by their difference, and is termed the plasticity index (P.I.). The values gained have been located in the chart below to show the outcome: -
In accordance with the diagram above the natural moisture content of the sample is in the range of when the sample is in a semi-plastic solid state.
Our liquid limit (L.L.) is 56%. The values stated below help us to describe the plasticity of the soil sample: -
Low Plasticity : L.L. <35%
Intermediate Plasticity: L.L. = 35% - 50%
High Plasticity: L.L. = 50% - 70%
Very High Palsticity: L.L. = 70% - 90%
Extremely High Plasticity: L.L. > 90%
In accordance to these results I can state that the soil sample has high plasticity, which will in turn be related to in Casagrandes Plasticity Chart.
The relationship between the plasticiy index and the liquid limit was used in “Casagrandes Plasticity Chart” (which has been drawn on graph paper with our results, on previous pages). The Casagrandes Plasticity Chart is to establish the sub-groups of fine soils. The division between iorganic silts (or organic soils) is by an empriical line (A-Line) the equation of which is Ip= 0.73(L.L. – 20). Clays fall above the line and silts below it. According to our results the soil sample falls into the values of a high plasticity clay. The diagram on the next page describes to us the classification of the soil sample:-
Casagrandes System of Soil Classification
This system has been developed for roads and airfields. The soil type is designated by two capital letters as illustrated by the chart bellow.
According to the chart our soil sample is a CH.
If we refer to the Example of Casagrandes classification for roads and airfields chart (which if the following page) the CH character is stated as having the following properties: -
Soils with liquid limits greater than 50% and generally with clay content greater than 40%. Can be readily rolled into threads when moist. Greasy to touch. Show considerable shrinkage and drying. All highly compressible soils. 40% was used for the clay particles for the activity of the soils due to this suggestion.
Another significant thing to look at is the Liquidity Index. The relationship between the soil’s natural moisture content and its consistency limits is given by the liquidity index (L.I.). Our liquidity index was –0.034.
The significant values of L.I. are: -
L.I. < 0: soil is in semi-plastic solid or solid state.
0 < L.I. <1: soil is in plastic state.
L.I. > 1: soil is in liquid state.
So therefore our soil is a semi-plastic solid or solid. From the limits above we can say that if L.I. is about unity the condition of the material is in the liquid state. If L.I. is about 0 then the material is a semi-plastic solid or solid.
The consistency limits represent the plasticity characteristics of the soil as a whole. Plasticity, however, is mainly determined by the amount and nature of the clay minerals present. The different clay minerals possess different degrees of flakiness. Also, even ‘clays’ may only comprise 40 – 50 per cent clay minerals. The degree of plasticity of the clay fraction itself is termed the activity of the soil, which was calculated previously in the results. Some typical values of activity for some common clay minerals and soils are given in the table bellow. Our value for activity of the soil was 0.725.
Activity of clays
According to our findings our sample is a Glacial clay and loess with mineral of Kaolinite. For inactive clays the index values are typically bellow 0.5. as index reaches unity it displays swelling and shrinking characteristics.
So overall the soil sample is a CH in Casagrandes plasticity chart which means clay with high plasticity. The clay has a semi-plastic solid or solid form and is a Glacial clay with Kaolinite minerals. The soil can be readily rolled into threads when moist. It is greasy to touch. It shows considerable shrinkage and drying and therefore will exhibit possibly damaging shrink/swell characteristics with moisture content changes.