ENV-2E1Y Fluvial Geomorphology: 2002 - 2003
Assessment for Course Work Item 2
1. INTRODUCTION
This document summarises the requirements for Course Work Item 2.
Please note that in the Practical Handout the deadline was given as week 2 of next term. This is incorrect as the Handout was prepared in early summer before the deadlines were approved by the Assessment Co-ordinator. The correct deadlines are posted on the Assessment Committee Noticeboard, on the WEB Pages and are as indicated below.
The requirement for this section of the course is that one of the following practicals forms the basis of a write-up:
1) Atterberg Limit Practical
2) Consolidation Practical
3) Shear Strength Practical
4) Slopes Stability Practicals (2)
Please note
1) that most of the practicals involve cross reference to another practical at the analysis stage.
2) this item of course work is not just a write-up (indeed the description of the method should not be included as it appears in the handout). You will be expected to analyse your results according to the specific requirements for each practical and in most cases use or results to make predictions of the behaviour of the material in other situations.
3) In all cases you will be expected to demonstrate that you have read outside the formal part of the course and to be able to objectively review the assumptions inherent in the methods. Credit will be given particularly if you show realism- i.e. if you indicate a deficiency in the assumption, then you should indicate whether it would ever be practical to eliminate the assumption, and perhaps give an idea of what sort of error you might expect from the assumption.
4) some of the practicals have the opportunity to get bonus marks. A maximum of 5% bonus will be awarded, and this will be used to scale the basic mark already awarded. I.e. if you received a mark of 65% and a bonus of 3%, then the aggregate mark would be 66.95 (or 65*1.03).
If you choose to write up any of the following (i.e. the default option), the deadline is 17:00 on Friday Week 12 - i.e. it is handed in in class.
a) Atterberg Limit Practical
b) Consolidation Practical
c) Shear Strength Practical
Unless you indicate to NKT by Friday Week 11, it will be assumed that you will be writing up one of these practicals.
Deadline: Friday Week 12 in practical
If you choose to write up the practicals in Semester 2,, then the deadline is Wednesday week 3. However, no dispensation can be given for bunching of coursework with other courses. You must specifically opt for this option and those so opting will have the deadline of
WEDNESDAY Week 3: Semester 2
2. ATTERBERG LIMIT PRACTICAL
2.1 CALCULATIONS
2.2 LIQUID LIMIT
First determine the true moisture content from all the over dried samples.
a) Casagrande Method
Use Logarithmic - Linear graph paper to plot results (with the linear scale as the Y - axis and log scale as the X - axis). Plot the moisture content as the Y - axis against the logarithm of the number of blows. You should have about 5 readings. Draw the best fitting straight line (if you are writing up the practical you should fit the line by regression) and read off the moisture content corresponding to 25 blows. This, by definition, is the Liquid Limit.
b) Fall Cone Method
Use linear - linear graph paper to plot the results from this determination, plotting the penetration as the Y - axis and the moisture content as the X - axis. Plot the best straight line through the points (obtained by regression if you are doing the write up), and read off the value of moisture content corresponding to a penetration of 20 mm. This is the Liquid Limit.
2.3. PLASTIC LIMIT
Obtain the average of readings of the moisture contents used to determine plastic limit. This is the plastic limit.
2.4 WRITE UP
If you choose to write up this practical, you should complete all the computations etc. described above and also compute the derived indices e.g. Plasticity Index etc. (see lecture notes). Further you should estimate the slope of the consolidation line from your results and compare it with that measured in the consolidation practical. You are also expected to critically review the advantages of the two methods for Liquid Limit Determination (see for example the article by Wood - reference in lecture notes). I shall expect to see evidence of reading from textbooks and comments about soil classification in general.
You will also be expected to include printouts of the computer analysis of the Consolidation and Shear Box Practical as part of your report, although you do not have to do anything more on them.
There may be additional items which may be of help for the write-up . details of these will be posted on the WEB Page.
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3. CONSOLIDATION EXPERIMENT
3.1 WRITE-UP
If you choose to do this write-up you must complete the following computations. Further I shall expect you to discuss the relevance of the assumptions made in the classical theory of consolidation making reference where relevant to lecture notes and other texts.. You should also include comments on how you believe the soil fabric might react during consolidation.
You should also include the colour print outs from the Shear Box analysis, although you do not have to comment on them (5% of basic mark will be given for this).
A) Plot the settlement against time curve for the increment completed in the laboratory period, and hence estimate the time for completion of primary consolidation. Compare the result obtained with that from the settlement against the log-time curve. Comment on the results.
B) For the complete series of loads, plot the voids ratio (e) against log s and e against s. In a later practical you will be able to use a computer program to do the preliminary calculations for you. otherwise you can use the manual method as described below. Comment on the shape of these curves and the validity of the assumptions made in consolidation theory.
C) Using the values of cvc and mvc calculated from your results, estimate the permeability of the sample when it was first loaded to 20 lb.
D) Estimate the time to complete primary consolidation of a 3m layer of the soil having a previous stress history to that tested in the laboratory when loaded as follows: Initially the top of the layer is 8m below the surface of the ground and 5.17m below the water table. There is sand above the layer of unit weight 17 kNm-3. The top 1m of sand is removed and fill placed to a height of 5.5m. The unit weight of the fill is 18.59 kNm-3.
E) From the e - log s graph estimate the gradient of the virgin consolidation line, and compare the value with that estimated in the Atterberg Limit Practical.
NOTE:
1) Loads on hanger are in lbs
2) All weights are in grams
3) All lengths in table are in mm (Take care of units when evaluating reduced height)
In the last few years, a computer program has been written to analyse much of the data. However, you may wish to try doing the calculations by hand so that you understand what is going on (particularly if you are writing up this practical). If you decide to rely on solely on the computer print out you can get a maximum of 90% for the write up. If you also complete the manual calculations, a further 10% is available.
You should first work out the final voids ratio and reduced height (ho) as shown below and enter these in the bottom row of the Table below.
Weight of Dish (W1) = g
Note: enter values in grams otherwise there may be confusion in later calculations.
Weight of Dish + wet clay (W2) = g
Weight of Dish + dry clay (W3) = g
W2 - W3
Moisture content (m) = ------=
W3 - W1
(the above is sometimes expressed as a decimal for use in the equations following. However, when reporting a value at other times it is often expressed as a percentage (i.e. multiply m by 100).
hence void ratio e = mGs = 2.65m =
reduced height = cm. = mm.
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hence h1 = eho = H = h1 + ho = mm.where d = diameter of consolidometer (8.55 cm) and gs is the unit weight of the solid particles (i.e. 2.65 gm/cc).
First enter the values of e and ho into the highlighted boxes. The value of ho is constant so replicate the value in all rows. Now evaluate the settlement rn in the bottom row (make sure you watch the signs: - think about what is happening: should the increment of settlement be negative or positive for the particular increment). Now work out the new value of h2 in the penultimate row = (h1 +r1)., From this it is a simple matter to complete the remainder of the penultimate row (e.g. the new voids ratio etc.), and from this the settlement in the previous increment may be computed and so on. So calculation begins in the bottom row and works progressively up the table until all columns are complete. You may wish to try this to check that you get the same results as the program as part of your revision for the examinations.To start the calculation, enter values of n in the increment column: enter 1 in the bottom row, 2 in the next row up and so on. These values of (n) are the ones referred to as subscripts in columns 7, 8 and 9 in the table below.
Dateand
Time / stress
(kPa)
* / Dial Reading / Total Settlement
(mm) / Increment of settlement
(mm)
rn / n / hn =
hn-1
+rn-1 / Ho reduced height (mm) / Total thickness
H =
ho + hn / void ratio
e
(1) / (2) / (3) / (4) / (5) / 6 / (7) / (8) / (9) / (10)
6
5
4
3
2
1
· To convert load in lbs to stress in kPa multiply by 8.525 (this is related to the geometry of the consolidometer).
3.2 Bonus Marks
If you wish you can earn up to an additional 5% (i.e. the maximum mark would be 105%) by considering the following. NOTE: The bonus will be up to 5% of the mark already awarded to you. In the past bonus marks of between 1 and 4% have been awarded.
Explain how you would use a Spreadsheet to automatically analyse the results from this practical. It does not have to be as sophisticated as the one used, merely an illustration of what formulae you should use in which box. There is a slight catch because at the outset you do not know the number of data lines to add. Think about it....
One final point about this assignment:. This is not just a Practical Write-Up. It includes that as a component part, but several problems have been included to give you practice in manipulating data from your results. Finally, we will be expecting you to critically, objectively, and realistically review the procedure for determining the consolidation behaviour of sediments and what can be deduced from the results.
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4. MEASUREMENT OF SHEAR STRENGTH
4.1 WRITE_UP
Your write-up should include the following calculations and comments etc.
1) Present the two graphs listed above for each of the two tests. In the write-up you should not only allow for any offset in taking up the slack, but should convert the axes from the raw readings) so that the Y-axis in the first graph now displays shear stress and the X-axis deformation (this will be the % displacement from the original length of 60 mm). For the displacement graph, the Y-axis should be converted to a % volume change and X axis should be deformation.
2) Estimate the voids ratio at the beginning and end of each test.
This year, the calculations are done automatically for you on the computer program. You should also attempt these by hand to confirm.
3) Comment on the shape of the curves, and the values of e. How do the two tests differ? Is this what is expected? What refinements could be applied to the analysis to improve the accuracy of the results?
4) You will have obtained data from just one normal load for both the loose and dense sample. Other groups will be using different normal loads. During the practical when all results are computed, the final values from all members of the class are stored in a common file which all members of the class can access. This also includes data from previous years (for earlier years, all suspect readings have been removed). There should essentially be two groups of sample:- one: dense group and one loose group. Plot the complete set of data points for the class for the dense samples as shear strength against normal load, and repeat for the loose samples. By determining the best fitting line to the points, estimate if any cohesion is present, and also what the angle of internal friction is. NOTE: unless you use the same scale for both axes, the angle measured will be incorrect (unless of course you compute the angle trigonometrically).
5) Critically review the different methods available for testing the shear strength of soils both in the laboratory and in the field giving the advantages and disadvantages of the various methods. [I shall be looking for evidence of external reading in this section].
6) What modifications would be needed to the test to investigate the shear behaviour of clays?
7) What problems do you see in measuring the shear strength of soils from the field, rather than laboratory prepared ones as used in this practical?
8) It is hoped that there will be a demonstration of the triaxial test in a lecture period. Two sets of tests will be done, one drained, and one undrained. Your write-up should include a discussion of these tests. Some notes on this test are included at the end of this handout.