ENV-2E1Y FLUVIAL GEOMORPHOLOGY
Examples of Previous Examination Questions and notes on how to solve numeric questions for Slope Stability and Related Topics.
Exam Questions Pre-1999
ENV-2E1y -GEOMORPHOLOGY
Selected Examples of previous examinations questions.
The following are a selection of the questions which have appear over the years in Geomorphology.
You should note that the previous Course Number was ENV-2B07, before that ENV 264, and prior to that it was ENV 210
Note: in several years similar types of question have been set. Accordingly the wording and values in any one question will not necessarily be exactly as set, but will cover the full range of possibilities. Sometimes only parts of former questions are given below, as actual full questions have been developed from aggregating different combinations in various years. YOU SHOULD ALSO NOTE THAT SEVERAL WORKED EXAMPLES WERE GIVEN IN THE HANDOUTS, AND THESE SHOULD BE CONSULTED TO.
A. Numeric Questions - as actually set
1. Describe how you would assess the consolidation characteristics of a soil.
A layer of homogeneous clay (unit weight 18 kNm-3) is 9 m thick and underlies a 7m layer of sand (unit weight 19 kNm-3). The water table is 3m below the surface, while beneath the clay is a permeable, incompressible layer of sand.
If the water table falls to the interface between the upper sand and the clay, estimate the reduction in thickness of the clay layer given the following data relating the coefficient of volume compressibility (mvc) to stress level:-
stress mvc
(kPa) (kPa-1)
100 0.00768
120 0.00597
140 0.00461
160 0.00358
200 0.00210
250 0.00170
The unit weight of water may be approximated to 10 kPa.
[Numeric Part 60%] - this question was set in 1992 and solution follows example given in section 3.9.2 of handouts.
2. A 20m high slope is shown in Fig. 1. Estimate the factor of safety along the potential failure surface if the soil has the following properties:-
f = 20o
c = 20 kPa The actual figure was just a general profile of a slope
g = 19.3 kNm-3
Estimate the depth to which tension cracks could develop at the slope crest, and the effect these would have on the stability of the slope.
Clearly state the assumptions and limitations of the method of analysis you use, and indicate how you would improve the estimates of the factor of safety.
[ numeric part 70%] This question was set in 1992 and a very similar one in 1989 In 1989, (40 minutes allowed) the areas of the slices were given as shown in Fig. 1 as were the positions of the slices. In 1992, neither of these were given as 60 minutes was now allowed for the question. The solution begins by following exactly the procedure used in the Slope Stability Practical. However, one can estimate the depth of tension cracks from
and if one is cunning, one of the slice boundaries would intersect the failure surface at exactly this depth below the surface - see sections 5.7.7 and 5.8.6 of handout. Two effects of tension crack. (1) a reduction of the length of the failure surface - i.e. neglect all slices to right of tension crack and recalculate; (2) water filling crack will cause a lateral pressure tending to increase likelihood of failure.
Fig. 1 Question 2 Profile and Geometry of Slope
In 1989 the following data about the weight of the various slices was given. In 1992 you were expected to evaluate these as 50% more time was allowed.
Slice Number / Area of Slice (m2)1 / 18.2
2 / 43.2
3 / 47.2
4 / 27.8
5 / 2.4
3. A 4 m thick layer of fine sand of unit weight 19 kN m-3 overlies a 1.2 m thick layer of clay (16.67 kN m-3). The water table is at the surface. The results from a one-dimensional consolidation test on a sample of the clay from 4.6 m below the surface are as follows:-
10 / 1.450
20 / 1.425
40 / 1.400
60 / 1.388
90 / 1.340
160 / 1.170
320 / 0.970
480
/ 0.850
What is the overconsolidation ratio of the soil?
If an additional layer of sand 5 m thick (unit weight 16.4 kN m-3) were now deposited on the surface, how much settlement could be expected in the clay layer?
(Take the unit weight of water to be 10 kN m-3).
see Question 5: 1980. First plot up graph on log paper. Where there is a kink in the curve, this indicates the previous maximum stress in the soil. Now work out the present in situ stress (see section entitled Estimation of effective vertical stress at depth in handout). The ratio of maximum to in situ stress gives the over consolidation ratio. Now work out the new stress (after adding sand) and use graph to work out change in voids ratio. Finally convert this change in voids ratio to a change in thickness
4. A 6 m layer of a sandy sediment is underlain by a thick layer of clay. The area is known to have been glaciated at some point in the past. Results from a standard consolidation test on a sample of clay from 10 m below the present ground surface are summarised in the table below.
Comment on the shape of the curve and estimate the over consolidation ratio of the sample in the field if the water table is 7 m below ground surface. The unit weight of the clay is 15 kN m-3 and the degree of saturation of the sand is 6.3%, and its voids ratio is 0.8. Estimate the overlying thickness of ice if, during glaciation, the water table was at the sediment surface. The following assumptions may be made:-
(i) The sand sediment is incompressible for stresses up to 5000 kPa,
(ii) The unit weight of water is 10 kN m-3
kPa / voids ratio60 / 1.402
120 / 1.380
240 / 1.357
480 / 1.279
960 / 1.173
1920 / 1.068
3000 / 1.000
300 / 1.075
30 / 1.150
see Question 1: 1981. This question is similar to question 4 above except that there is an added complication that the sand is only partially saturated. The initial in situ stresses can be found using the various formulae in the basic set of definitions. Once both the in situ and previous maximum stresses have been found, the difference can be related to the overlying burden of ice (unit weight of ice is in data sheet). One point to watch though is that the sand is fully saturated during the glaciation period, so allowance for this extra buoyancy must be made.
5. With the use of sketches to illustrate your answer, describe how a clay consolidates.
Partially processed data from a consolidation test undertaken on a soil sample taken from a depth of 16m below the surface at the position show in the simplified bore hole log (Fig. 2) are show in the following data.
normal stress sample thickness
(kPa) (mm)
10 28.00
25 27.80
50 27.65
100 27.50
200 27.16
400 26.40
800 25.65
1600 24.90
3200 24.14
At the end of the test the voids ratio was 1.414.
Determine the over-consolidation ratio of the soil, and the voids ratio associated with the previous maximum pressure.
If previous changes in the vertical pressure have only occurred through changes in the water level, determine the lowest level the water table has reached.
Describe how the shear behaviour of a sample located at position A in Fig. 2 would differ from that tested.
see Question 3: 1990 - this question requires the manual solution of the consolidation data as done in the practical write up by many of you - see practical sheet handout. The key point for the last past of the question is that the overconsolidation ratio of the sample at A will be much higher than that at the base - you will need to compute the values. Then noting the comments in section 4.9 of the handout you can make statements about the likely behaviour of the respective samples under shear. Remember the critical value of OCR is 1.7
Fig. 2 [Question 5]
6. Indicate how the Atterberg Limits of soil may be determined. Indicate how a knowledge of these limits may be used to predict the shear and consolidation behaviour of the soil.
A uniform bed of clayey soil having a specific gravity of 2.65 covers an area which has been glaciated in the past. Apart from partial desiccation of the top 0.625 m to a degree of saturation of 61%, the soil stratum has remained undisturbed since the retreat of the glacier. A sample is taken from 2.125m below the surface and found to have a moisture content of 40.0%. Estimate the in situ unit weight of the sample of soil, and also the in situ stress on the sample (you may assume that the water table is also 0.625m below the surface, and that the unit weight of water is 10 kN m-3).
The liquid limit is measured at 54.7% while the plastic limit is 33.2%. Plot these points on a suitable graph, and hence, or otherwise, estimate the undrained shear stength of the sample of soil in situ. Would you expect a drained test on the sample starting from the in-situ stress conditions to expand or contract?
In a laboratory consolidation test, the following results were obtained. If the water table was initially at the surface during cover by the glacier, estimate the maximum thickness of ice.
TABLE 1
stress void ratio
(kPa)
20.0 1.060
40.0 1.048
60.0 1.041
80.0 1.039
100.0 1.011
120.0 0.989
140.0 0.970
160.0 0.953
180.0 0.939
125.0 0.946
98.0 0.949 ------end of test
At the final stress indicated in the consolidation test, the sample is sheared in a drained test. Estimate the change in volume during the shearing.
Comment on the results.
see Question 3: 1993 - First evaluate in situ unit weight of the soil using the general definitions formulae (the soil is full saturated), and then work out the current in - situ stress. Plot the Liquid Limit and Plastic Limit information as a Liquidity index plot against shear strength remembering the key values of 1.7 and 170 kPa for the shear strengths at the two limits. From this get the in situ shear strength. Now plot the consolidation data, and plot the point corresponding to the in situ voids ratio and stress. You will find this plots below the normal consolidation line. Now plot the Critical Stress Line which is parallel to the normal consolidation line and displaced towards axis by a factor of 1.7 (the critical OCR). In a drained test, the stress path will move either vertically upwards or downwards until the critical state line is reached. Read of the change in voids ratio and convert this to a change in thickness (i.e. proportion reduction in thickness
= ) and hence from initial thickness of clay layer, work out settlement.
7) Describe how you would assess the consolidation characteristics of a soil. [1995 paper]
A region of Holocene deposits between Acle and Yarmouth has a borehole section as shown in Fig. 3. Originally the water level was at the surface, but as some time in the past drainage took place and the current mean water table is located at the base of the upper sand. There is no evidence to suggest that further shrinkage is taking place in the upper clay, while the upper sand may be considered as being incompressible.
The borehole was drilled with a piston sampler and several undisturbed samples were taken for unit weight determination, the results of which are also shown on Fig. 3. A sample for consolidation was taken from mid depth in the lower clay layers and a piezometer installed at the same depth. The equilibrium water level in the piezometer tube was initially found to be 1.548m below present ground level and found to drop to 1.706m below ground level after 10 years. Results from the consolidation test are shown in Fig. 3.
As part of the environmental reconstruction of the area, a sample of the surface clay was reconstituted at high water content in the laboratory and allowed to settle in a tall sedimentation tube. The mean bulk unit weight of this sample was measured at 14 kN m-3.
Estimate how high above the present ground level was the original ground surface before drainage started, and also estimate approximately how long ago this was. Have far into the future can it be expected that 90% of consolidation has been completed, and how much will be this additional settlement.? Clearly state all assumptions you make.
The unit weight of water may be assumed to be 10 kN m-3
Fig. 3 Borehole log through sediment
TABLE 1. Consolidation characteristics of the soil.
Effective Stress(kPa) / Voids Ratio
5 / 2.202
10 / 2.172
20 / 2.142
40 / 2.112
80 / 1.918
160 / 1.638
320 / 1.359
640 / 1.080
[Hint: divide the lower clay into three sections] - we have covered most of this question in lectures
[Description 20%; Assumptions 10%; Calculation 70%]
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8. Describe the tests you would carry out to determine the consolidation characteristics of a soil.