UNIVERSITEIT TWENTE
Faculteit CTW/ CivT: Afdeling (BInfra)
TENTAMEN: GRONDMECHANICA (VAK: 226565)
29 augustus 2006
9.00u – 12:00u
Let op:
. Beantwoord alle 5 vragen. Besteed max. 35 min/ vraag (±10 min. per deelvraag) en laat de
moeilijkste vragen tot het eind. De puntenverdeling is gelijk voor alle 5 vragen (20% per
vraag).
. Alleen het gebruik van het boek SOIL MECHANICS & FOUNDATIONS van M. Budhu. (of
SOIL MECHANICS van R. F. Craig) is toegestaan.
Tentamen is opgesteld door Dr. ir. U. F. A. Karim
QUESTIONS:
DRAW A CLEAR DIAGRAM OF THE PROBLEM IF RELEVANT AND WRITE VERY NEATLY AND CLEARLY.
PAY ATTENTION TO THE ANSWERS UNITS [ ], INTERNATIONAL DECIMAL CONVENSION (--.-- ): USE
MAXIMUM 2 DECIMAL POINTS, PAY ATTENTION TO SIGNS ALSO IN THE ANSWERS (+/-) BASED ON
SOIL CONVENSION (COMPRESSION, DOWNWARD DISPLACEMENT AND COMPRESSIVE FORCES ARE
POSITIVE)
Qu. 1. A clay sample with a mass of 683 g was coated with paraffin wax to protect and
prevent the sample from drying out. The combined mass of the clay and the wax was 690.6
g. The volume of the clay and wax was measured in a calibrated immersion tank (by putting
the waxed sample in water) to be 350 ml. These sample preparations and measurements
were part of a bulk density determination test on the sample. The sample was then broken
open and moisture content and particle specific gravity tests gave respectively 17% and 2.73.
The specific gravity of the wax was 0.89. Determine:
(1) the unit weight of the soil in [kN/m3],
(2) the dry density of the soil also in [Mg/m3]
(3) the void ratio [-],
(4) the degree of saturation in [%]
Qu.2. A 4.5 m square foundation base poses a uniform vertical stress of 200 kPa on a soil.
Determine:
(1) At what depth below its centre is the increase in vertical stress only 20% of the
foundation load (approximately 20kPa) in [m],
(2) the increase in vertical stress at a point 3 m below the foundation and 4 m from its
centre (along the midline of the square-shaped foundation), also in [kPa]
Qu. 3. In a 2-layered construction ground, the following is known from a laboratory and a field
investigation:
Top soil layer I: Sand, 5 m thick, water table is at ground level, unit weight 18 kN/m3.
Bottom soil layer II: Clay, thickness: deep, unit weight 20 kN/m3.
Assuming full saturation and unit weight of water of 9.81 kN/m3, determine:
(1) the effective stresses at depths of z= 5 m (layer I) and z= 10 m (within layer II) below
ground level in [kPa],
(2) the over consolidation ratios [-] at these depths assuming a maximum past geological
stress on the current ground level of 100 kPa.
(3) The effective stress change [in kPa] (above or below!) the existing effective vertical
stress at 5 m depth after a long time due to a slow drop over time in the water table by
2.5 m due to long-term continuous pumping to keep the water level at that depth.
Assume no change in the unit weight of the sand layer above and below the water
table and that no capillary action is observed. The over consolidation ratio is 1.
(4) The effective stress change shortly after loading at 10 m depth resulting from a
sudden rise in the water table by 2.5 m [in kPa] due to failure of a nearby earth dam.
The over consolidation ratio is 1.
Qu. 4. A soft normally consolidated clay layer 15 m thick with water table at ground level has
the following characteristics: saturated unit weight 17.2 kN/m3, over consolidation ratio 1,
moisture content 45%, particle specific gravity 2.68, compression and rebound parameters
respectively Cc = 0.495 and Cr = 0.05. This layer is overlain by a foundation directly placed
on the ground imposing a vertical effective stress increase at the centre of the layer of 10
kPa. Calculate the approximate value of the settlement of the foundation in [mm].
Qu. 5. Consolidated Undrained (CU) shear box tests on a series of soil samples lead to the
following results:
Test no.
1
2
3
4
total normal consolidation stress [kPa]
100
200
300
400
total shear stress at failure [kPa]
98
139
180
222
From these data estimate
(i)
the cohesion in [kPa];
(ii)
the shear angle of friction at failure in degrees [o]
(iii)
Show the Mohr circle for test 2 only. Can this circle shrink and fall below the
failure line?
(iv)
what is the value of the shear angle measured if the 4 tests are carried on
unconsolidated undrained samples (UU test) in degrees [o]
2