UNIVERSITEIT TWENTE
Faculteit CTW/ CivT: Afdeling (BInfra)
TENTAMEN: GRONDMECHANICA (VAK: 226565)
2 februari 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.
. Alleen het gebruik van het boek SOIL MECHANICS & FOUNDATIONS van M. Budhu. (of SOIL
MECHANICS van R. F. Craig) is toegestaan: het gebruik van alle andere studiematerialen en
PC’s niet.
Tentamen is opgesteld door Dr. ir. U. F. A. Karim
QUESTIONS:
DRAW A CLEAR DIAGRAM OF THE PROBLEM IF RELEVANT AND WRITE NEATLY AND CLEARLY.
PAY ATTENTION TO THE ANSWERS UNITS [ ], DECIMALS (--.-- ): MAXIMUM 2DECIMALS, SIGNS (+/-).
Qu. 1. The wet weight of 500 cm3 of soil is 8.7 N and its dry weight is 7.66 N. Assuming the
specific gravity is 2.7 and the unit weight of water is 9.8 kN/m3, determine:
(1) the water content [%],
(2) the degree of saturation [%],
(3) the void ratio [-],
(4) the bulk unit weight[kN/m3].
(5) the solids unit weight [kN/m3]
Qu.2. A cylindrical oil storage-tank with a circular base poses axi-symmetric vertical stress
increase of 60 kPa and lateral stress increase of 24 kPa under the centerline of the tank at the
center of a 3 m thick elastic soil layer. Assuming that the soil has an elastic modulus of 5000
kPa, determine:
(1) the maximum increase in vertical strain at the centre of the layer (Poisson’s ratio =
0.3) [%],
(2) the corresponding vertical displacement [mm],
(3) the volumetric strain at the instant of loading at the centre of the layer (assume
Poisson’s ratio of 0.5) [%] and the corresponding volume change [mm3]
(4) Which of the situations (1) or (3) above can be classified drained (D) and which as
undrained (U). State at least 4 conditions (boundary loading and/or drainage conditions
and/or soil properties) in a soil mass corresponding to the U situations.
Qu. 3. In a 2-layered construction ground, the following is given from a laboratory and a field
investigation:
Top soil layer I: Sand, 6 m thick, water table 1 m below ground level, void ratio 0.5, degree of
saturation above the ground water table is 40 % and 100 % under.
Bottom soil layer II: Clay, thickness: deep, water content 80 %, degree of saturation 100 %.
Assuming the specific gravity for both soils is 2.7 and the unit weight of water is 9.8 kN/m3,
determine:
(1) the effective stresses at depths of 1 m, 6 m (layer I) and 12 m (layer II) [kPa],
(2) the over consolidation ratios [-] at these depths assuming a maximum past geological
stress on the current ground level equivalent to 2 times the current weight of the top
soil layer.
Qu. 4. A 3 m thick clay layer has the following characteristics: saturated unit weight 18
kN/m3, over consolidation ratio 7, initial void ratio 1.03, fully-saturated, compression and
rebound parameters respectively Cc = 0.26 and Cr = 0.03, the consolidation parameter Cv =
0.12 m2/yr. This layer is overlain by 3 m thick dry sand with a dry unit weight 17 kN/m3. A
square foundation 2 m x 2 m directly placed on the ground carries a load imposing a stress at
the contact with the sand layer of 260 kPa. Calculate the following:
1) i- final effective stress at the centre of the clay layer due to the initial soil overburden
and foundation caused stress change of 23kPa at a depth of 4.5 m [kPa], and, ii- the
pre-consolidation effective stress [kPa],
2) final primary consolidation settlement [mm] of the clay layer using the suitable
calculation equation corresponding with answers from 1,
3) the displacement [mm] and the time [days] corresponding to 90 % consolidation in
the field: in a laboratory test on the same clay a sample 20 mm thick with double
drainage is tested and 90 % consolidation occurred in 380 minutes.
4) Check with some method if Cv value corresponding to the laboratory results is correct.
Qu. 5. To determine the strength properties of a clay soil (unit saturated weight 20 kN/m3) a
series of shear-box CU tests (consolidated-undrained) is performed. For the first test, the
normal pressure is 36 kPa and the sample fails when the shear stress is 13.2 kPa. The second
soil sample is tested under a normal stress of 72 kPa, and failed when the shear stress is 24
kPa. The third sample is tested under a normal stress of 120 kPa and failure occurred when the
shear stress is 38.4 kPa. From these data estimate:
a) effective cohesion [kPa]
b) effective shear angle [o]
c) the failure shear stress [kPa] of the same soil in the field at a depth of 10 m where the
ground water table is at the ground surface.
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