Investigation in to Some Dynamic Properties of Soils Around Ziway
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Date
2015-02
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Addis Ababa University
Abstract
The town of Ziway is located in South Easter part of Ethiopia. It has a latitude 08’00
N and longitude 38’43E with the elevation between 1640 and 1670 meter above sea
level. The topography of the town is almost flat. The town is located in seismic zone
four according to EBCS 8, 1995. Therefore, the objective of the research is to
investigate some dynamic properties of soils found in Ziway town. To achieve its
objective samples from different parts of the city were collected and laboratory tests
were done on the collected samples.
The index property investigation in this study includes; particle size distribution,
Atterberg limits and specific gravity. The grain size analysis test result showed that
the dominant proportion of soil particle in the research area is silt and silty sand,
which have clay content ranging from 1 – 28%, silt fraction 37 - 47%, sand fraction
24 - 57% and gravel content from 0.8 – 2.0%. Liquid limit ranging from 30 – 33%,
plastic limit ranging from 26 – 29%, plastic index from 0 – 7% and specific gravity
ranging from 2.50 to 2.62.
According to the Unified Soil Classification System, most of soils found in Ziway
town are silt and silty sand. A series of cyclic laboratory tests were conducted at
field density. The tests were conducted as a function of cyclic strain amplitude of
0.01 % to 5.0 % under an axial stress of 100 kPa, 200 kPa, 300 kPa and 400 kPa. The
stress strain responses were recorded, and the shear modulus and damping ratio
were obtained for each soil sample. The value of shear modules is in the range of
1.0 to 8MPa for medium strain level (0.01 %< γ <0.1 %) and 8.0 to 25MPa for higher
strain level (γ >0.1 %). The damping ratio values range between 2.85 to 10 % for
medium strain level (0.01 %< γ <0.1 %) and 10.0 to 27.0 % for higher strain level
(γ >0.1 %).
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Additionally those parameters are compared with sand and saturated clay soil
curves developed by Seed H.B and Idriss (1970), Hawassa silt soil studied by
Ayalew G. (2011) and Adama silt and silty sand soil studied by Abu G. (2011).
The results have shown that the obtained normalized shear modulus values are
lower than those suggested by Seed H.B and Idriss (1970), especially at lower strains
(γ ≤ 0.01 %). For higher strains the obtained normalized shear modulus values agree
with those suggested in literature. This shows that testing conditions and sample
preparation appear to have significant effect on the shear modulus values, especially
at small strain level (γ ≤ 0.01%).
The obtained damping ratio values are almost lower than the sand soil curves
developed by Seed, H.B. and Idriss,I.M(1970). But those values are in the range of
saturated clay by Seed, H.B. and Idriss,I.M(1970). This shows that the position of
damping ratio curves for silt and silty sand soil results are in between the curve for
sand and clay. Additionally those values are in good agreement with the curves of
local soils of Hawassa and Adama.
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Earthquake Engineering