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Sand:
high shear strength
small settlement
high permeability (porosity)
Clay
soft, big settlement
low shear strength
low permeability
Darcys Law
Terzaghis Effective Stress
= - u
the effective stress is the controlling stress that determines thedeformation and
failure of soil
Shear strength
Soil Physical Characteristics
soil are formed from physical and chemical weathering of rocks
particle size is used to distinguish various soil textures, such as gravels, sands, silts and clays
a sieve analysis is used to determine the grain size distribution of coarse grained soils
a hydrometer analysis is used to find the particle size distribution of fine grained soils
particles size distribution is represented on a semi-logarithmic plot of percentage passing
versus particle size
physical and mechanical behaviour of fine grained soil is linked to four distinct states in
order of increasing water content
o solid
o semi-solid
o plastic
o liquid
the shrinkage limit (SL) {semi solid and solid}, plastic limit (PL) {semi solid and plastic} and
liquid limit (LL) {plastic and liquid} mark the boundaries of the four states.
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WEEK 2 LECTURE
USCS: unified soil classification system
(USA)
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Soil Types according to formation
alluvial soils
are fine sediments that have been eroded from rock, transported by water, and have settled
on river and stream beds.
collovial soils
are soils found at the base of mountains that have been eroded by the combination of water
and gravity
calcareous soil
contains calcium carbonate
expansive soils
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are clays that undergo large volume changes from cycles of wetting and drying
glacial soils
are mixed soils consisting of rock debris, sand, silt, clays and boulders
glacial till
is a soil that consists mainly of coarse particles
marine soils
are sand, silts and clays deposited in salt or brackish water
mud
is clay and silt mixed with water into a viscous fluid
Soil Physical Properties
water content
void ratio
porosity
degree of saturation
dry unit weight
specific gravity
bulk unit weight
Composition of Soil
V=Vs+Vw+Va
Vv=Vw+Va
W=Ws+Ww
W=weight
V=volume
Vv=volume of void
Definitions and Relationships
Water content: w=
(the ratio of the weight of water to the weight of solids)
Void Ratio:
(the ratio of the volume of void to the volume of solids)
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(the denser the soil, the smaller the void ratio)
Porosity:
(the ratio of the the volume of void to the total volume of soil)
Degree of saturation:
(the ratio of the volume of water to the the volume of void)
specific gravity:
(is the ratio of the weight of the soil solids to the weight of water of equal volume)
bulk unit weight: (
)
Soil type (kN/m3) (kN/m3)Gravel 20-22 15-17
Sand 18-20 13-16
Silt 18-20 14-18
Clay 16-22 14-21
dry unit weight: (
)
Physical Characteristic of Soils
Relative density:
Void ratio:
(the denser the soil, the smaller the void ratio)
Description0-15 Very loose
15-35 Loose
35-65 Medium
65-85 Dense
85-100 Very Dense
Comparison of Coarse Grained and Fine Grained Soils for Engineering Use
Coarse grained soils
good loadbearing capacities
good drainage qualities
strength and
stiffness
increases
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not significantly affected by moisture changes
engineering properties controlled by grain size and their structural arrangements
Fine grained soils
poor loadbearing capacities poor drainage qualities practically impermeable
change volume and strength with moisture changes
frost susceptible
Particle Size Distribution (Sands)
soil passed through sieves of different sizes
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Apart from use for soil classification purpose, particle size analysis are also used to select
o aggregates for concrete
o soils for construction of dams and highways
o soils as filters and material for grouting and chemical injectiono soils as engineered fill materials.
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Particle size of fine graded soils
Physical State and Index Properties of Fine-Graded Soils
can exist in one of four distinct states
o solid
o semisolid
o plastic
o liquid
wateris the agent that is responsible for changing the state of soils
Plasticity index
o PI=LLPL
Liquidity index
o LI=
o gives quantitative indication of the strength of a fine grained soil
at a specific water content
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The liquid and plastic limits are also called Atterberg limits
Liquid limit is defined as the water content at which the groove cut will close following 25
blows
plastic limit is found by rolling a small clay sample into threads and finding the water content
at which threads approximately 3mm diameter will start to crumble
experiment results from around the world shows that clays, silts and organic soils lie in
distinct regions of a graph of plasticity index versus liquid limit.
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Unified Soil Classification System
(USCS)
First and/or second letters Second
letter
Letter Definition
G gravel
S sand
M silt
C clay
Examples:
GW indicates soil is well graded gravel with little or no fines
GC- clayey gravel
SP- poorly graded sand
CL- clay soil with low plasticity
CH-clay soil with high plasticity
Typical values of Atterberg limits
sand is non-plastic
clays have higher LL, PL and PI
Letter Definition
P poorly graded (uniform particle sizes)
W well-graded (diversified particle sizes)
H highplasticity
L low plasticity
O organic
http://en.wikipedia.org/wiki/Gravelhttp://en.wikipedia.org/wiki/Gravelhttp://en.wikipedia.org/wiki/Sandhttp://en.wikipedia.org/wiki/Sandhttp://en.wikipedia.org/wiki/Silthttp://en.wikipedia.org/wiki/Silthttp://en.wikipedia.org/wiki/Clayhttp://en.wikipedia.org/wiki/Plasticity_(physics)http://en.wikipedia.org/wiki/Plasticity_(physics)http://en.wikipedia.org/wiki/Plasticity_(physics)http://en.wikipedia.org/wiki/Organic_materialhttp://en.wikipedia.org/wiki/Organic_materialhttp://en.wikipedia.org/wiki/Organic_materialhttp://en.wikipedia.org/wiki/Plasticity_(physics)http://en.wikipedia.org/wiki/Clayhttp://en.wikipedia.org/wiki/Silthttp://en.wikipedia.org/wiki/Sandhttp://en.wikipedia.org/wiki/Gravel8/12/2019 Geotechnical Notes
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How to obtain soil samples for testing?
ground investigation
Investigation Methods
trial pits or test pits
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Lecture 4: Compaction
Compaction principle:
The objective of soil compaction is to reduce the void in soil to minimum
Ideal: build structures on dense soils becauseo dense soils are stronger
o result in lower settlement than loose soils
to increase the density or unit weight of a soil
o by examining the dry unit weight expression
the only variable is void ratio e
Field Compaction Equipment
(a) Sheepsfoot Roller
o good for fine-grained soils
(b) Drum Roller
o good for coarse grained soils
Dynamic Compaction
primarily involves process of dropping heavy weight on the ground to achieve densification of
the soil
the weight of hammer varies from 8-35 tons
height of hammer drops varies between 7.5m to 30.5m
the degree of compaction achieved depends on
o weight of hammer
o height of drop
o spacing of the locations at which the hammer is dropped
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Rapid Impact Compaction (RIC)
imparts energy by dropping a 7.5 ton weight from a controlled height of about 1m onto a
patented foot
energy is delivered at a rate of 40-60 blows per minutes
the drop height, number of blows and penetration per blow are monitored and/or
controlled by a data acquisition.
Hand Operated Compactor
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Compaction Quality Control
to check that field compaction meets specification
o (1) sand replacement test (Sand cone)
1.fill the jar with a standard sand (a sand
with known density) and determine the
weight of the sand cone apparatus with
the jar filled with sand
ASTM recommends Ottawa sand
as the standard
2. determine the weight of sand to fill the
cone (W1)
3. Excavate a small hole in the soil and
determine the weight of the excavated
soil (W2)
4. Determine the water content of the
excavated soil (w)
5. Fill the hole with the standard sand by
inverting the sand cone apparatus over the hole and opening the valve
6. Determine the weight of the sand cone apparatus with the remaining
sand in the jar (W4).
Calculate the unit weight of the soil follows
weight of the sand to fill hole: Ws=W1-(W2+W4)
volume of hole:
weight of dry soil: dry unit weight:
o (2) balloon
consists of a cylinder with a centrally placed balloon
cylinder is filled with water
procedure for the balloon
fill the cylinder with water and record its volume, V1
excavate a small hole in the soil and determine the weight of the
excavated soil (W)
determine the water content of the excavated soil (w)
use the pump to invert the balloon to fill the hole
record the volume of water remaining in the cylinder, V2.
calculate the unit weight of the soil as follows
o (3) nuclear density meters
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Objectives for Soil Compaction
increasing bearing capacity of foundations
decreasing undesirable settlement of structures
reduction in hydraulic conductivity
increasing stability of soil structures
Factors affecting compaction:
1. compaction effort
2. soil type and gradation
3. moisture content
4. dry unit weight (dry density)
Compaction Quality Control
types of equipment available to check the amount of compaction achieved in the field
o proctor compaction test
o in-situ density test
Standard Proctor Compaction Test
In the standard compaction test, a dry soil specimen is mixed with water and compacted in a
cylindrical mould of volume of 944 x 10-4
m3(standard Proctor mould) by repeated blows
from the mass of a 2.5kg hammer falling freely from a height of 305mm.
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The soil is compacted in 3 layers, each of which is subjected to 25 blows.
Dry unit weight water content curves
Specification for earth structures (Slopes, embankment, footings, etc.) usually requires a
minimum of 95%of proctor maximum dry density (dry unity weight) to be achieved.
compaction
o (1) increases soil strength
o (2) lowers compressibility
o (3) reduces permeability
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Dry unit weight water content curves
Compaction Specifications
work type specification
o contractor to decide what to do and how to do it
performance based specification
o contract must achieve a certain degree of compaction based on field and lab data
specifications will refer to % relative compaction
o relative to proctor test
% Relative Compaction (R~90-100%)
Is it possible to achieve more than 100% relative compaction? R>100%?
o most speficiations for earthwork require contractor to achieve a compacted field
with dry unit weight of not less than 95% of the maximum dry unit weight
determined in the laboratory by either the Proctor test
o this is a specification for relative compaction, which can be expressed as
R(%)= where R=relative compaction
Compaction Quality Control
A geotechnical engineer needs to check that field compaction meets specifications
(1) Proctor compaction test
to establish the dry unit weight-water content relationship
and establish the maximum dry density and the optimum water content
(2) in situ density test to measure the achieved density of the compacted soil
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Week 5: Site Investigation
The purpose of a soil investigation
to evaluate the general suitability of the site for the proposed project
to enable an adequate and economical design to be made to disclose and make provision for difficulties that may arise during construction due to
ground and other local conditions
Phases of ground Investigation:
PHASE I: Collection of available information (desk study)
o such as:
a site plan
size
importance
structure
loading conditions
previous geotechnical reports
topographic maps
air photographs
geologic maps
etc.
PHASE II: Preliminary site reconnaissance or a site visit
o to provide a general picture of the topography and geology of the site
o it is necessary that you take with you on the site visit all the information gathered inthe Phase I to compare the current conditions of the site
PHASE III: Detailed soil exploration
o objectives are
to determine the geological structure
which should include the thickness sequence and extend of the soil
strata (layers/divisions/sections)
to determine the ground water conditions
to obtain disturbed and undisturbed samples for laboratory tests
to conduct in situ (on site) tests
Scope of a site investigation
The scope of a site investigation depends on the type, size and importrance of the structure,
the client, the engineers familiarity with the soils at the site and local buildings codes
structures that are sensitive to settlement such as machine foundations usually require a
thorough soil investigation compared to a foundation for a house
a client may wish to take a greater risk than normal to save money and set limits on the type
and extent of the site investigation
if the geotechnical engineer is familiar with a site, he/she may undertake a very simple soil
investigation to confirm his/her experience
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Investigation methods
trial pits of test pits
o trial pits are shallow excavations going down to a depth no greater than 6m
o the trial pit as such is used extensively at the surface for block sampling and
detection of services prior to borehole excavation
o all pits below a depth of 1.2m must be supported
hand or powered augers
o boreholes/drillholes
o used to determine the nature of the ground (usually below 6m depth) in a
qualitative manner
o and then recovers undisturbed samples for quantitative examination
Standard penetration test (SPT)
the SPT test is made in boreholes by means of the standard 50mm diameter split sampler
the sampler is driven to penetration of 450mm by repeated blows of a 63.5kg monkey falling
through 760mm
only the number of blows for the last 300mm of driving is recorded as the SPT N value
The SPT test is a very useful means of determining the approximate in situ density of coarse
grained soils
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The SPT N values provide useful information for
foundation design
Sampling
sampling can either beo undisturbed
o disturbed
the principal sampling methods used in boreholes are:
o SPT test(see above)
o core sample
must be sealed with paraffin to maintain the water conditions and then end
sealed to prevent physical interference
the most common of these is the U100 (see below)
U100 is 450mm long, 100mm in diameter undisturbed sample
tube has a cutter at one of the end and the driving equipment at the
other
The Standard Penetration Test (SPT)
provides a disturbed sample of soil and a
blow count which approximately
correlates to density/strength of soil, by
driving a hollow split spoon sampler into
the ground
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behind the cutter is the core catcher
o incorporating 3 arms that go into the sample as it is
withdrawn to prevent the sample from falling out
o bulk samples
o water samples
Sampling for Different Soils
Clays
o normally need undisturbed samples
o U100 every 1.5m or change of stratum
blow count and penetration should be noted
o if unable to obtain a U100 then bulk samples as above
o if U100 does not full penetrate SPT test is required
Sands and Gravels
o undisturbed samples are not practical due to the lack of cohesion
o SPT every 1m or change of stratum
number of seating blows should also be recorded
o bulk samples to be taken between SPTs
Silts
o alternate SPT and U100 samples at 0.75m intervals
Shear Vane Test/ Vane Shear Test
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is a test to find shear strength of a given soil
o useful method of measure shear strength of clay
it is cheaper and quicker method
laboratory test useful for soils of low shear strength (less than 0.3kg/cm2) for which triaxial
or unconfined tests cannot be performed
the tests gives the undrained strength of the soil
the undisturbed and remoulded strength obtained are useful for evaluating the sensitivity of
soil
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Field Vane Shear Test
