II- Index Properties
Consistency of Soil
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1 Basic States: * Depending on the moisture content, soil behaves like:
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2 Atterberg Limits or Consistency Limits:
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* The moisture content (wc) at which the transition from one state to another is defined as:
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LIQUID LIMIT:
- The test is performed on material passing the 0.425 mm (No. 40) sieve that has been completely remolded to destroy the in situ structure.
- It is the water content at 25 blows and can be determined by using Casagrande Cup. The slope of the line is defined as the flow index.
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For each water content, the soil
is placed in the cup in such a manner that the soil fills
the same volume as water would if placed in the cup
while in the impact (drop) position. |
The grooving tool is used with the
beveled edge at the front of the tool to create a groove
down the center of the pat. The tool creates the
geometry of the slope and controls the volume
of soil in the cup. The cut should be made from the
back to the front of the cup. Care must be
taken to keep the grooving tool at 90 degrees to
the cup curvature at all times. Wipe the excess material from the front edge of the cup. |
Turn the
crank at a rate of 2 +/ - 0.1 blows per second until the groove closes by 13 mm (1/2 in.), |
PLASTIC LIMIT
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Is the moisture content at which the soil crumbles when rolled into threads of 1/8 in. in diameter
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PLASTICITY INDEX
PI = LL - PL
PLASTICITY CHART
PI vs. LL.
* Line A differentiates between the silt and the clay
SHRINKAGE LIMIT
The
shrinkage limit (SL) is the water content where further loss of moisture will
not result in any more volume reduction. The test to determine the shrinkage
limit is ASTM International D4943. The shrinkage limit is much less commonly
used than the liquid limit and the plastic limit.
LIQUIDITY INDEX
The
liquidity index (LI) is used for scaling the natural water content of a soil
sample to the limits. It can be calculated as a ratio of difference between
natural water content, plastic limit, and plasticity index: LI=(W-PL)/(LL-PL) where W is the natural water content.
ACTIVITY
The
activity (A) of a soil is the PI divided by the percent of clay-sized
particles(less than 0.075mm size) present. Different types of clays have
different specific surface areas which controls how much wetting is required to
move a soil from one phase to another such as across the liquid limit or the
plastic limit. From the activity one can predict the dominant clay type present
in a soil sample. High activity signifies large volume change when wetted and
large shrinkage when dried. Soil with high activity are
very reactive chemically. Normally, activity of clay is between 0.75 and 1.25
and in this range, clay is called normal. It is assumed that the plasticity
index is approximately equal to the clay fraction (A = 1). When A is less than
0.75, it is considered inactive. When it is greater than 1.25, it is considered
active.
Is used to identify the
swelling potential of clay soils.
3- Moisture Content - Dry Unit Weight Relationship (Compaction)
Soil Compaction
Many civil and environmental engineering applications such as
highway embankment, earth dams, landfills, airfields and others require that
soft soils be compacted to increase their unit weights. Compaction improves
soil properties by increasing its strength, reducing permeability and
settlement/swelling potential.
Smooth wheel rollers, sheepsfoot
rollers rubber-tired rollers, and vibratory rollers are usually used in
compacting soils in the field. Vibratory rollers are used mostly for the
identification of granular soils.
Compaction. in general. is the identification of soil by removal of air. which requires mechanical energy. The degree of compaction
of a soil is measured in terms of its dry unit weight. When
water is added to the soil during compaction. it
acts as a softening agent on the soil particles. The soil particles slip over
each other and move into a densely packed position. The drive unit weight after
compaction first increases as the moisture content Increases (Figure 1). Note
that at a moisture content wc
=0, the moist unit weight g
is equal to the dry unit weight
(dry ).
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Hammer drop pattern when using the 101.6 mm (4 in.) diameter mold |
Hammer drop pattern when using the 152.4 mm (6 in.) diameter mold. |
Determination of the Maximum Dry Density
(MDD) and Optimum Water Content (ωopt ).
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Field Compaction |
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Rammers
Rammers deliver a high impact force (high amplitude) making them
an excellent choice for cohesive and semi-cohesive soils. Frequency
range is 500 to 750 blows per minute. Rammers get compaction force from
a small gasoline or diesel engine powering a large piston set with two sets
of springs. The rammer is inclined at a forward angle to allow forward
travel as the machine jumps. Rammers cover three types of compaction:
impact, vibration and kneading. |
Sheepsfoot
rollers •
Has many round
or rectangular shaped protrusions or “feet” attached to a steel drum •
8% ~ 12 % coverage •
Contact
pressure is from 1400 to 7000 kPa •
It is best
suited for clayed soils. •
Compactive effort:
static weight and kneading. |
Pneumatic
(or rubber-tired) roller •
80% coverage
under the wheel •
Contact
pressure up to 700 kPa •
Can be used for
both granular and fine-grained soils. •
Compactive effort:
static weight and kneading. •
Can be used for
highway fills or earth dam construction. |
Smooth-wheel
roller (drum) •
100% coverage
under the wheel •
Contact
pressure up to 380 kPa •
Can be used on
all soil types except for rocky soils. •
Compactive effort:
static weight •
The most common
use of large smooth wheel rollers is for proof-rolling subgrades
and compacting asphalt pavement. |
In-Place Density
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Sand Cone Method
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Rubber
Balloon Method
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Nuclear Density Method
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Advantages * Large sample |
Advantages * Large sample |
Advantages * Fast |
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Disadvantages Many steps |
Disadvantages * Slow |
Disadvantages * No sample |
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Errors * Void under plate |
Errors * Surface
not level |
Errors * Miscalibrated |
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Cost * Low |
Cost *
Moderate |
Cost * High |
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4- SOIL CLASSIFICATION SYSTEMS
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Sieve Analysis
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Hydrometer Analysis
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Grain
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