The test for the index properties of soils helps
us to ascertain the behavior of soils in construction. It may include the
physical and mechanical properties of soil.
In these very project carried out by me and the geologist
present, we use the fill material (laterite) to test for the Atterberg limits
of which we will discuss the following in this chapter
·
Liquid limit
·
Plastic limit
·
Linear shrinkage limit
Other features tested on the material collected are the
·
Compaction test
·
The C.B.R (California Bearing Ratio) test.
·
To be discussed are the aims for such test [to determine
the optimum moisture content and the maximum dry density] along with its
process and each conclusion.
ATTERBERG/LIQUID LIMIT (LL)
The liquid limit test is a test that is carried out to
determine the ability of the fill material (soil) to retain or absorb water and
still retain its strength.
The
test starts with the sieving of the material (soil sample) into a flat pan.
Then a certain amount of water is then poured into the sieve material after
which it is then mixed together with our hands for about 5-10 minutes until
there is a proper mix between the water and the material. Then the material is
collected into a sample bag where it is then wrapped and stored for about
24hours. Then after 24hours is achieved, the stored material is then placed on
a glass board where it is remixed again this time for about 4-5 minutes after
which the material is then collected and placed on the cassagranda (liquid test
device) has a handle and beside the handle is the meter where the number of
blows would be read from. After the material is placed on the cassagranda a
spatula is used to smoothen the surface of the material. After that is done,
the grove is then used to create a marginal demarcation in the middle. This
would help to determine the endpoint of the attainment of the liquid limit by
the material about 25 blows is usually hit in the liquid limit test.
In
measuring the (LL), if the soil strength is above 35%, then the (LL) is good.
But if it’s below 10% then the (LL) is low.
In
determining this values the sample after undergoing this (LL) test is then
collected in a sample can which would be measured as the initial weight (wet sample), then it would be
placed in the oven for 24hours after which
it would be then measured again as the final weight or dry sample
weight.
Then we evaluate the values after the liquid limit
calculation in the chart and thus deduce the O.M.C and M.D.D (optimum moisture
content and maximum dry density) of the material.
3.2 PLASTIC LIMIT (PL)
The plastic limit is a
feature that is measured to determine the behaviour of moulds (holes) in the
soil.
The test starts similarly
with that of the liquid test where the material (soil) is filtered and
collected over a pan and then water is being added and then subsequently mixed
and wrapped in a sample bag to retain its moisture and this also would be kept
for 24hours. After 24hours is over the damped material is collected and mixed
again for about 4 – 5 minutes with a spatula on the glass board, after mixing,
we use our hands to collect the sample
and then roll it gently on the glass board until a length of 3mm is attained.
Then each of the 3mm sample is then placed into the measuring can where it is
then measured on an electronic weighing scale as the wet sample after which it
is then placed into the oven for about 24hours and then it would be removed and
measured as the dry sample weight and other values would be calculated to
determine the O.M.C and M.D.D.
In measuring the (PL) of
the soil material, when the (PL) is below 10%, then the (PL) is low but when it
is above 40%, then the (PL) is high and these behaviour is usually observed
physically with cracks after compaction (when the PL is low) but there would be
no cracks when the (PL) is high.
·
LINEAR SHRINKAGE LIMIT (S)
The linear shrinkage limit test
is a test that is carried out to determine the strength of the material and
also to determine the reaction of the soil material in its absorption or
reduction of water.
These similarly are started off
by sieving the material, then water is been added into the sieved material and
mixed well until damped. After which it would be wrapped and stored for about
24hours, after which it would be collected and mixed again on the glass board
after which the linear shrinkage mould would then be used to collect the
sample. The spatula is used to carry out and smoothen the surface of the
shrinkage mould after which a meter rule/ruler is used to measure the length of
the mould plus material and also it would be weighed as weight of wet sample,
then it will be placed in an oven for about 24hours, after that it would be
removed and also the length of the shrinkage (observable feature) will be
measured and then weighed as the weight of the dry sample. Other factors would
be calculated so as to obtain the O.M.C and M.D.D.
3.4 COMPACTION/PROTON
TEST
The aim of the compaction is to
determine the bearing capacity of the fill material (soil) and also to
ascertain the soil strength. The test starts with sieving of the material using
the pan (3/4) to remove the stone particles, then we place a tray place a tray
on an electronic weighing scale and weigh about 300kg of the sample which would
be compacted. Then we divide the sample into 4 parts each weighing 75kg to be
compacted individually, then we pour the first part of sample into a flat tray
where we then mix the material with water (a measured volume of water) that is to be compacted with 4%, 6%, 8% and
10% volume of water which would be used to obtain the M.D.D and OMC (maximum
dry density and optimum moisture content).
In deriving the volume of water to be used in
the compaction test we use the format below:
Total weight of sample = X/100
Where X = volume of water
= 3000kg/100 = 30kg
Obtain the volume of 12%
of water to be used for the compaction test
= 30 x 12
= 360cm (measured using a
cylinder)
About 360cm of water
would be used to compact about 12% of the fill material.
After
the material has been added and the mixing is done, we ensure that the entire
material is well damped then we use the scoop to pick the sample and then pour
it into the compaction mould and the rammer which weighs about 25kg is used to
hit the 25 blows into the mould and this would be done in 3layers. These
results in the formula for compaction (3/5) that is 3layers and 25blows also a
tare can would be used to collect some sample. This also would be the major
determinant in obtaining the O.M.C and these would be measured as the initial
weight (wet sample) and also the final weight (dry sample, after being removed
from the oven after 24hours).
After the compaction of the material (in the
compaction mould), the surface is then smoothen by the use of a spatula. The
mould is then weighed in an electronic weighing scale. The process is repeated
for the next 4 volumes of water and these is to enable us to get 2 rising point
and falling point to be used in plotting the graph which would give the O.M.C
and M.D.D.
3.5 C.B.R (CALIFORNIA
BEARING RATIO) TEST.
The
C.B.R test is a test that is carried out to determine the strength and
stability of the fill material (soil) and also the shearing resistance of the
soil when a load is placed upon it, telling us it`s response and also the
layering if other base materials that would be placed on the fill material.
The
process starts with the soaking of the compacted material in a compaction mould
placed in a water bath for about 48hours with filter papers on both ends of the
mould (top and base). Next the C.B.R machine is set up with dial gauge (used in
measuring penetration) on both of the 2 gauges set about 0.0sec, then the gap
between the load and the penetration pin and
the piston are set so as to accommodate the mould that would be placed
inside the C.B.R machine. After the setup is done then we power on the motor
and then wait till there is contact between the penetration piston/plunger and
the compaction mould and these can be observed physically by the movement of
the dial gauge.
Then
in taking the reading values of the penetration of the material from the 2 dial
gauge we count every 0-25 revolutions of the top dial gauge to 1 revolution of
the lower dial gauge. While we also read from 25-50 revolutions at the top to
the lower dial gauge.
3.6 SIEVE ANALYSIS
The aim of the sieve analysis is
to determine the quality and durability of the soil material and the process
starts with the washing of the material using sieve pan no. 75 and 2.36 after
which the washed sample is then dried in the oven for about 24hrs after which
the samples are then poured into the sieve pan (each with various sizes) after
which we shake the pan for about 3-4mins and then each is poured into the pan
and it is then weighed. Before it is being weighed the pan is first measured so
as to determine the weight of the pan before any material is poured on it. A
brush is used to remove any remaining particle on any of the sieve pan so as to
ensure that there are no errors in calculating the results.
Mortar compressive
strength test
Equipments/Materials Used: mixing machine, 2.5mm
mould, scrapper, Dangote cement, water cylinder and a water bath.
Procedure:
Get the sample (Lafarge cement)
from the delivery truck and weigh 490g, then mix with sand (1350g) and water
(237ml) and place in a mixing machine for about 10minutes. After mixing you put
the mortar in moulds of 2.5mm and compact using a tamping rod (25blows) , a
scrapper is used to smoothen the surface and allowed to dry for 24hours. After
the 24hours, you remove it from the mould and cure for 3days, 7days and 28days
for it to attain its maximum strength. Then you crush using the crushing
machine and the values gotten.
Note: The value gotten
depends on the type of cement used.
Cement Setting Time Test
Equipments/materials
used: vicant, water cylinder, vertical plug and a mould.
Procedure:
Collect a cement sample (Dangote
cement) and weigh 650g and mix with water, after mixing you place it in a
mould. Using the vertical plug placed in the vicant to check its testing value
(it must be between 8-11cm, if below 8cm the water is excess and the mixture
thrown away and the process repeated. But if above 11cm, water is added till it
gets in range). The vertical plug is then removed and a vertical pin is placed
for the vicant penetration and time of commencement recorded, the penetration
is recorded at 20minutes interval till the cement sets. The time it starts
setting and ends is recorded and the calculation done to get the average
strength.
Grading
Grading is determined by passing
a sample of aggregate through a series of sieves each of which has a smaller
opening than the one preceding it. In this way all particles of approximately
the same size are caught on a sieve and can be determined by weighing the
amount present. Thus it can be determined whether the load is deficient perhaps
in fine materials or contains too much oversize material.
The grading of concrete aggregate, particularly
fine aggregate, has marked effect on the workability of the concrete. If the
grading varies from load to load, the mixer operator has difficulty controlling
the amount of cement and water he must add to the mix to keep the workability
constant. The mixer operator should never adjust the workability by simply
adding more water, since this will weaken the concrete.
To obtain the grading of the aggregate, take a
weighed representative sample and pass it through a nest of sieves, beginning
with the sieve having the largest openings. Make sure the sieves are well
shaken to ensure that all the fine material passes through. Then weigh the
amount of material retained on each sieve and report as the percentage retained
on each sieve size.
Slump Test
Equipments/material used: slump cone (300mm high,
200mm diameter at bottom and 100mmdiameter at top), tamping rod (600mm long,
15mm diameter and bullet pointed), a scoop and a non-absorbent board.
Note: before starting the test all equipment must
be clean and inside the slump cone moistened.
The slump test is a measure of
the consistency (workability) of concrete and is also a simple means of
ensuring that the concrete on the site is uniform.
Variation in water content of the
concrete usually results in variable strengths. Other factors which can affect
the slump are the grading and the particle shape of the aggregate and the cement
content. A consistent slump usually means that the concrete is “under control”.
If the result varies, it means that something else has varied, usually the
water.
To make the test, the cone is
placed large end down on the non-absorbant board surface and held firmly with
the board pegs. The cone is then dilled in three layers (of approximately the
same volume) by scooping concrete from the sample. Each layer is rodded 25times
with the tamping rod (25 blows), the rod being allowed to penetrate the layer beneath.
The strokes should be distributed over the surface of the layer uniformly
(note: do not work the rod up and down continuously in one place). To ensure
equal layers of concrete the first two layers should be 1/5 (after rodding) and
½ the cone height respectively (300mm).
After the top layer has been
compacted, the surface of the concrete should be struck off level with the top
of the cone and any surplus concrete removed from around the base.
The slump cone should be lifted,
carefully but firmly, straight up so that the concrete is allowed to subside.
To measure the slump, place the
pointer from the 300mm caliberated rod and measure the amount of subsidence.
Compressive/Crushing Test
The strength of concrete is
determined by making specimens, curing them and then crushing them to determine
their maximum failure. The preparation of the specimen is the most important
phase of the test, for a badly prepared specimen will nearly always give a low
result.
Compressive test specimens are
normally with cubes of 150mm in diameter 300mm high, although other sized
moulds of 100mm by 150mm cubes are also used. These different sizes and shapes
will give different compressive stresses at failure even though the same
concrete is used.
Cube moulds should be made of metal
or plastic and should be rigid enough to hold their shape during preparation of
the specimen. The mould must be clean and the inside surfaces coated very
lightly with a film of oil.
To mould the specimen, a sample
is collected and put into the mould as soon as possible in three layers, each
layer being rodded 25times with a tamping rod or a vibrator (30mm in diameter)
in two layers. After compaction, the concrete is struck off and finished with a
wood float. The cover plate is then attached and the mould marked or it’s
number recorded or an identification tag wired onto the mould.
The specimens should be stored
for 18-24hours at a storage temperature between 130 C and 330 C
and carefully stripped and the side marked with waterproof crayon or paint to
show the date of casting and reference number. The concrete is then placed into
a water bath or fog room where the temperature is maintained at 230 –
20 C (curing).
After 28days, the cube is removed
and taken to the crushing machine where it is loaded individually and crushed.
The load failure is the displayed on the machine screen and recorded or
printed.
Note: tests for 3days and
7days can be can be conducted using the same concrete to test run if the 28days
failure is in range
PILE FOUNDATION
AND DESIGN
PILE FOUNDATION
Pile
foundation is a substructure that carries and transfers the load of a structure
to the bearing/ solid ground located at some depth below the ground surface. It
is a load carrying and transferring system.
It is a formation that strengthens the bearing
capacity of unstable and weak soil for construction purposes. The main
components of the pile foundation are the pile cap and piles.
FUNCTIONS OF PILE FOUNDATION
·
To transmit a foundation load to a solid ground.
·
To resist vertical, lateral and uplift load.
PILE
This is a long slender formation
which transfers load to deeper soil or rock of high bearing capacity displacing
shallow soil of low bearing capacity. It is a convenient method of foundation
for works over water such as jetties or bridge piers.
RIAL FOR PILING
The main
types of materials used for piling bare wood, steel and concrete. Piles made
from these materials are driven, drilled or jacked into the ground and
connected to the pile caps.
CLASSIFICATION OF PILE WITH RESPECT TO TYPE OF MATERIAL
·
Timber Piles
·
Concrete Piles
·
Steel Piles
·
Composite Piles
DRIVEN PILES: Are considered to be
displacement piles, in the process of driving the piles into the ground, the
soil is moved radially as the pile shaft enters the ground. There may also be a
component of movement of the soil in the vertical direction.
Driven piles with the pile driver (crane)
Bored piles (Replacement piles) - Are generally
considered to be non-displacement piles; a void is formed by boring or excavation
before pile is produced. Piles can be produced by casting concrete in the void.
Some soils such as stiff clays are particularly
amenable to the formation of piles in this way, since the borehole walls do not
require temporary support except cloth to the ground surface. In unstable
ground, such as gravel, the ground requires temporary support from casting or
bentonite slurry.
Prefabricated concrete piles (reinforced) and
pre-stressed concrete piles
·
Do not corrode or rot
·
Are easy to splice. Relatively inexpensive.
·
The quality of the concrete can be checked before
driving.
·
Stable in squeezing ground, for example, soft clays,
silts, and peats pile material can be inspected before piling.
PROCESSES INVOLVED IN
CONSTRUCTING AN INTER-CHANGE (BRIDGE)
The first step is to run
a geophysical survey on the proposed site (after reconnaissance). Soil
investigation and geotechnical survey is carried out to check the soil profile
and lithology of the ground before boring of the ground commences using the
designed rig (we have 450mm, 600mm, 800mm,1000mm and 1200mm in diameter of
rigs).
After the various survey
was carried out, the ground surface was found to have clayey and silty
materials followed by weathered rocks and then fresh rocks which served as base
rock for the pile foundation. The boring is done to come in contact with the
base rock (fresh rock) at varying depths based on when in contact with the base
rock, after that a temporal casing is put to make the ground stable to avoid
the hole from being filled with loose particles and possible collapse, a
re-enforcement (rods woven like a basket) is then placed in the bored hole with
regards to the depth. A tremie pipe is placed on the hole (serves as funnel for
proper casting) before casting is been done immediately using a mobile concrete
mixer to avoid the hole from being filled with lose particles or rain water,
the process is repeated for all the bored hole.
Note: at times during
drilling, the rig might hit an aquifer or wet soil leading to seepage and the
hole filled with fluids which will lead to collapse or failure, also the
casting must be done properly to avoid the concrete mixing with loose particles
and clays which will lead to weakening of the pile and possible failure. If
properly casted, the clays (montmorrilorite) will be pushed to the surface.
The pile cap is done next
after about 2meters of the casted pile is exposed and re-enforced with rods of
sizes ranging from 8mm to 35mm. The pile cap then forms the foundation for the
abutment to be raised. After the abutment is raised at both ends, columns are
then raised to support the beam which is placed on the abutments for it to rest
on it.
PILE CAP
COLUMNS
BEAMS
ABUTMENT
This is a
structure or the part of a structure which bears the load or pressure of an arch.
It is a structure that supports the end of a bridge.
It
anchors the cables of a suspension bridge.
RETAINING WALLS.
This is a structure that retains the soil and
water behind it, hence, it bears mostly sideways load. It must resist
overturning, sliding as well as any vertical load.