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Consolidation of Soils Testing and Evaluation Collected by: Ing. Jaafar Mohammed E-mail: [email protected] [email protected]
Consolidation of Soils Testing and Evaluation
Collected by: Ing. Jaafar Mohammed
E-mail: [email protected] [email protected]
Introduction
Consolidation is a process by which soils decrease in volume. According to Karl Terzaghi consolidation is any process which involves decrease in water content of a saturated soil without replacement of water by air.
In general it is the process in which reduction in volume takes place by expulsion of water under long term static loads. It occurs when stress is applied to a soil that causes the soil particles to pack together more tightly, therefore reducing its bulk volume.
When stress is removed from a
consolidated soil, the soil will rebound,
regaining some of the volume it had lost in
the consolidation process. If the stress is
reapplied, the soil will consolidate again
along a recompression curve, defined by
the recompression index. The soil which
had its load removed is considered to be
overconsolidated. The highest stress that it
has been subjected to is termed the
preconsolidation stress.
The over consolidation ratio or OCR is
defined as the highest stress experienced
divided by the current stress. A soil which is
currently experiencing its highest stress is
said to be normally consolidated and to have
an OCR of one.
A soil could be considered underconsolidated
immediately after a new load is applied but
before the excess pore water pressure has had
time to dissipate.
In case of coarse grained soils like
sands and gravels, the removal of this
pore water is easy since water freely
moves from one region to another
within these soil types.
However, in case of fine grained soils
like silty or clayey soils, consolidation
is a time consuming process.
In case of fine grained soil on which a structure is
to be built, high water content is not desired as the
weight of the structure may cause sinking
(consolidation settlement) of the structure in due
time. Typically the permeability (ability of water
to move through the soil voids) of fine grained
soils is low, hence it takes a long time for
consolidation process.
So two aspects of consolidation settlement are
important:
The rate at which the consolidation is taking place and
The total amount of consolidation.
It is very important to note that unlike settlement
in sands and other coarse grained soil,
consolidation settlement of fine grained soil does
not occur immediately. Hence, it is common
practice to ensure that the consolidation process
is expedited and that most of the consolidation
takes place during the various phases of
construction.
If the soil is such that it has never experienced pressure of the current magnitude in its entire history, it is called a normally loaded soil.
The soil is called pre-consolidated (or over-consolidated) if at any time in history, it has been subjected to a pressure equal to or greater than the current pressure applied to it. In case of normally consolidated soils, the consolidation will be greater than that for a pre-consolidated soil.
That is because the pre-consolidated soil has previously experienced greater or equal pressure and has undergone at least some consolidation under that pressure. So a pre-consolidated soil is preferred over a normally consolidated soil.
Types of Consolidation
Primary Consolidation:
It is the reduction in volume due to expulsion of water from the voids. Expulsion of water from the voids depends on permeability of soil and it is therefore time dependent.
Secondary Consolidation:
When all the water is squeezed out of the voids and primary consolidation is completed, further reduction in volume of soil is called secondary consolidation. It may be due to plastic deformation of the soil particles or some other reasons. The value is however very small and commonly neglected.
The process of consolidation is often confused with the process of compaction.
The difference between consolidation and compaction can be appreciated using three-phase diagrams as shown below:
Elastic Settlement or Immediate Settlement
This settlement occurs immediately after the load is applied. This is
due to distortion (change in shape) at constant volume. There is
negligible flow of water in less pervious soils. In case of pervious
soils the flow of water is quick at constant volume. This is
determined by elastic theory (E & μ are used). It occurs due to
expulsion of pore water from the voids of a saturated soil.
The movement of pore water depends on the permeability and dissipation of pore water pressure. With the passage of time the pore water pressure dissipates, the rate of flow decreases and finally the flow of water ceases. During this process there is gradual dissipation of pore water pressure and a simultaneous increase of effective stress as shown in Fig above. The consolidation settlement occurs from the time water begins move out from the pores to the time at which flow ceases from the voids. This is also the time from which the excess pore water pressure starts reducing (effective stress increase) to the time at which complete dissipation of excess pore water pressure (total stress equal to effective stress). This time dependent compression is called “Consolidation settlement”.
Primary consolidation is a major component of settlement of fine grained saturated soils and this can be estimated from the theory of consolidation.
In case of saturated soil mass the applied stress is borne by pore water alone in the initial stages:
At t = 0 ∆σ= ∆u ∆σ´= 0
With passage of time water starts flowing out from the voids as a result the excess pore water pressure decreases and simultaneous increase in effective
stress will takes place.
The volume change is basically due to the change
in effective stress ∆σ´. After considerable amount
of time (t = ) flow from the voids
ceases the effective stress stabilizes and will be is
equal to external applied total stress (∆σ ) and this
stage signifies the end of primary consolidation.
At t = t1 ∆σ = ∆σ ′ + ∆ u
At t = ∆σ = ∆σ ꞌ
∆u = 0 (End of primary consolidation)
Secondary Consolidation Settlement
This is also called Secondary compression (Creep). “It is the change in
volume of a fine grained soil due to rearrangement of soil particles
(fabric) at constant effective stress”. The rate of secondary
consolidation is very slow when compared with primary consolidation.
Terzaghi's Spring Mass Analogy
Terzaghi’s model consists of a cylindrical vessel with a series of piston separated by springs. The space between springs is filled with water the pistons are perforated to allow for passage of water. Piezometers are inserted at the centers of different compartment to measure the pressure head due to excess pore water
pressure.
Terzaghi has correlated the spring mass compression process with the consolidation of saturated clay subjected to external load ∆σ.
The springs and the surrounding water represent the saturated soil. The springs represent the soil skeleton networks of soil grains and water in the vessels represents the water in the voids. In this arrangement the compression is one dimensional and flow will be in the vertical direction.
When pressure is applied this will be borne by water surrounding the spring ∆σ= ∆u at time t =0
u is called excess hydrostatic pressure due to this water level in all the Piezometer reach the same height ‘h’ given by.
∆σ= ∆u and ∆σ´=0 --------- t=0
There will be no volume change. After sometime ‘t’
there will be flow of water through perforation
beginning from upper compartment. In the lower
compartment the volume of water remains constant
since the flow is in upward direction.
Due to flow of water in the upper segment there will
be reduction in volume due to this springs get
compressed and they being to carry a portion of the
applied load. This signifies a reduction in excess
hydrostatic pressure or pore water pressure and
increase in effective stress in the upper segments.
Whereas there will be no dissipation of excess
hydrostatic pressure in lower compartments.
At time t1, t2,--------t= the variation of excess
hydrostatic pressure are as indicated by the
Isochrones shown in Fig below. The isochrones
indicate that with passage of time there is flow of
water from the lower compartments leading to
gradual dissipation of excess hydrostatic pressure.
At time t = when no more Pore-water flows
out the excess hydrostatic pressure will be zero in
all compartments and the entire load is carried by
springs.
Settlement of a Soil Layer
The settlement is defined as the compression of a soil layer due to the loading applied at or near its top surface.
The total settlement of a soil layer consists of three parts:
Immediate or Elastic Compression
Compression due to Primary Consolidation
Compression due to Secondary Consolidation
The immediate or elastic compression can be calculated using the elastic theory if the elastic modulus of the soil layer is known.
Compaction increases the density of an unsaturated soil by reducing the volume of air in the voids.
Consolidation is a time-related process of increasing the density of a saturated soil by draining some of the water out of the voids.
Consolidation is generally related to fine-grained soils such as silts and clays.
Coarse-grained soils (sands and gravels) also undergo consolidation but at a much faster rate due to their high permeability.
Saturated clays consolidate at a much slower rate due to their low permeability.
Consolidation is gradual reduction in volume,
on load application, of a fully saturated clay due
to drainage of pore water
Consolidation continues until all the excess pore
pressure generated by the increase in total stress
has completely dissipated
The consolidation test is used to find
compressibility parameters (σꞌvp, Cc, Cs, and Cv)
One-dimensional consolidation
The seepage forces experienced by the soil structure will
vary with time, and the soil structure itself may deform
under the varying loads it sustains.
The time-dependent process during which a soil
specimen responds to compression is commonly called
the process of consolidation. Laboratory tests for
measuring such compressibility of a soil are conducted
either in a consolidometer (sometimes known as an
oedometer) or in an axial compression cell (usually
known as a ‘triaxial’ cell).
The apparatus consists essentially of a rigid metal
cylinder with closed base, containing a soil sample in the
shape of a thick circular plate sandwiched between two
thin porous plates.
The porous plates, generally made of ceramic or some suitable stone, allow the passage of water into or (more usually) out of the soil sample, but their pores are not sufficiently large to allow any of the soil particles to pass through them. Both porous plates are connected to constant head reservoirs of water, and vertical loads are applied to the sample by means of a closely fitting piston. A test consists of the instantaneous application of a constant increment of load and observation of the consequent settlement of the piston (and hence consolidation of the sample) with time.
The soil sample in the consolidometer is at all times carrying the total vertical load transmitted by the piston; and, as such, forms a load-carrying system. But the soil is a two-phase continuum, and it contains two separate materials (water and the structure of soil particles) which can be thought of as two independent structural members in parallel, as, for example, two members in parallel could make a double strut. The two members have markedly different stress – strain characteristics, and we can only discover the share of the total load taken by each (at any one instant) by considering both their statically equilibrium and strain compatibility .
One-dimensional Consolidation
Since water can flow out of a saturated soil sample in any direction, the process of consolidation is essentially three-dimensional.
However, in most field situations, water will not be able to flow out of the soil by flowing horizontally because of the vast expanse of the soil in horizontal direction.
Therefore, the direction of flow of water is primarily vertical or one-dimensional.
As a result, the soil layer undergoes one-dimensional or 1-D consolidation settlement in the vertical direction.
1-D Consolidation – The Spring Analogy
The Spring Analogy (Continued..)
The spring is analogous to the soil skeleton. The stiffer the spring, the less it will compress.
Therefore, a stiff soil will undergo less compression than a soft soil.
The stiffness of a soil influences the magnitude of its consolidation settlement.
The valve opening size is analogous to the permeability of the soil. The smaller the opening, the longer it will take for the water to flow out and dissipate its pressure.
Therefore, consolidation of a fine-grained soil takes longer to complete than that of a coarse-grained soil.
Permeability of a soil influences the rate of its consolidation.
Consolidation Test
(Oedometer)
The one-dimensional consolidation test procedure was first suggested by Terzaghi. The test is performed in an oedometer.
The objective of the oedometer consolidation test is to determine consolidation characteristics of soils with low permeability. The test determines two important consolidation parameters of clays,
i.e. coefficients of volume compressibility, mv, and
coefficient of consolidation, cv
The standard oedometer consolidation test for saturated clays is the main feature of this experiment. The test is carried out by applying a sequence of vertical loads to a laterally confined specimen having a height of about one quarter of its diameter. The vertical compression under each load is observed over a period of time, usually up to 24 hours. Since no lateral deformation is allowed it is a one-dimensional test, from which the one-dimensional consolidation parameters are derived.
A schematic diagram of an oedometer
Theory
The soil sample is placed inside a metal ring with a porous stone at the top of the sample and another at the bottom. The samples are usually 63.5mm in diameter and 25.4mm thick. Load on the sample is applied through a lever arm and compression is measured by a micrometer dial gauge. The sample is kept underwater during the test. Usually each load is kept for 24 hours. After that, conventionally, the load is doubled, thus doubling the pressure on the sample, while measurement of the compression continues. At the end of the test, the dry weight of the test sample is determined.
Time-deformation Plot During Consolidation
for Given Load Increment (source: Das 1979)
The general shape of the plot of deformation of the
sample versus time for a given load increment is
shown in Figure above. The plot shows three
distinct stages that may be described as follows:
Stage I: initial compression, which is mostly due to
preloading.
Stage II: primary consolidation during which, due to
expulsion of pore water pressure, is gradually
transferred into effective stress.
Stage III: secondary consolidation after complete
dissipation of excess pore water pressure - some
deformation of the sample is caused by plastic
readjustment of soil fabric.
Apparatus
1. Casagrande's type oedometer (Figure below) which includes : a consolidation ring, internal diameter 75 mm, height 20 mm;
a fixed ring, consolidation cell;
a dial gauge reading to 0.01 mm having a travel of at least 10 mm;
a loading device, see Figure below.
2. Glass plate 100 mm x 100 mm (approx.),
3. Apparatus for moisture content determination ,
4. Top pan weighing balance reading to 0.1 g,
5. Vernier callipers,
6. Packet of 75 mm diameter filter papers,
7. Silicone grease or petroleum jelly,
8. Set of standard weights,
9. Stop watch or clock readable to 1 sec,
10. Palatte knife.
Section of a Loading Device
Procedure
1. Preparation of the sample see Figure (c) and (d). Weigh the consolidation ring and glass plate separately to an
accuracy of 0.1g. (Form .1)
Lubricate the inside of the ring with a thin smear of silicone grease or petroleum jelly.
Measure the height of the ring to 0.05 mm at four equally spaced points using the Vernier callipers and calculate the mean height.
Measure the internal diameter of the ring to 0.1 mm in two perpendicular directions using the Vernier calipers. Calculate the mean diameter and the area in mm2.
Extrude a small amount of soil from the compaction mould using the mechanical extruder.
Press the cutting ring, bevelled sharp cutting edge downwards, into the soil until its upper most rim is just below the soil surface.
Extrude more of the soil so that the bottom of the ring is well clear of the edge of the mould.
Trim off the top of the soil with the palatte knife.
Cut off the soil below the base of the consolidation cutting ring with the spatula.
Place the glass plate on the top surface and gently slide the specimen clear using a palate knife to assist the process.
Invert the ring containing the soil sample and trim off the upper surface of the clay level with the bevelled edge of the consolidation ring with the spatulas.
Any voids should be carefully filled with pieces of clay without compressing the sample.
Weigh the glass plate, ring the clay sample to the nearest 0.1 g.
Notes : the height of the ring can be accepted as the initial height of the clay sample.
2. Preparation and assembly of consolidation
apparatus.
Put a wet filter paper onto the porous disc at the
base of the consolidation cell and place the sample,
contained in the ring, on it with the bevelled cutting
edge of the ring uppermost.
Cover the top of the sample with a second wet filter
paper and use the retaining screws to secure the
collar of the consolidation cell to the base to hold the
consolidation ring and sample firmly together.
Place the top porous stone and loading plate on tope
of the filter paper.
3. Assembly in load frame
Place the consolidation cell in position on the cell platform of the oedometer.
Connect the loading yoke of the oedometer with the top platen of the consolidation cell and adjust the counter balance weight of the beam so that it is slightly above the horizontal position.
Place a 100 g weight on the top pan of the weight hanger to give a very small positive downward load on the sample in the consolidation ring (seating load).
Check the beam ratio value and set it to 9:1.
Fill the consolidation cell with water at room temperature.
Clamp the compression dial gauge in to position, allowing space for swelling as well as compression of the sample and record the initial dial gauge reading.
Screw up the beam support jack so that the beam is held fixed, ready for the start of the test.
4. Test procedure
Normally, in the consolidation test, a loading sequence is adopted to give a range of compression stresses suitable for the soil type and also for the effective pressure which will occur in situ due to the overburden and the proposed construction. The initial pressure should be large enough to ensure that the sample in the consolidation cell does not swell.
A loading sequence of stages selected from the following range of pressures is considered appropriate (see BS 1377, 1990, Part 5, p. 5 section 3.5.1.). 6, 12, 25, 100, 200, 400, 800, 1600, 3200 kPa. A typical test comprises four to six increments of loading, each held constant for 24 hours
and each applied stress being double that of the previous stage. Unloading decrements are usually half the number of loading increments.
The single stage consolidation test to be performed will be for a stress of 100 kPa. Determine the value of mass (in kg) needed on the
weight hanger pan to produce a stress of 100 kPa on the specimen (σ’vo) see Appendix A.
With the screw jack support in supporting position, load the weight hanger with the necessary weights and set the dial gauge to zero. Remove the weight used for seating load.
Check that the timing device (stop watch or clock) is working correctly, note the time of day and activate the timing device whilst at the same time lowering the beam support jack to allow the consolidation to begin.
Take readings of the compression gauge at the following time sequence (minutes) 0.25, 0.5, 1, 2, 4, 9, 16, 25, 36, 49, 64, 81, 100 and 121 min.(Form No. 6.2) A final reading, after approximately 24 hours can be taken by the technical staff.
As the sample undergoes compression record
the data and plot a graph of compression dial
gauge readings versus time . (Figure 6.5) After
24 hours, when the consolidation will be
virtually complete, unload the sample and record
the following data.
weight of consolidation ring + sample, wet
weight of consolidation ring + sample, dry*
*after drying to constant weight in an oven at 105oC.
From this data the final moisture content and void ratio
of the sample may be determined.
Determine the values of
from the graph of compression vs time on Figure below by :
Draw the straight line of best fit to the early portion of curve (usually within the first 50% of compression) and extend it to intersect the ordinate of zero time. This intersection represents the corrected zero point, denoted by do.
Draw the straight line through the do point which at all points has abscissae 1.15 times as great as those on the best fit line drawn in (a). The intersection of this line with the laboratory curve gives the 90% compression point, d90.
Read off the value of t90 from the lab. curve corresponding to the d90.
Determine the value of the coefficient of volume compressibility, mv(m²/MN) (see Equation 1) from the settlement data for this loading.
Determine the value of the coefficient of consolidation, cv(m²/yr) (see Equation 2).
The record of data obtained from a full
consolidation test with several stages of loading
and unloading. Plot a graph of void ratio versus
log10 of applied pressures.
For the single stage test you are required to plot
only settlement versus time in order to find t90 by
Taylor’s curve fitting method. For the
determination of t90 and cv for each stage over
several stages a separate graph of settlement
versus timewill have to be drawn for each stage.
Calculation of mass (m) or equivalent mass (in kg)
supported by the specimen
Figure :(a) Section of a Typical Consolidation Cell, and (b)
Details of a Consolidation Cell.
ConMatic IPC,
Automated Consolidation System,
120/220V 50/60Hz— HM-2470A.3F
The ConMatic IPC is a fully-automated, incremental pressure controller for performing incremental consolidation and one-dimensional swell tests. The ConMatic IPC allows consolidation and constant load and volume swell tests to be run automatically, freeing up technicians for other tasks and reducing the duration of the testing procedures by more than half—effectively saving time and manpower and increasing lab profitability. One ConMatic automated system can replace the production of several manual machines— running incremental consolidation tests according to ASTM D2435 Method B, where successive load increments are applied after 100% primary consolidation.
Once a sample has been placed onto the test platform and
the test conditions set, the ConMatic IPC, used in
conjunction with a computer and Humboldt's HMTS
software, performs all consolidation tests, including
moving to the next stress level, without operator
assistance. The system automatically moves through the
different test parameters specified by the user with
incremental consolidation tests typically being completed
in 24 to 48 hours. The HMTS software records readings
from the force and displacement transducers to control the
unit's exceptionally accurate stepper motor. Test results
are recorded and rendered in real-time on the computer
screen while test data is stored and calculations are
performed automatically.
The HMTS software provides:
Live tests and live graphing capabilities (real-time)
Complete test reporting including all calculations and graphs required for testing
Review and exporting of tests using Microsoft Excel
Smart Test Function: automatically picks up where it left off if the test was not finished due to unexpected events within your computer
The unique design of the ConMatic IPC system enables the user
to choose from multiple tests and run them independently and
simultaneously.
Fixed Ring Consolidation Cell
Complete cell assembly features stainless steel construction and self-trimming cutter ring. Cutter ring rests inside clamping ring on lower porous stone, which is larger than the sample. The top porous stone and loading pad rest on the sample. The assembly is fixed on the cell base and enclosed within an acrylic cylinder open to the atmosphere, which permits saturation of the sample. The cell comes complete with all the parts illustrated in the drawing below.
ConMatic Consolidation Machine,
120/220V 50/60Hz— HM-2432A.3F
Compact and easy-to-use, the HM-2432A.3F pneumatic consolidation load frame is used to estimate the rate and amount of settlement anticipated for a proposed structure. The unit applies loads instantly without impact for stress-controlled consolidation testing; and, maintains the load regardless of sample compression. Its small footprint saves valuable lab counter space while maintaining its versatility by supporting fixed ring, floating ring, or permeability cells. Available with standard mechanical dial gauge, digital indicators or with strain transducers (LSCT) coupled to one of our data loggers. Complies with ASTM D2435, D4546; AASHTO T216; BS 1377 part 5. Shipping wt. 49 lbs. (22kg)
Features include:
Highly sensitive and accurate in lower load ranges
Integral digital readout simplifies checking applied load and setup of predetermined load
Adjustable upper cross beam
Instantaneous loading without impact
Flexible load choice
Not sensitive to shock
Choice of English or Metric models
Dead-Weight Consolidation Frame— HM-1100A
Able to survive in even the harshest laboratory environments, the HM-1100A will provide you with reliable service day-in and day-out. The design features a rugged frame design manufactured from aluminum with stainless steel vertical rods, horizontal cross arms and beam support rods. The load arm incorporates 9:1, 10:1, and 11:1 beam ratios for greater flexibility and loading weight requirements. Using the 10:1 ratio on 2.5" (63 mm) diameter samples, the system is capable of producing load up to 48 tsf (4,597 kPa).
Features include:
Triple beam ratios minimize loading weight requirements
48 tsf (5,148 kPa) maximum load capacity
Aluminum and stainless steel construction for corrosion resistance and long life
Wide range of consolidation cells available in fixed ring, floating ring, permeability and backpressure designs
Loading weights available in both, tsf and kg versions
Available with standard mechanical dial gauges, digital indicators or with strain transducers (LSCT) coupled to one of our data loggers.
Consolidation testing to ASTM D2435, D4546; AASHTO T216 and BS
1377 part 5 can be carried out using our manual loading frame, the
HM-1100 or one of the pneumatic loading machines, the HM-2432 or
HM-2470A.
Typical Consolidation Data Acquisition Setups Using
Humboldt MiniLoggers
Floating Ring Consolidation Cell
Complete cell assembly features stainless steel
construction with self-trimming cutter ring.
Similar in construction to a fixed ring cell with
the exception that the lower porous stone fits
inside the cutter ring and can move vertically
within it. The sample ring is also free to move
vertically. The cell comes complete with all the
parts illustrated in the drawing below.
Consolidation Settlements
Preconsolidation Pressure
The preconsolidation pressure for an overconsolidated soil should not be exceeded in construction, if possible.
Consolidation settlements will small if the effective vertical stress in the soil layer remains below its preconsolidation pressure.
If effective vertical stress in the soil layer exceeds its preconsolidation pressure, the consolidation settlements will be large due to further yielding of the soil layer.
The estimation of preconsolidation pressure is greatly affected by the amount of disturbance experienced by the soil sample.
Estimation of Preconsolidation Pressure
Preconsolidation pressure (σ’c) for an OC soil can be estimated from the e-log( σ’v) curve using Casagrande’s procedure.
Casagrande’s procedure has the following steps:
Extend the straight line part (BC) of the curve.
Determine the point of maximum curvature (point
D) on the recompression part (AB) of the curve.
Draw a horizontal line and a tangent at point D and
bisect the angle between these two lines.
The intersection point of the bisector and the
extended straight line BC gives the preconsolidation
pressure (σ’c).
Preconsolidation Pressure: Effect of Disturbance
Figure on the right shows e-log( σ’v) curves for two soil samples – sample A is relatively undisturbed and sample B is disturbed.
An increase in the degree of disturbance results in a decrease in the slope of the compression line.
It also makes the point of maximum curvature
difficult to establish due to a much more gradual
transition from recompression to compression.
As a result, there may be errors in the
estimated value of the preconsolidation
pressure.
In-situ e-log(σ’v) curve
Even a perfect soil sample has some degree of disturbance. Therefore, the in-situ compression line is likely to have a slightly higher slope than that obtained from an oedometer test.
Schmertmann found out that the oedometer
compression line meets the in-situ compression
line at a void ratio of approximately 0.42 times
the initial in-situ void ratio e0.
Once the preconsolidation pressure is obtained
using Casagrande’s procedure, points E and F on
insitu compression line can easily be fixed.
Consolidation Settlement – 1-D Method
For the calculation of consolidation settlement using the 1-D method, the value of either the coefficient of volume compressibility (mv) or the Compression Index (Cc) is required.
Let’s consider a layer of saturated soil of thickness H subjected to an increase in total vertical stress of Δσ as shown in the figure on the right.
At the completion of consolidation, the soil layer will experience an increase in effective vertical stress of Δσ’ and as a result, its void ratio will reduce from e0 to e1.
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