DETERMINATION OF THE PERMEABILITY OF GRANULAR SOIL:TENTATIVE STANDARD METHOD USING A CONSTANT-HEAD PERMEAMETER

OBJECTIVE

To determine the coefficient of permeability.

SCOPE

Permeability refers to the ease with which water can flow through a soil. This property is necessary for the calculation of seepage through earth dams and the calculation of the rate of settlement of clayey soil deposits.

This method covers the determination of the coefficient of permeability for the laminar saturated flow of water through granular soils such as sand and fine gravel containing not more than 10% passing the No. 200 (0.75mm) sieve.

The method is based on measuring the volume of water flowing through a soil specimen in a certain time, under conditions of constant head, steady state of flow, full saturation of the soil and direct proportionality between discharge velocity and hydraulic gradient. (Darcy’s law)

The discharge velocity is measured as the volume of water passing through the specimen per unit time divided by the cross-sectional area of soil. The coefficient of permeability is determined as the slope of the curve representing the discharge velocity as a function of the gradient.

APPARATUS

Perspex cylinder with all the connections Glass tube manometers Clean water and a sink De-airing tank with a vacuum pump Stop watch, balance Clean gravels and measuring cylinder

METHODOLOGY

Preparation for test

A sieve analysis of the soil to be tested was made. The permeameter connections were checked and the gaskets were sealed. The cylinder was then placed on the base and tightened down.

The cell was filled with de-aired water and filter gravel was placed at the bottom of the cylinder to a thickness of about 5cm.

The soil was put in the permeameter and stirred with a rod to settle the soil particles. The soil level was about 7cm from the top of the cylinder.

The upper filter gravel was placed to a thickness of 5cm and the top plate was put on.

The 4Kg weight was placed on the loading shelf and the height of the specimen measured.

The permeameter inlet was connected to the constant head tank and permeameter was topped up by the de-aired water, letting the air escape through the bleeder valve.

Test Procedure

The de-airing tank was operated under high volume until it was filled with de-aired water.

The constant head tank was filled with de-aired water and disconnected from the vacuum. The connection to the constant head tank was opened until water overflowed from the latter tank.

The permeameter inlet and outlet was opened and the level of the outlet was adjusted to give a mean head

loss ( .The mean temperature was then measured and recorded.

The permeameter outlet was then used to start filling the measuring cylinder while starting the stopwatch simultaneously. When enough water was collected, the water was stopped and the time (t) and the volume of water (v) recorded.

RESULTS AND ANALYSIS

MANOMETER READING (cm)

TIME(secs) WATER DISCHARGED (cm3)

TEMPERATURE(0C) H1 H2 H3

120 30 19 94.3 83.7 65.5240 41 19 93.7 79.2 59.0360 61 19 93.2 71.9 42.0480 81 19 92.6 63.7 22.8

Y = mx + cY = -0.0001x + 0.0022

Value of k = 10-4

K20 = Kt

Kt = 10-4

ηt = 0.01030

η20 = 0.01005 K20 = 10-4 × = 1.0249 × 10-4

DISCUSSION

After correction, the coefficient of permeability was found to be 1.0249 × 10-4. The value of permeability varies from soil to soil with clay having the lowest permeability and sand has the highest. Repetition of the experiment severally can give more accurate results of the coefficient of permeability. A more permeable soil has a higher rate of seepage. This fact is important during the construction of dams. Dams are normally constructed using soils with the lowest rate of seepage.

Probable sources of error during the determination of the value of K include poorly calibrated instruments, parallax error when reading the values of head, delays in starting and stopping the stopwatch, air trapped in sample or sample not 100% saturated, soil was washed from the sample, some of the head loss occurred in the apparatus rather than in the sample, sample disturbed by flowing water at inlet and difficulty of accurately measuring heads relative to tail water and significant figures.

CONCLUSION

The objective of the experiment was met as the coefficient of permeability was able to be determined.

REFERENCES

Geotechnical engineering principles and practices, V. N. S. Murthy

FALLING HEAD PERMEABILITY TEST

OBJECTIVE

To determine the coefficient of permeability of the given soil sample, using falling head method.

SCOPE

This is the method of measuring the permeability of a soil sample using a falling head permeameter, which is an instrument in which water is passed through a soil sample and the hydraulic gradient and quantity of water flowing into the sample are measured. This instrument is used for measuring the permeability of clays, silts and fine sands.

Permeability is defined as the rate of flow of water under laminar conditions through a unit cross-sectional area perpendicular to the direction of flow through a porous medium under unit hydraulic gradient and under standard temperature conditions. The principle behind the test is Darcy’s law for laminar flow. The rate of discharge is proportional to (i x A).

q= KiAWhere; q= Discharge per unit time.A=Total area of c/s of soil perpendicular to the direction of flow.i=hydraulic gradient.k=Darcy’s coefficient of permeability (The mean velocity of flow that will occur through the cross-sectional area under unit hydraulic gradient)

APPARATUS

Standard compaction mould Stand with screw Stop watch Burette Soaking tank Meter Rule Washing bottle

METHOD

The sample was trimmed in the mould and weighed and the moisture content determined. The mould was then mounted into the cage with the gauge on the top and bottom and the top screwed down. The mould was then placed in a soaking tank which was slowly filled with air free

distilled water. The air-free distilled water was carefully placed in the tank so that it was not aerated by agitation as the tank was filled. Since the vacuum in the sample draws air from the sample and pulls air-free water up to the sample completely saturating it, the operation was carried out slowly to avoid air entrapment. The saturated sample was then connected up through the tubing filled with air-free distilled water to the burette which was also filled with air-free distilled water. At the beginning of the test, water was allowed to fall through the sample and the time required for it to pass was recorded. The burette was refilled with water and the test was repeated many times, each time being recorded.

ANALYSISThe coefficient of permeability is obtained from the formula

Where K = Coefficient of permeability (cm/sec) a = Cross-sectional area of manometer tube (sq.cm) L = Length of sample under test (cm) A = Cross-sectional area of sample (sq.cm) H1 = Initial head of water (cm) H2 = Head of water in cm indicated at the end of a particular period of time t2 = Time corresponding to H2 (sec) t1 = Start time (sec) 2.3026 = conversion factor, log to log10

Slope =

TIME IN SEC HEAD (CM) LOG H0 90.5 1.955 86.3 1.9410 83.0 1.9215 80.2 1.9020 77.6 1.89

Slope = 0.0033a = 0.95cm2

A = 78.50cm2

L = 10.4cm

K = 9.568 × 10-4 cm/sec

DISCUSSION

The coefficient of permeability was found to be 9.568 × 10-4 cm/sec. Probable sources of error during the determination of the value of K include poorly calibrated instruments, parallax error when reading the values of head, delays in starting and stopping the stopwatch, air trapped in sample or sample not 100% saturated, soil was washed from the sample, some of the head loss occurred in the apparatus rather than in the sample, sample disturbed by flowing water at inlet and difficulty of accurately measuring heads relative to tail water and significant figures. Repetition of the experiment severally can give more accurate results of the coefficient of permeability.

Whereas the falling head method of determining permeability is used for soil with low discharge, the constant head permeability test is used for coarse-grained soils with a reasonable discharge in a given time. For very fine-grained soil, capillarity permeability test is recommended. Permeability tests are used to determine estimation of quantity of underground seepage water under various hydraulic conditions, quantification of water during pumping for underground construction, stability analysis of slopes, earth dams and earth retaining structures and to find out the rate of consolidation and settlement of structures.

CONCLUSION

The experiment was successful as the coefficient of permeability was determined conclusively.

REFERENCES

a) Soil Mechanics; T. William Lambeb) Lab Manual

ONE DIMENSIONAL CONSOLIDATION TEST

SCOPE AND OBJECTIVE

Consolidation is the process of time-dependent settlement of saturated clayey soil when subjected to an increased loading. The amount of water that escapes depends on the size of the load and compressibility of the soil. The rate at which it escapes depends on the coefficient of permeability, thickness and compressibility of the soil.This test is performed to determine the magnitude and rate of volume decrease that a laterally confined soil specimen undergoes when subjected to different vertical pressures. From the measured data, the consolidation curve (pressure-void ratio relationship) can be plotted. This data is useful in determining the compression index, the recompression index and the preconsolidation pressure (or maximum past pressure) of the soil. In addition, the data obtained can also be used to determine the coefficient of consolidation and the coefficient of secondary compression of the soil.This method covers the determination of the magnitude and rate of the consolidation of a saturated or near-saturated specimen of soil in the form of a disc confined laterally, subjected to vertical axial pressure, and allowed to drain freely from the top and bottom surfaces.

Significance

The consolidation properties determined from the consolidation test are used to estimate the magnitude and the rate of both primary and secondary consolidation settlement of a structure or an earth fill. Estimates of this type are of key importance in the design of engineered structures and the evaluation of their performance.

APPARATUS

Consolidation device (including ring, porous stones, water reservoir, and load plate) Dial gauge (0.002mm/div) Sample trimming device Glass plate Metal straight edge Stop watch Moisture can Filter paper

Fig. 1.0 Cassagrande odometer apparatus.

METHOD

Preparation of test specimen

A short length of the soil sample was extruded from the sample tube by means of the jack and frame. A consolidation ring of suitable dimensions and metal tray was cleaned and dried. The ring was then slightly lubricated. The extruded length of the sample was then cut off flash with the end of the tube.

Type A soils

A representative sample of testing was extruded and cut off, care being taken to ensure that the two faces of the disc of soil were parallel to each other. The thickness of the disc of the soil was greater than the height of the consolidation ring.Using the consolidation ring as a template, the edges of the disc were trimmed off carefully until the ring slid over the soil. The last fraction of the soil was pared away by cutting the edge of the ring as it was pushed down slowly and evenly over the sample with no unnatural voids against the inner face of the ring. The top and bottom surfaces that projected above and below the edges of the ring were trimmed off until they were level and flush with the top and bottom edges of the ring.The thickness of the consolidation specimen was measured and the specimen in its ring was placed on the metal tray and weighed immediately.

Assembly of apparatus

The bottom porous plate was centered dry in the consolidation cell. The ring complete with the specimen was placed centrally on top of the porous plate with a filter paper against each face of the specimen. The filter papers were moist. The top porous plate and the loading cap was placed centrally on top.The loading cell was then placed in position on the bed of the loading apparatus and the counter-balanced loading beam was adjusted carefully to a level position with the appropriate load transmitting member in contact with the loading cap.The gauge was clamped into position for recording the relative movement between the base of the consolidation cell and the loading cap. The gauge was arranged in such a way that it allowed a small amount of swelling for the specimen and the remainder of the range of travel being taken to allow for compression.

Loading sequence

The loading sequence followed was 50, 100, 200 and 400 (KN/m2). The initial pressure was applied to the specimen at a convenient moment (zero seconds) as indicated by the stopwatch and the compression reading was taken. Further readings of the compression gauge were taken at

suitable intervals of which helped the plotting of the readings after the application of

pressure. The consolidation cell was filled with water immediately after the application of pressure.The specimen began to swell and the pressure was increased to the next higher value. When the swelling continued the pressure was increased and compression readings were taken at the time intervals. The compression gauge readings were then plotted versus the square root of elapsed times.The pressure was maintained until the plotted readings indicated that primary compression had taken place and this was after 24hours. At the end of the 24hours readings of time and compression were made after each load increment.

Unloading

On completion of the compression gauge readings, under the maximum applied pressure the load was removed from the test specimen and the consolidation cell removed from the apparatus. The mass of the metal tray was taken and the specimen in its ring was removed from the cell and taken to the oven to dry. It was dried to a constant mass. ANALYSIS

Mass of wet soil= 136gMass of dry soil= 100g

Moisture content =

The equivalent height of the solid particles, Ho

The voids ratio, e was calculated using the following formula:

Applied pressure(kN/m2)

∆H(mm) H (mm) Percentage thickness (%)

Ho(mm) Voids ratio e

Log of applied loading

0 0 20.0 100 9.769 1.05 050 0.994 19.006 95.03 9.769 0.95 1.699100 0.282 18.724 93.62 9.769 0.92 2.000200 0.298 18.426 92.13 9.769 0.89 2.301400 0.336 18.09 90.45 9.769 0.85 2.602

THE COEFFICIENT OF CONSOLIDATIONSThis was calculated with two approaches,

Square root of time fitting method Logarithm of time fitting method

a) Square root of time fitting method

GRAPHS OF DIAL GAUGE READINGS AGAINST SQUAREROOT OF TIME

T90 = 29.16

mm2/min

T90 = 14.44

T90 = 8.41

T90 = 21.16

b) The logarithm of time fitting method:

The two straight portions of the laboratory graph when the compression gauge readings are plotted against the log of time were extended to intersect to give the point of 100% primary compression.The corrected zero point was located by marking off the difference in ordinates between any two points in the initial portion of the curve with the times in the ratio of 1 to 4 , and laying off an equal distance above the upper point.With then zero and 100% points known, the 50% primary compression point can be located with its time, t50 (min).The consolidation coefficient was computed from the following equation:

GRAPHS OF DIAL GAUGE READINGS AGAINST LOGARITHM OF TIME

The calculation sheet for the coefficients of consolidation and compression ratios is attached at the back

DISCUSSION

Dry density

= g/cm3

Bulk density

= g/cm3

The consolidation tests give a directive on various consolidation properties that can be used to determine the rates of primary and secondary settlement in a building and also to determine the most appropriate foundation designs that can be used for a given building. The possible errors that would cause inaccurate determinations of consolidation characteristics include specimen not

completely filling the ring, friction between specimen and consolidation ring specimen disturbance during trimming, inappropriate load increment factor amongst others.

CONCLUSION The experiment was successful as the rates of consolidation for the different loadings were determined and also their respective coefficients of consolidation.

REFERENCES

a) Engineering Properties of Soils Based on Laboratory Testing; Prof. Krishna Reddy, UICb) D W Taylor, Research on the Consolidation of Clays, Serial 82