Ppr12.132alr

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Fall Cone Test Parameters and Their Effects on the Liquid and Plastic Limits

of Homogeneous and Non-Homogeneous Soil Samples

M. Reza Emami Azadi

Assistant Professor, Dept. of Civil Eng., Shahid Madani University of Azarbaijan, Tabriz, Iran; Tel/Fax: 0098-411 3340311,[email protected]

S. R. Monfared

Dept. of Civil Eng., Shahid Madani University of Azarbaijan, Tabriz, Iran [email protected]

ABSTRACT In this experimental work, we investigated the effects of various fall cone parameters such as cone apex angle, diameter, overload, cone surface roughness on Atterberg Limits of both homogeneous and non-homogenous (layered) type soil samples. The liquid limit of cohesive clayey silt as well as silty-clay soil samples were determined based on fall cone tests with four different cones with 30o and 60o apex angles, two different diameters and also different surface roughness. The results showed that the apex angle of cone may play a more important role in determining the Liquid limit of soil. Furthermore, it is found that the dropped cone behavior in soil sample changes from a dynamic to quasi-static by increasing the cone apex angle from 30o to 60o. It is also shown that the penetration vs. moisture content response curves in non-homogenous type soil samples have changed rather considerably near the boundary of layers compared to the homogeneous soil counterparts. The results of this study have also shown that this variation would depend on the layering structure of soil. The current study has also shown that for non-homogeneous soils 20mm penetration may not be accurate for determining the liquid limit of soil compared to homogeneous samples. The findings also indicated that fall cone test results may also be applied for determining the plastic limit of clayey and silty soil samples. The obtained results based on different cones might also be used to determine the undrained shear strength (Su) of clayey or silty soil samples at the lower bound limits of penetration vs. moisture content curves.

KEYWORDS: Fall Cone Tests; Moisture Content; Liquid Limit; Plastic Limit, Cone Apex Angle, Homogeneous Soil, Layered Soil

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INTRODUCTION In the past decades, several studies have been conducted on fall cone tests to determine

Atterberg Limits of cohesive soils such as works by Karlsson (1961,1977), Wood and Worth (1978), Lawrence(1980), Wood(1985), Wasti and Bezirici (1986), Harison (1988), Koumoto and Houlsby (2001), Brown and Downing (2001), Feng (2004), Rashid(2005), Muntohar and Hashim (2005), Prakash and Sridaharan (2006), Hazell(2008), Landris Lee and Freeman (2009),Sivakumar et al.(2009), Ying and Wang (2009), Saad (2011) and Kayabali (2012). These studies showed that the liquid limit of soil samples may be determined more accurately by fall cone tests or other methods (e.g. Namdar, 2008) in comparison with the test method using Casagrande device. The differences observed may be attributed to the fact that in Casagrande test, the dynamic effects during various blows may exist. Also the nature of test may involve some error due to measurements and rate of blows. Another problem is related to the fact that Casagrande's test method may not applicable for very silty or sandy soils.

The previous works by Houlsby (1982) and Koumoto and Houlsby (2001) have also indicated the effects of cones with different weights or apex angles on only homogeneous soils. The most recent works by Muhunthan and Sariosseiri (2008) and Landris and Freeman (2009) have also indicated that using the cones with two different weights, the plastic limit of cohesive soil sample may be determined quite satisfactorily.

More recently studies by Muntohar and Hashim (2005) and Hashim (2010) have also shown the influence of cone surface roughness on the penetration depth of the dropped cone during tests. Houlsby (1982) had introduced an adhesion parameter showing the effect of surface roughness on fall cone performance during tests. Koumoto and Houlsby (2001) have shown that this effect may be more significant for a penetrometer cone with an apex angle of 60o.

The works by Lawrence (1980) and Wast i(1987) showed that from fall cone tests for a penetration depth of 2.0-2.2mm, the plastic limit of clayey soil (PL) may be obtained by extrapolating the penetration depth vs. Liquidity Index Curves. Such works indicated minimum penetration depths achieved during fall cone tests which were usually much greater than 2.0mm.

The recent technical report by GEONOR (2010) offers a new automated fall cone device with data acquisition system added and with a different size of mold. This report indicate that this device can determine the un-drained shear strength of cohesive soil specimens as well as their Atterberg Limits based on a calibration K parameter, penetration depth and weight of dropped cone.

EXPERIMENTAL WORK

Test Procedure

This new experimental work was carried out at the soil mechanics Laboratory of Faculty of Eng., Azarbaijan T.M. University from 5 Feb.2011 until 15 Jan.2012. The silty clay, clayey silt and sandy soils were obtained from the university site near Azarshahr (see also Emami Azadi, 2008). We first performed a sieve analysis to determine the grain size diagram and to obtain the Cc and Cg parameters. Hydrometer analyses were performed on clayey and silty soil samples to obtain the grain size diagram of the soil used in the subsequent tests. To perform hydrometer analysis of clayey and silty soils, first Specific Gravity (Gs) values of soil samples were

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Table 1: Gs data from pycnometer tests for sand, clay and silt samples

Table 1 shows the specific gravity (Gs) values obtained from pycnometer tests at Azaruniv. Chemical Lab. for sand, silty clay and clayey silt soil samples. Gs values range from 2.58 to 2.64 for sand to clayey silt and silty clay soils. The results have been used in Hydrometer tests to determine the fine grain size distribution of the silty clay and clayey silt samples of university site near the Azarshahr and the Urmieh Lake.

Figure 3: Grain size curve of the sand sample used in one of the layers.

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. 17 [2012

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Figure 4: Grain size curve of the clayey silt sample used in one of the layers.

Table 3: Hydrometer test data for clayey silt sample

Fig.4 shows the grain size diagram of the clayey silt obtained from hydrometer test (see Table 3 below). It can be seen that the percentage finer than the particle size of 0.002mm (i.e. clay particle size) are less than 20%. The diagram of grain size distribution in Fig.4 indicates that slightly more than 75% of the soil is finer than 0.075mm that is silt and clay fines. This means that about 75% of soil particles are clayey silt and about 25% are very fine sand with size less than 0.425mm.

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Figure 5: Grain size curve of the silty clay sample used in one of the layers.

Table 4: Hydrometer test data for silty clay sample

Fig. 5 also shows the grain size diagram of silty clay soil obtained from Hydrometer test(see Table.4). The diagram of grain size distribution in Fig.4 indicates that slightly more than 80% of the soil is finer than 0.075mm that is silt and clay fines. It can be seen that the percentage finer than the particle size of 0.002mm (i.e. clay particle size) is about 23%. The soil mainly consisted of about 82% particles with size less than 0.075mm and according to the Atterberg limits obtained here and using a plasticity chart the soil is classified as (CL).

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Figure 8a: ELE Fall Cone Test Apparatus (with standard cone) used for determining LL and PL of soil samples

Figure 8b: ELE Fall Cone Test Apparatus (with modified cone & new container) used for determining LL and PL of soil samples

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Figure 8c: Standard ELE and Modified Cones used for determining LL and PL of soil samples

Figure 8d: Standard ELE and Modified Holding Bars for Fall Cone Test Apparatus

Figure 8e: ELE Fall Cone Test with Modified Cone In Homogeneous Soil for determining LL and PL of soil samples

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SUMMARY OF THE RESULTS

STANDARD FALL CONE TESTS Fig.8a and b shows ELE fall cone test apparatus used for determining the Liquid Limit and

Plastic Limit of silty clay and clayey silt samples with standard and modified cones and a standard mold with size of 55mm in diameter and 40mm in depth and also a modified container with 15cmx30cmx20cm dimensions. Cones 1,2,3 and 4 and the holding bars (see Figs.8c and d) with a small attached pin together weigh 80.33gr, 80.01gr, 184.97gr and 162.16gr, respectively. Cone 1 was the standard ELE cone with 1.81cm diameter and an apex angle of 30o and smooth surface while cone 2 had the same diameter and apex angle but with rough surface. Cones 3 had diameter twice as standard cone 1 with apex angles of 60o and smooth surface. Cone 4 had the same dimensions of cone 3 but with rough surface. The test procedure used in this part of study was according to BS1377. Penetration of each cone is measured within 5sec from its release into the mold. Fig.8e shows a fall cone test using modified cone on in homogeneous soil sample with 12cm thickness in the modified container of 15cmx30cmx15cm dimensions.

Figure 9: The test results from standard fall cone tests on clayey silt samples (cone.1)

Table 5: Fall cone tests results for clayey silt soil as a uniform layer using standard cone and a mold size of 55mm x 40mm

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Fig. 9 shows the liquid limit test data obtained from fall cone tests on clayey silt soil samples. It can be seen that the moisture vs. cone penetration curve has a rather non-linear form and at 20mm penetration depth of the standard 80.33gr cone, LL is obtained to be about 28.2%. If the curve is extrapolated by a non-linear curve fitting technique, then at a penetration depth of about 2.0mm, the plastic limit of this clayey silt sample is found to be about 20.9%. Table.5 indicates the corresponding data obtained during the tests by standard cone for determining LL, PL and Su of soil. Undrained shear strength of the clayey silt samples are determined here using formula given initially by Hansbo(1957) as:

/ℎ (2)

where Wc is the weight of the steel cone itself plus the steel bar holding the cone with a small pin, K is a calibration factor which is obtained as 1.013-1.06 for standard ELE cone with apex angle of 30o and weight of about 0.78N and hi is the penetration depth of cone into the soil specimen in the standard mold. The fall cone factor K may be obtained based on Hansbo’s method as:

(3)

Here ξ can be computed based on the shear strain rate during the fall cone tests as 0.74 for cone with apex angle of 30o. The calibration factor K may be given as 1.52 for standard cone 1 with apex angle of 30o rather smooth surface and 1.013 for cone 2 with the same apex angle but with a rough surface. K is obtained to be about 0.325 for cone 3 with smooth surface. For cone 4, K is computed as 0.252. It can be seen that the undrained shear strength (SU) range of cohesive soil samples can be determined easily using the Eq.2 based on Hansbo's calibration approach. The results might give an approximate indication of Su and can be compared with the most expensive and accurate direct shear tests results. Similarly, we have calibrated Eq.2 for the modified cones (2,3,and 4) with different apex angles, weights and also varying surface roughness. Karlsson (1977) states that a 10-mm penetration of a 60° cone of mass 60 g corresponds to a soil shear strength of 1.7 kPa. This value corresponds to the mean value of fall cone factor K as obtained above for smooth and rough surfaces (cones 3 and 4) as 0.288 which then gives Su as 1.73kPa which is in very good agreement with his findings and also with the most recent findings by Hazell (2009). For simplicity and sake of comparison, we adopted lower-bound values: K=1.013 for cones 1 and 2 (with apex angles of 30o) and K=0.25 for cones 3 and 4 (with apex angles of 60°) based on GEONOR report, respectively.

Plastic Limit value may be obtained based on Lawrence (1980) and Wasti & Bezirci (1987) as follows:

(4)

where w1,w2, M1 and M2 denote the moisture contents corresponding to 20mm penetration obtained from fall cone tests using two different cones and the weights of these two cones, respectively.

Su for the liquid condition of clayey silt sample is computed from fall cone test data to be around 2.0kPa. While for Su>23.2kPa the clayey silt sample is near its plastic range. At a

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penetration depth of close to 2.0-2.3mm the soil is at plastic limit (PL) this may correspond to Su about 100 times that of LL.

Figure 10: The test results from standard fall cone tests on silty clay samples (cone.1)

Table 6: Plastic Limit test Data of Clayey Silt Sample

Fig.10 shows the standard fall cone test results as plotted in dotted lines and best fitted parabolic curve to the data. As seen the fall cone penetration vs. moisture content of the silty clay soil samples of Azarshahr has a non-linear relationship. The liquid limit obtained in this case corresponding to hi=20mm is about 36%. Plastic Limit of silty clayey soil may be obtained in this case by extrapolating the fall cone data up to a hi=2mm to 3mm as PL=18-21% compared to PL value obtained as 21.22% obtained from plastic limit tests.

Table.6 shows the plastic limit tests data of clayey silt according to ASTM D4218. The plastic limit of silty clay soil sample (of 20gr in weight) is obtained as PL=23.59% of three different specimens each divided and converted into four threads of diameter of 3.2mm. This

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value can be compared with PL=21-23% range obtained from standard fall cone tests for hi=2-3mm. In verification tests for PL of silty clay as performed later in March,2012 using the BS1377 method, the authors obtained average PL value of 22.53% which had shown variations of about 5-6% compared to the average values of the initial tests. These discrepancies are well in the range of expected margin as obtained by Brown and Downing (2001) and Sherwood (1975), Stone and Phan(1995) and Sivakumar et al. (2009). For Clayey Silt soil specimens the initial tests did not yield a proper PL value due to early shrinkage of the threads during rolling process and higher sensitivity of silt compared to clay, but later more careful verification tests gave average values of PL as 23.59% for such soil samples. The latter might also indicate that a more accurate procedure as discussed by Koumoto and Houlsby (2001), Sivakumar et al.(2009) and Rashid (2010) might be adopted based on fall cone tests.

Figure 11: The test results from modified fall cone tests on clayey silt samples

(cone.2)

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Figure 12: The test results from modified fall cone tests on silty clay samples

(cone.2)

MODIFIED FALL CONE TESTS WITH OVERLOAD Figs.11 and 12 show the results of fall cone test using modified cone with a rough surface

but the same apex angle as 30o and the same diameter on clayey silt and silty clay soil samples, respectively. These fall cone test were also performed using a standard mold size 55mm x 40mm. The weight of this modified steel cone (cone.2) was 83.2gr. The test procedure used was the same as standard cone (cone.1) according to BS.1377. As seen, the penetration depth vs. moisture content curve for clayey silt soil sample has almost linear form with a LL=28.57% at hi=20mm. Plastic limit value of this soil may be determined in the range of PL=20.5-21.5% by extrapolating this curve to a penetration depth of hi=2.0-2.3mm. Fig.14 shows a nonlinear relationship of hi-w curve for silty clay soil sample. Test results are indicated with dashed blue line and the fitted parabolic curve has been shown in this graph with a solid dark line. It can be seen that LL=36.81% may be determined for silty clay soil sample at hi=20mm while PL range for this soil type may be obtained in the range of 17-20% from extrapolating the hi-w curve for hi=2.0-3.0mm. Again this silty clay soil has exhibited a semi-plastic condition at hi=5mm.

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Figure 13: The test results from modified fall cone tests on clayey silt samples

(cone.1+50gr overload)

Fig.13 shows the fall cone test results on clayey silt soil sample using a standard cone of ELE with an overload of 50gr. For this cone.2 test, the liquid limit at hi=20mm can be obtained as 27.4%. The corresponding undrained shear strength can be found as 30kPa from Fig.16. It can be seen that Su again in this cone tests decreases exponentially from around 200kPa at hi=8mm to about 10.kPa at hi=34.5mm. Plastic Limit of this clayey silt from extrapolation of hi-w curve in Fig.13 can be obtained in the range of 17.5-20.0% at hi=2.0-2.4mm range. This is in close agreement with the results obtained from the standard fall cone.1 tests as described above. From the plastic limit tests, it is hence found that PL values obtained for the clayey silt soil samples may correspond to hi=2.3-2.4mm.

Figure 14: The test results from modified fall cone tests on silty clay samples

(cone.1+50gr overload)

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Fig.14 shows the modified fall cone tests on silty clay soil samples using a standard cone plus an overload of 50gr in a standard mold of size 55mm x40mm. As shown, for hi=20mm the liquid limit from the tests results plotted here is obtained as about 35% for this soil type. The corresponding undrained shear strength value can be found from about 3.4kPa.

Figure 15: The test results from modified fall cone tests on clayey silt samples (cone.2+50gr overload)

Figure 16: The test results from modified fall cone tests on silty clay samples (cone.2+50gr overload)

Figs.15 and 16 show the results of the modified fall cone tests using cone.2 plus a 50gr overload on clayey silt and silty clay soil samples in a standard mold size 55mm x40mm, respectively. The liquid limit values for clayey silt and silty clay soil types from Figs.15 and 16 can be obtained at hi=20mm as LL=27.61% and LL=35.27%, respectively. These results are also in very good agreements with the LL values obtained from the tests using cone.1 plus the same 50gr overload. However, there is a slight discrepancy due to more roughness of the steel cone.2

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compared to cone.1. The plastic limit range values can also be obtained approximately by extrapolating the Figs.15 and 16 as 18-20.5% and 16-20% for hi=2.0-3.0mm range. Table.7 compares the LL values obtained from various fall cone test using cones.1 and 2 with extra overload of 50gr and without it. It can be seen that with extra 50gr overload the LL values have shown about 2-3% reduction in clayey silt soil and about 4-5% reduction in silty clay soil samples.

Table 7: Comparison of LL values from Various Fall Cone Tests

Figure 17: Comparison of the test results from modified fall cone tests on clayey silt samples using 4 different cones

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Figure 18: Comparison of the test results from modified fall cone tests on silty

clay samples using 4 different cones

MODIFIED FALL CONE TESTS WITH DIFFERENT APEX ANGLE AND ROUGHNESS

IN HOMOGENEOUS SOILS

Fig.17 compares the tests results on clayey silt samples using the cones 1,2,3 and 4 Cone-1 is standard ELE cone made of stainless steel with 30o apex angle and smooth surface. While cone-2 had the same apex angle and the same diameter as cone-1 but with a rather rough surface.Cone-3 had a diameter as twice the cone-1 with an apex angle of 60o and cone-4 with the same dimensions as cone-3 but with a rough surface. The modified cones were also manufactured of stainless steels with different grades in Tabriz. As shown in Fig.17, the fall cone penetration vs. soil moisture curves for the cones 1 and 2 with 30o apex angle have non-linear form in general but are quite close. This might indicate that the effect of surface roughness is not so dominant in this case. Whereas the fall cone penetration vs. soil moisture curves using the modified cones 3 and 4 differ considerably from the previous fall cone test results as shown in Fig.17. For the same amount of soil moisture the penetration depth of falling cones 3 and 4 are much less than cones 1 and 2. This would indicate that the apex angle of cone may have more pronounced effect on the fall cone test results. Fig.17 also indicates that the effect of roughness of the fall cone surface for cones 3 and 4 is more significant than cones 1 and 2. This can be due to larger area and also the different friction and end bearing effects of the cones 3 and 4 with 60o apex angle compared to cones 1 and 2 with 30o apex angle.

Similar trends can be seen in Fig.18 for silty clay soil samples. While the cones 3 and 4 (with larger diameters and weights but with apex angles of 60o) have shown completely different behavior compared to the cones 1 and 2 with 50gr overload disks. The findings in this case endorse the inherent fact that the weight of cone has opposite effect compared to apex angle of falling cone on penetration response curve. In all the test results shown in Figs.17 and 18, near end of curves (i.e. at moisture content values far above LL) the cone penetrates rapidly into soil

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Figure 21: A Non-Homogeneous Soil Sample with 3 Layers (Profile-1)

Figure 22: Comparison of the test results from modified fall cone tests on Non-Homogeneous

Soil using 4 different cones (Profile-1)

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hi=40mm again the penetration is greater in non-homogeneous soil profile-2 compared to homogeneous uniform soil-2. Whereas for cones 3 and 4, a different response is observed during the tests which indicate that after hi>40mm the penetration in homogeneous soil-2 is faster and larger compared to non-homogeneous soil-2. This may be due to the increase end-bearing and skin friction resistance in the case of cones with apex angle of 60o in the second layer of profile-2 compared to its first layer.

DISCUSSION Based on dynamics of fall cone tests, it may be perceived that as the moisture content of the

soil varies (say from just above PL to far above LL values), the penetration response of cone into soil changes from a quasi-static to dynamic behavior. At lower water content of soil sample, the cone penetrates less and so its speed and acceleration during plunge is less than the case of soil with higher moisture content say w>LL. At the upper limit of w, the soil sample has liquefied and hence the cone penetrates more rapidly to greater depths say hi>=60mm. Koumoto and Houlsby (2001) have modeled this behavior in terms of dynamic equation of motion. The water's viscosity damps out a part of energy of the plunging cone. However, for the case of plastic soil or semi-solid state of soil, the falling cone impacts with the soil surface and due to considerable stiffness of soil layer in this case slows down quickly while it penetrates just few mm into the upper layer. Most of the kinetic energy of the falling cone in this case is dissipated in the form of soil's elastic and plastic strain energy. A small part may be transferred into heat energy due to skin friction between the penetrating cone and the interface soil particles.

CONCLUSION The present experimental work has shed more light into fall cone test approach and

increased our knowledge to some extent on this particular subject. In particular, for the first time we conducted successfully the fall cone tests on non-homogeneous soil samples with three layers. We also used four different types of cones with different apex angle and surface roughness and also extra overloads to evaluate the effects of these important parameters on determining the soil's Liquid and Plastic Limits.

The results of the tests conducted here showed that the effect of apex angle of the cone on LL of soil sample may be quite considerable. The cone penetration vs. moisture content of soil response curves are significantly influenced by the apex angle of the cone. For cones 1 and 2 with apex angel of 30o , higher penetration depths are usually observed in the same soil samples tested here compared to cones with apex angle of 60o in The amount of discrepancies in terms of penetration depth hi increased with an increase of water content of soil samples for different cones used in this study. The influence of the surface roughness of cone seems to be less for the cones (1, 2) with apex angle of 30o compared to the cones (3,4) with apex angel of 60o . The influence of an overload disk on hi-w curves or LL or PL seems to be rather more than the roughness effect as tested for cones with the same diameter. But for cones with diameter the influence of cone's weight may overwhelms the surface roughness effect.

It is observed that the effect of cone's weight or overload on LL of silty clay soil samples was in fact more than that for clayey silt soil samples. The effect of cone surface roughness is seen to be somewhat higher for silty clay samples compared to the clayey silt samples. It is also found that for cone with twice diameter and larger weight or overload, the surface roughness may increase LL more than that for standard cone.

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For both homogeneous and non-homogeneous soil samples, the results of fall cone tests conducted during course of this work showed that the penetration depth hi usually varies non-linearly with increase of moisture content of soil.

It is shown that PL values of soil samples can be determined rather accurately by extrapolating the hi-w curves from fall cone test for a penetration depth in the range of 2.0-3.0mm.

It is also verified that using a method established earlier by Lawrence (1980) and Wasti et al.(1987) we could determine the plasticity index and so plastic limit of soil samples tested by means of two different cones (1,3) or (2,4).

It is also shown that the undrained shear strength (SU) range of cohesive soil samples can be determined easily using the Eq.2 based on Hansbo's calibration approach. The results might give an approximate indication of Su and can be compared with the most expensive and accurate direct shear tests results.

The results of fall cone tests on homogeneous soil samples with depth 12cm compared to standard cylindrical ELE container of 4cm depth which was also used showed an increase in effective depth corresponding to LL from 20mm to 21.0-23.5mm.

It is concluded that for non-homogeneous and layered soil samples, the penetration depth of cone (hi) depends on moisture content of soil (w), the type of soil in the first and the second layer and the boundary conditions or the layers of soil. It is generally observed that after hi=40mm due to changes in the boundaries the trend of hi-w curve might change.

It is observed that for considerable variations in water content or weight of cone or apex angle the fall cone response in test soil samples might change from quasi-static to dynamic. It may be verified that dynamic equations given by Koumoto and Houlsby (2001) can be applied for varying weight and apex angle conditions for homogeneous soils in the above tests. However, for layered soil profiles 1 and 2, this computational approach might be rather approximate due to changes in the soil layers.

ACKNOWLEDGMENT This research work was carried out at Azarbaijan T.M. University Soil Mechanics Lab.

Assistance of Mr. Ziayee at Soil Mechanics Laboratory and Chemical Laboratory of Azarbaijan T.M. University are greatly appreciated. Gratitude is also given to Dr. H. Soltani for his support by providing the geotechnical laboratory room and also for his useful comments.

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Effects of Urmieh Salt Water on the CBR Test Results of GSCW and GSBW Soil Samples ", Electronic Journal of Geotechnical Engineering (EJGE), http://www.ejge.com/2008/Ppr0892/Ppr0892.htm.

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16. Koumoto, T. and Houlsby, G.T. (2001) "Theory and Practice of the Fall Cone Test ", Geotechnique 51, No.8, pp.701-712.

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