New methods of determination of Atterberg limits of Philippine soils

Delfin C. Suministrado1

Introduction

Many theories in soil mechanics have been formulated and advanced to model
the interactions of soil with various tools, wheels and other soil-engaging instruments.
Generally, the purpose is to evaluate the strength behavior as affected by soil types
and moisture conditions. Along with these, soil classification systems have also been
devised to better understand soil characteristics with regards to their suitability as an
engineering material.

If a soil does not contain a substantial proportion of clay and its specific surface is
relatively small, the grading system involving particle size distribution and the
determination of specific volume may be enough for engineering purposes. However,
soils containing a considerable quantity of clay require additional tests. One set of these
tests which is used to classify soil for clay content and strength characteristics involves
the determination of soil consistency.

Consistency refers to the resistance of soil to deformation or rupture. It is the

manifestation of the physical forces of cohesion and adhesion acting within the soil at
various moisture contents. In the early 1911, a Swedish agricultural engineer, Albert
Atterberg, developed a classification system and method by which soil consistency
may be determined. The method is based on the determination of soil water content at
distinct transitions between the different states of soil consistency. The boundaries of
these states expressed in terms of soil moisture content are now collectively referred
to as Atterberg limits.

There is a continuing worldwide research and development work on Atterberg

limits, on their significance in soil classification, on their application in assessing
engineering behavior, and on the formulation of new methods of their measurement.
In the Philippines, as local agricultural engineers and other soil scientists have yet to
understand the basic principles and importance of these indices of soil behavior and
property, it is expedient to be aware of the current international trends about this
aspect of soil mechanics and consider their implications to local situation. It is also
necessary to conduct more research work in the country to help advance the
knowledge about Philippine soils for applications in agriculture and engineering.

1
Professor V, Agricultural Machinery Division, Institute of Agricultural Engineering, College of
Engineering and Agro-industrial Technology, UPLB.
The Atterberg limits of soil

Current engineering practice mostly uses only two of these Atterberg limits,
the liquid and the plastic limits, while the shrinkage limit is referred to occasionally.
The liquid limit (LL or wL) is the water content of the soil at which the soil passes
from a plastic to a liquid state. The plastic limit (PL or wP) is the lowest moisture
content at which soil passes from the plastic state to the crumbly semi-solid condition.
The liquid and plastic limits define the range of water content over which the soil
exhibits plastic behavior. This range itself is referred to as the plasticity index (Ip).
The shrinkage limit is the moisture content that defines where the soil volume will not
decrease further if the moisture content is reduced.

Significance and applications of Atterberg limits

The liquid limit, plastic limit and plasticity index of soils are used extensively,
either individually or together with other soil properties, to correlate with engineering
behavior such as compressibility, permeability, compactibility, shrink-swell
characteristics and shear strength. The plasticity chart of Casagrande which makes use
of these values is now an accepted handy tool for classifying soils (Holtz and Kovacs,
1981).

Recent research investigations conducted worldwide add clarification to the

importance of knowing Atterberg limits of soils to both engineering and agriculture.
In Turkey, Lav and Ansal (2001) carried out experiments to determine the
correlations between consolidation properties based on test results obtained from 300
soil samples. They observed that the void ratio, water content, liquid limit, and dry
unit weight yielded sufficiently reliable correlations. Smith and Buchan (2002) who
conducted a comprehensive study of undisturbed soil samples collected in the North
Atlantic explained the advantage of using the indices and their derivatives in
estimating the geophysical and engineering behavior of clay-rich sediments in sea-bed
environmental conditions instead of performing analysis of undisturbed soil samples.

Sridharan and Nagaraj (2005) who worked with Indian soils, found that the
plastic limit bears a good correlation with the compaction characteristics, namely
optimum moisture content and maximum dry unit weight, much better than liquid
limit or plasticity index. Islam, et al. (2006) who used both the percussion method for
liquid limit determination and the thread rolling method for plastic limit determination,
reported about the physical properties and Atterberg limits of selected soils in
Mymensingh district in Bangladesh, and established relations between plastic limit
and soil gradations. Yukseleni and Kaya (2006) investigated the relationships between
cation exchange capacity (CEC) and various other soil engineering properties and
reported on a quick method of estimating CEC. Dolinar, et al. (2007) who used the
fall-cone method for liquid limit determination and the thread rolling test for plastic
limit determination, reported about the correlation between soil surface area and
Atterberg limits of five randomly selected soil samples. Using data from across the

2
US, Hawaii and Alaska, Seybold, et al. (2008) developed linear correlation models
between Atterberg limits and some basic soil properties such as clay content and CEC
in order to predict liquid limit and plasticity index.

For more than half a century since Casagrande proposed the plasticity chart, it
has not been given much scrutiny. Recently, a new plasticity chart, which also aims
to classify soils using the Atterberg limits, has been proposed by Polidory (2003). It
differs from Casagrande's plasticity chart, especially in terms of its silt and clay zones,
whose positions are reversed compared with Casagrande's chart. He explained that,
contrary to what is commonly believed, in inorganic soils—liquid limits being
equal—the plasticity index increases as the clay content decreases.

A revision of Casagrande’s chart has also been advanced by Gutierrez (2006,

2007) who proposed the use of regression analysis in establishing the linear behavior
of Atterberg limits of soils which come from the same geotechnical ‘formation’. He
demonstrated that the correct representation in the plasticity chart must involve the
liquid limit, LL, and plastic limit, PL, instead of the current LL and plasticity index, Ip.

Liquid limit determination

For the liquid limit, the percussion or Casagrande method has been the
standard in use in the United States and it must have been the de facto standard in the
country as Philippine engineers had borrowed many engineering traditional practices
from the Americans. This method was developed and standardized by Casagrande
(1932). To determine the liquid limit, the standard metal cup containing a pat of clay
is given repeated standard blows by being dropped through a height of 10 mm onto a
hard rubber or micarta base. When the soil is at its liquid limit, the groove made on
the soil with a standard grooving tool would close at its base over a distance of 13 mm
in exactly 25 blows. In real practice, liquid limit is obtained by interpolation from a
‘flow curve’ which relates water content and the number of blows that closes the
groove (Wood, 1990).

Many other countries have however already shifted to the drop-cone or fall-
cone penetrometer technique to determine liquid limit. To perform the test, a cone is
allowed to fall freely under its own weight from a position at rest with the point of the
cone just touching the surface of the soil sample. As the cone is released, it
accelerates initially and then decelerates to rest and penetrates a distance at which the
soil resistance is equal to the weight of the cone (Wood, 1990). The moisture content
of a soil at which a 60 gm cone of 60° tip angle would produce a penetration of 10
mm is designated as the liquid limit in Sweden and other Scandinavian countries.
Although Japan uses the same type of cone as that of Sweden, the adopted depth of
penetration is at 11.5 mm. A 30° cone of mass 80 gm at 20 mm penetration is the
standard in the UK. Table 1 below shows the variations of the cone standards adopted
by some countries to determine liquid limits of soils.

3
Table 1. Standards for the fall cone liquid limit test in different countries (Sampson and
Netterberg, 1984; Koumoto and Houlsby, 2001).

Cone details Penetration Penetration at

Country Apex angle, ° Mass, gm time, s liquid limit, mm
Sweden, Norway 60 60 10 10
Canada 60 60 5 10
Japan 60 60 5 11.5
India 31 148 ? 25.4
UK 30 80 5 20
New Zealand 30 80 5 20
France 30 80 ? Variable
China 30 76 5 17
USSR+ 30 76 5 10
Georgia Institute of 30 75 ? 10
Technology (USA)+
+
The USSR and Georgia Institute of Technology cones are not fall cones, instead the cone is loosened
slowly until the weight is carried by the soil.

The preferred method by the ISO is the fall-cone while the Casagrande method
is considered as an alternate. The ISO recognizes that results using the percussion or
Casagrande method are subject to the performance and judgement of the operator.

The estimated soil strength at the liquid limit is 1.71 kPa according to the
Swedish definition and 1.67 kPa according to the British definition. Theoretical
values were reported by Koumoto and Houlsby (2001) to vary from 1.38 kPa to 4.52
kPa. Wroth and Wood (1978) suggested a strength of 1.7 kPa and indicated a possible
range of 0.7 -2.65 kPa. Liquid limit values obtained using the Casagrande apparatus is
assumed to correspond to a strength level of 2 kPa (Wood, 1990) which is within the
range suggested by Wroth and Wood above.

Plastic limit determination

The current standard of most countries for plastic limit determination is the
thread-rolling method. The process involves rolling the clay with fingers until it
formed threads. The threads are put together and rolled again, and the process is
repeated until the clay could no longer be rolled but broke into pieces. Plastic limit is
defined as the moisture content at which the thread of soil crumbles at 3 mm diameter.

The method is currently the same worldwide but investigations on new

techniques which would be less arbitrary and less dependent on the skill of the
operator have been continuously pursued. At least three measurement procedures
based on the drop-cone principle have already been proposed (Campbell, 1976; Feng,
2000; Kodikara, et al. 2006). The method developed by Feng (2000) which uses the
30° cone of mass 80 gm, the UK standard, has already been evaluated by some soil
researchers along with other cone types as they searched for better methods of plastic
limit determination (Weaver and Hullugalle, 2001; Feng, 2004; Rashid, 2005;

4
Kodikara, et al. 2006; Tanpo, et al., 2007). The fall-cone method of Feng (2004) is
already at a relatively high stage of development as a plastic limit measurement
method. Also, a patent for method and apparatus for determining plastic limit of soil
based on the drop-cone principle has been filed in Great Britain along with parallel
patent applications to many other countries around the world.

On the basis of the 100-fold variation of the shear strength of soil from liquid
limit to plastic limit, the undrained shear strength of soil at plastic limit is equivalent
to about 170 kPa. The model of Koumoto and Houlsby (2001) involving drop-cone
technique, however gave a value of only 138 kPa and an average of 108 kPa. Wood
(1990) proposed an average value of around 200 kPa.

A unified cone penetrometer method to determine soil plasticity?

There is a general trend to adopt the principle of the drop-cone method for
both the liquid limit and plastic limit determination for its reliability and consistency.
Although the technique has already been standardized in many countries for liquid
limit, it is only these later years that similar research and development investigations
are being conducted for its application to the plastic limit.

Koumoto and Houlsby (2001) made a theoretical analysis of the cone

penetrometer technique and conducted experimental work of its operation for liquid
limit determination. On the basis of their work, they proposed that the 60°, 60 gm
fall-cone at 10 mm penetration be used to internationally redefine the water content at
liquid limit.

For plastic limit, the standard thread rolling technique has long been criticized
for its subjectivity, poor repeatability and unreliability. The simple but tedious
procedure appears as a primitive test in the age of electronics and computers. Current
proposals to adopt the drop-cone method for plastic limit determination is gaining
attention and favor. Although Koumoto and Houlsby (2001) had proposed that the
60°-60gm fall cone at 1 mm be used as standard for plastic limit, they realized the
practical problem of the 1 mm depth itself. Instead, Feng (2001, 2004) had developed
and proposed the use of the 30°-80gm cone at 2 mm depth.

Around the vicinity of the Philippines, Feng (2004) of Taiwan has been
working to refine the use of the small ring and fall-cone to determine the plastic limit
in continuation of his earlier work where he evaluated the relationship between the
fall-cone penetration and the water content of clays (Feng, 2000). Rashid (2005) in
Malaysia reported on the fabrication and testing of three cones with different sizes
and weight and compared their performance with the thread rolling method. He found
that the 20°-101.47gm, the 30°-240gm and the 30°-80gm cones could all feasibly
perform plastic limit test but concluded that the 20° cone gave the best correlations
with the standard thread rolling method. In Thailand, the group of Tanpo, et al. (2007)
is currently working to develop the use of the cone penetrometer to determine both the

5
liquid and plastic limits. In Australia, Weaver and Hulugalle (2001) found that
quicker and more consistent results were obtained by a less experienced operator with
the drop-cone penetrometer method than the thread rolling method.

Despite arguments presented by many researchers, Prakash and Sridharan

(2006) opposed the shift to the drop cone. Contrary to the trend of finding the cone
penetrometer method as the appropriate and unified method of determining the
Atterberg limits, they reported opposite findings and made different conclusions.
They claimed that the percussion method of liquid limit determination and the 3 mm
thread rolling method of plastic limit determination appropriately measure the factors
responsible for soil plasticity while the cone penetration method cannot represent the
soil plasticity fully. They claimed that the mechanisms that come into play during
testing relate to undrained strength due to both undrained cohesion and undrained
friction. They therefore stressed that the percussion and 3 mm thread rolling methods
must be the only ones used to determine the plasticity of soils.

Atterberg limits of selected samples of Laguna soils

Soil samples were obtained from different locations in the province of Laguna,
Philippines and used in the research work on the critical evaluation of the techniques
of Atterberg limits determination. The data of the soil textural analysis of eight soil
samples used by Polinga (2001), Suministrado (2001), Gareza (2005) and
Suministrado and Gareza (2007) are given in the Table 2 below.

Table2. Textural analysis of selected soil samples (Suministrado, 2001; Gareza, 2003).
Percentage
Source of soil sample sand silt clay Textural grade
CEAT, UPLB 11 42 47 Silty clay
Banago, Nagcarlan 25 56 19 Silt loam
San Roque, Luisiana 25 35 40 Clay
San Juan, San Pablo 47 35 19 Loam
Poblacion, Nagcarlan 21 34 45 Clay
Sabang, Nagcarlan 27 49 24 Loam
San Francisco, Victoria 18 44 38 Silty clay loam
Dita, Liliw 21 46 33 Clay loam

Liquid limits of soil samples

The soil samples were tested for liquid limits using the procedures for the
percussion and drop-cone methods explained above. However, instead of the standard
three-point or four-point method to establish ‘flow curves’, more data points were
obtained, and by statistical correlation analysis, the values of the liquid limits were
calculated.

6
 For the Casagrande liquid limit, the linear log-normal relationship of the
number of blows at closure of the groove and the moisture content was used to
determine the liquid limit values.

For the drop-cone method, a 30°-80gm penetrometer at 20 mm depth of

penetration was used. In the analysis of data, the linear log-log relationship of the
depth of cone penetration and moisture content was adopted. This analytical
procedure was a modification from the log-normal relationship earlier used in the
papers reported by Polinga (2001), Suministrado (2001) and Gareza (2003). The
change of procedure was made on the basis of the observations reported by Feng
(2000, 2001), Koumoto and Houlsby (2001) and Dolinar et al. (2005) who performed
mathematical and laboratory verifications to determine the appropriate kind of
relationship. Their findings were further supported by results of the thermomechanical
analysis of a family of soil models made by Collins and Kelly (2002). With this
change of the method of analysis of the same sets of raw data, the liquid limit values
in this paper are slightly different from those found in the earlier papers.

Polinga (2001) and Suministrado (2001) had critically compared the use of the
two methods of liquid limit determination. They had reported data obtained by novice
operators on the measurement techniques using both the percussion (Casagrande) and
the drop-cone methods (See Table 3 below). The relative advantage of the drop-cone
method is clearly evident from the calculated correlation coefficients. Data set 1
involves 20 data points each for both the Casagrande method and the drop-cone
method while Data set 2 involves at least 5 data points for the Casagrande method and
6 data points for the drop-cone method.

Table 3. Liquid limit data reported by Polinga (2001) and Suministrado (2001) on three Laguna
soil samples.
Soil sample Data set 1 Data set 2
Casagrande Drop-cone Casagrande Drop-cone
CEAT, UPLB 56 (0.06)1 52 (0.18) 62 (-0.85) 62 (0.99)
Banago, Nagcarlan 46 (0.00) 54 (0.32) 50 (-0.21) 49 (0.98)
San Roque, Luisiana 83 (-0.42) 79 (0.61) 84 (-0.07) 81 (0.99)
1
The number in parentheses are correlation coefficients.

Gareza (2003) extended the above work by using five other soil samples.
Equipped with the basic theoretical knowledge and adequate practice in performing
the test procedure, the operator obtained the values with higher correlation
coefficients compared to the earlier data of Polinga (2001) and Suministrado (2001).
The test results are shown in Table 4 below. All liquid limit values were calculated by
statistical regression using 10 data points each.

7
Table 4. The liquid limits of the soil samples obtained by Casagrande and drop-cone methods
and the corresponding correlation coefficients (Gareza, 2003).
Method
Soil sample Casagrande Drop-cone
Liquid limit R Liquid limit R
San Juan, San Pablo 42 -0.81 46 0.98
Poblacion, Nagcarlan 58 -0.88 60 0.97
Sabang, Nagcarlan 55 -0.85 57 0.94
San Francisco, Victoria 47 -0.82 52 0.96
Dita, Liliw 59 -0.86 61 0.97

Gareza (2003) also performed procedures to determine the time required to

conduct the measurement methods. For the percussion method, he observed that it
took 15 to 40 seconds from the time the soil was placed into the Casagrande cup until
the time when the operator was about to obtain sample for initial weight
determination. For the drop-cone method, he reported a range of 10 to 30 seconds
starting from the time when the soil sample is placed into the cup until the time when
the initial weight of the soil sample was determined. As he had previously evaluated
the average rate of moisture loss at 0.014 g/min and 0.011 g/min for the Casagrande
cup and the penetrometer cup respectively at the 80% relative humidity conditions in
the laboratory, he concluded that the total moisture loss by evaporation during the
conduct of each test was negligible.

Furthermore, he reported that an average time of 40 minutes was required to

complete the 10-point Casagrande test starting at the moment the soil and water were
mixed until the last soil sample for moisture content determination was placed in the
drying oven. A relatively longer time, an average of 50 minutes, was required to
perform the 10-point penetrometer test. He attributed this time difference to the
greater amount of sample of soil and water to mix and fill the test cup.

Overall, the experimental results of Gareza (2003) on liquid limit tests also
validated the relative advantage of the drop-cone method over the percussion
technique of Casagrande.

The analyses of data showed that the liquid limit values by the drop cone
penetrometer method were generally higher than those determined by the Casagrande
method at low plasticity (See Figure 1). The difference appeared higher by about 4
units at liquid limit values of 50 and below, but tapered to about 2 units as liquid limit
increased to 60. Although Sampson and Netterberg (1984) had found that the drop-
cone liquid limit values are at least higher by 4 units than the Casagrande liquid limits
for South African soils, other workers claimed that the difference in values is
negligible. In fact, Sridharan and Prakash (1998) wrote that the liquid limit obtained
by the percussion method can be as low as about 26% less and as high as 71% more
than the liquid limit obtained by the cone method.

8
 90
85 Equality line

80
Drop-cone liquid limit

75 San Roque
Dita, Liliw
70
Pob lacion, Nag.
65
60 CEAT

55 San Francisco
Sab ang
50
Banago
45
San Juan
40
40 45 50 55 60 65 70 75 80 85 90
Casagrande liquid limit

Figure 1. Comparison of the Casagrande and drop-cone liquid limits of the eight soil samples
(Suministrado, 2001; Suministrado and Gareza, 2007).

Although Budhu (1985) has attributed this difference to the clay content of
soil, other researchers presented different observations. As cited by Sridharan and
Prakash (1998), the lateritic soils of Queiroz de Carvalho (1986) which contain
kaolinite as the only clay mineral gave higher values for the drop-cone method than
the percussion method. On the other hand, the montmorillonite-rich soils from
Turkey (Wasti and Bezirci, 1986) gave much higher liquid limit values for the
percussion method than the drop-cone method. Similarly, Sridharan and Prakash
(1998) had also observed that based on their own data, the kaolinitic clays had higher
liquid limit values obtained by drop-cone method than the percussion method, while
montmorillonitic soils had higher liquid limit values for the percussion than for the
drop-cone method.

Plastic limits of soil samples

Polinga (2001) and Suministrado (2001) reported plastic limits values by

thread rolling method performed by the novice operators on three soil samples. The
big disparity of the two sets of data as shown in Table 5 is apparent. Polinga (2001)
explained the difficulty of performing the thread rolling procedure particularly with
regards to the forces exerted by the hands while rolling the soil and the correct time to
stop when the soil has dried up to its plastic limit. As Feng (2004) explained, the
operator himself is required to judge the state of crumbling as he estimates whether or
not the rolled soil has already reached the 3 mm diameter.

9
 Using the drop-cone raw data of Suministrado (2001), statistical correlation
analyses were performed and the regression lines were extrapolated to determine the
moisture content at 2 mm penetration. This procedure is proposed by Feng (2004) and
is based on the assumption that the undrained shear strength at the plastic limit is 100
times the undrained shear strength at the liquid limit (Feng, 2000). Table 5 also shows
the plastic limit values by this drop-cone method.

Table 5. Plastic limit data reported by Polinga (2001) and Suministrado (2001) on three Laguna
soil samples.
Soil sample Data set 1 Data set 2
Thread rolling Thread rolling Drop-cone1
CEAT, UPLB 35 38 38
Banago, Nagcarlan 34 39 29
San Roque, Luisiana 66 49 40
1
Obtained from the extrapolated liquid limit drop-cone data at 2 mm penetration (Feng, 2004).

Gareza (2003) determined plastic limits values by the thread rolling method
and by the drop-cone method suggested by Rio (1988) which the uses a 30°-240gm
cone. In the procedure explained by Rio (1988), the plasticity index, Ip, was first
calculated by the formula:

Plasticity index, Ip = 2 Δ/log (240/80)

where Δ is the vertical difference of the best-fitted lines at 80gm cone and 240gm
cone at the 20 mm penetration. The plastic limit is obtained from the formula:

Plastic limit, PL = Liquid limit, LL – Plasticity index, Ip

A third set of values of the plastic limit was also calculated from the liquid
limit drop-cone raw data reported by Gareza (2003) similar to the extrapolation
procedure of Feng (2004). Statistical correlation analyses were performed and the
regression line was extended to determine the plastic limit moisture content values at
the 2 mm penetration. The three sets of plastic limit values are given in Table 6
below.

Table 6. Plastic limits of three Laguna soil samples (Gareza, 2003; Suministrado and Gareza,
2007).
Soil sample Plastic limit
Thread rolling1 Drop-cone 12 Drop-cone 23
San Juan, San Pablo 35 (2.14) 24 27
Poblacion, Nagcarlan 38 (4.99) 24 35
Sabang, Nagcarlan 37 (0.65) 26 40
San Francisco, Victoria 34 (1.97) 27 35
Dita, Liliw 39 (1.94) 31 39
1
Average of at least six trials; Numbers in parentheses are the variances.
2
Obtained by the 30°-80gm and 30°-240gm drop-cone method of Rio (1988).
3
Obtained from the extrapolated liquid limit drop-cone data at 2 mm penetration (Feng, 2004).

10
 The skill of the operator has been identified as a great factor that affects the
plastic limit values by the thread rolling method. The indicated variances given in
Table 6 offer some light on the degree of variation of the data obtained during the
laboratory tests. In Table 7 below, at least seven operators most of whom are
beginners performed the thread rolling method to determine the plastic limits of 12
soil samples. As beginners, the variances are relatively higher than those in Table 6.

Table 7. Plastic limit values by thread rolling method obtained by various operators (Gatchalian,
2007).
Source of soil sample Set 1 Set 2 Set 3 Set 4 Ave. Variance
Cabanbanan, Pagsanjan 34 34 38 42 37 3.72
Dayap, Calauan 41 42 46 41 42 2.46
Gatid, Sta. Cruz 30 33 43 27 33 6.98
Maahas, Los Banos 33 38 43 39 38 4.20
Malaking Ambling, Magdalena 33 39 45 36 38 5.01
Masapang, Victoria 36 38 43 40 39 2.95
Masiit, Calauan 50 47 51 52 50 2.16
Pagsawitan, Pagsanjan 33 36 40 31 35 3.91
Palayan, Nagcarlan 40 42 46 39 42 3.10
San Antonio, Pila 31 30 38 36 34 3.93
Sto. Domingo, Bay 41 52 49 47 47 4.62
Taytay, Nagcarlan 46 40 50 48 46 4.32

Rio (1988) outlined a procedure by which the plastic limit could be indirectly
determined from the 80 gm and the 240 gm cone penetration at 20 mm. He gave no
theoretical basis of the process although he must have drawn from the original idea of
Wroth and Wood (1978). A somewhat similar method was proposed by Kodikara et
al. (1986) who used 80gm and 230gm cone weights. As shown in Table 6 above, the
obtained plastic limit values by this method explained by Rio (1988) are much lower
than those of the standard thread rolling method and those of the drop-cone method
of Feng (2004). This procedure may not be an attractive alternate to the current
standard thread rolling method as this would require twice the amount of tests (Feng,
2006).

The comparison of the plastic limit data contained in Table 5 and Table 6 for
the thread rolling and the drop-cone methods involving eight soil samples is shown in
Figure 2. Some data points seem to lie along the equality line. But with many more
points lying towards inequality, there are two likely reasons. Either the plastic limit
values are not correct, or the theory itself that the plastic limit by the thread rolling
method and that of the drop-cone method are the same is not correct. However, with
all the previous claims and evidences in support of the equality of the two plastic limit
values at least for the 30°, 80 gm cone of the UK standard (Feng, 2004), the former
reason is the most likely problem that creates the discrepancy. The untrained
operators could have obtained wrong values of the plastic limits for the thread rolling
method, the drop-cone method or both. Also, the data used to extrapolate the plastic
limits may not be adequately representative of the appropriate data. It is to be
remembered that the raw data which were analyzed and extrapolated by statistical

11
method were not for plastic limit determination but were originally meant for liquid
limits such that the depths of penetration were not lower than 9 mm and were mostly
within the range of 10 mm and 35 mm.

60
Equality line
55
Drop-cone plastic limit

50
CEAT
45

40 Sabang
Dita, Liliw San Roque
35 San Francisco
Poblacion, Nag.
30

25 San Juan Banago

20
20 25 30 35 40 45 50 55 60
Thread rolling plastic limit

Figure 2. Comparison of the plastic limits values obtained (a) by the thread rolling method and (b)
by extrapolating the drop-cone data of Gareza (2003) and Suministrado and Gareza (2007) at
2 mm penetration.

Conclusions and Recommendations

In the overall, any test procedure which can satisfactorily provide the correct
values of the soil indices and which reduces if not totally eliminates subjectivity and
variation of the indices being measured must be considered.

The results of many research work on the Atterberg limits of soils conducted
worldwide provide the possibility of procedurally unifying the methods of
measurement of the two indices of soil plasticity. The use of the drop-cone method,
currently the standard for determining liquid limit in many countries, has been shown
capable of being extended to also measure plastic limit. With the established
advantages of drop-cone method over the percussion (Casagrande) method in
determining liquid limit, alongside the questionable values of plastic limits obtained
by the cumbersome thread rolling method, this unification can definitely provide for
the desired simplicity and time efficiency of soil plasticity tests.

The experiences of local investigators discussed in this paper whose data on a

limited number of soil samples obtained around Laguna, Philippines area similarly
confirm the observations on the advantages of the drop-cone method over the

12
percussion method for liquid limit and the thread rolling method for the plastic limit.
Therefore, if the country has to adopt a new standard, the 30°-80gm penetrometer of
UK is the most preferred. Although Koumoto and Houlsby (2001) had claimed that
the 60°-60gm has a relative advantage over the UK standard as it is less sensitive to
cone roughness, the prospect of performing another test involving another set of cone
and penetration standard is discouraging. The 30°-80gm cone which has been
thoroughly studied by Feng (2004) easily lends itself to the 20 mm penetration for
liquid limit and 2 mm penetration for plastic limit.

Research work along this line is definitely necessary if the standards on soil
plasticity index have to be redefined in the country. Currently, there is yet a dearth of
data on local soils regarding Atterberg limits. As we strive to accumulate similar basic
data on soil properties for purposes of application in agriculture and engineering, we
also have to continue the research and development work for proper measurement
methods and accurate and reliable instrumentation systems in order to obtain
meaningful results and data.

References

Atkinson, J. H. and Bransby, P. L. (1982). “The mechanics of soils, an introduction to

critical state soil mechanics,” McGraw-Hill Book Company, UK.
Bowles, J. E. (1986). “Soil physics,” 3rd Ed. John Wiley and Sons, Inc., New York,
USA.
Budhu, M. (1985). “The effect of clay content on liquid limit from a fall cone and the
British cup device,” Geotechnical Testing Journal, 8(2):91-95.
Campbel, D. J. (1976). “Plastic limit determination using a drop-cone penetrometer,”
J. Soil Science, 27, 295-300.
Casagrande, A. (1932). “Research on the Atterberg limits of soils,” Public Roads 13,
121-136.
Collins, I. F. and Kelly, P. A. (2002). “A thermomechanical analysis of family of soil
models,” Geotechnique, 53(7), 507-518.
Dolinar, B. and Trauner, L. (2005). “Impact of soil compaction on fall cone tests
results,” Journal of Geotechnical and Geoenvironmental Engineering, 131(1),
126-130.
Dolinar, B., Misic, M. and Trauner, L. (2007). “Correlation between surface area and
Atterberg limits of fine-grained soils,” Clay and Clay Minerals, 55(5), October
2007, 519-523.
Feng, T. W. (2000). “Fall cone penetration and water content relationship of clays,”
Geotechnique, 50(2), 181-187.
Feng, T. W. (2001). “A linear log d-log w model for the determination of consistency
limits of soils,” Can. Geotech. J., 38, 1335-1342.
Feng, T. W. (2004). “Using a small ring and a fall-cone to determine the plastic
limit, ” Journal of Geotechnical and Geoenvironmental Engineering, 130(6),
630-635.

13
Feng, T. W. (2006). Closure to “Using a small ring and a fall-come to determine the
plasti limit,” Journal of Geotchnical and Geoenvironmental Engineering,
132(2), 276.
Gareza, Adrian G. (2003). “Comparative study of Atterberg limits determination
methods using selected Laguna soils”, Unpublished undergraduate thesis,
CEAT, UPLB.
Gatchalian, Giancarlo Enrique G. (2007), “Comparative study of plastic limit
determination methods of selected Laguna soils”, Unpublished undergraduate
thesis, CEAT, UPLB.
Gutierrez, A. (2006). “Determination of Atterberg limits: uncertainty and
implications,” Journal of Geotechnical and Geoenvironmental Engineering,
132(3), 420-424.
Gutierrez, A. (2007). Closure to “Determination of Atterberg limits: uncertainty and
implications,” Journal of Geotechnical and Geoenvironmental Engineering,
133(5), 613-614.
Holtz, R. D. and Kovacs, W. D. (1981). “An introduction to geotechnical
engineering,” Prentice-Hall Inc., New Jersey.
Islam, Md. Tohidul, Islam, Md. Serazul and Hoque, Md. Nurul. (2006). “Properties of
some selected soil under Mymensingh District in Bangladesh,” Journal of
Agriculture and Rural Development, 4(1&2), 149-154.
Kodikara, J., Seneviratne, H. N. and Wijayakulasooryia, C. V. (2006). “Discussion of
‘Using a small ring and a fall-cone to determine the plastic limit’ by Tao-Wei
Feng,” Journal of Geotechnical and Geoenvironmental Engineering, 132(2),
276-278.
Koumoto, T. and Houlsby, G. T. (2001). “Theory and practice of the fall cone test,”
Geotechnique, 51(8), 701-712.
Lav, M. A. and Ansal, A. M. (2001). “Regression Analysis of Soil Compressibility,”
Turk J Engin Environ Sci, 25 (2001), 101 – 109.
Leroueil, S. and Le Bihan, J. P. (1996), “Liquid limits and fall cones,” Canadian
Geotechnical Journal, 33; 793-798.
Polidori, E. (2003), “Proposal for a new plasticity chart,” Géotechnique, 53(4), 397 –
406.
Polinga, C. A. (2001), “Determination of plastic limit and liquid limit of three soil
types of Laguna,” Unpublished research paper in Agricultural Engineering
262(Advanced Agricultural Soil Mechanics), CEAT, UPLB.
Porsinsky, T., Sraka, M., and Stankic, I. (2006), “Comparison of two approaches to
soil strength classifications,” Croatian Journal of Forest Engineering,
27(2006)1.
Prakash, K. and Sridharan, A. (2006). “Critical appraisal of the cone penetration
method of determining soil plasticity,” Canadian Geotechnical Journal, 43(8):
884–888.
Queiroz de Carvalho, J. B. (1986). “The applicability of the cone penetrometer to
determine the liquid limit of lateritic soils,” Geotechnique, 36(1), 109-111.
Rashid, S. S. A. (2005). “Determination of plastic limit of soil using modified cone
penetrometer method,” Master’s thesis, Universiti Teknologi Malaysia.

14
Rio, Walden S. (1988). Soil mechanics laboratory manual, National Bookstore,
Manila.
Sampson, L. R. and Netterberg, F. (1984). “A cone penetration method for measuring
the liquid limits of South African soil and its implications,” Proceedings of the
Eighth Regional Conference for Africa on Soil Mechanics and Foundation
Engineering, Harare.
Seybold, C. A., Elrashidi, M. A., Engel, R. J. (2008). “Linear regression models to
estimate soil liquid limit and plasticity index from basic soil properties,” Soil
Science, 173(1), 24-34, January 2008.
Smith, D. T. and Buchan, S. (2002). “Usefulness of classification and index properties
of clay-rich sediments in evaluating sea-bed environmental conditions,” Geo-
Marine Letters, 22(2): 60–65.
Sridharan, A. and Nagaraj, H. B. (2005). “Plastic limit and compaction characteristics
of fine-grained soils,” Ground Improvement, 9(1), 17-22.
Sridharan, A. and Nagaraj, H. B. (2006). “Prediction of engineering properties of fine-
grained soils from their index properties,” Indian Institute of Science,
http://hdl.handle.net/2005/209.
Sridharan, A. and Prakash, K. (1998). A discussion on the “Liquid limits and fall
cones” by Leroueil, S. and Le Bihan, J-P., Canadian Geotechnical Journal,
35(2), 407-408.
Suministrado, D. C. (2001). “Determination of Atterberg limits of Philippine soils,”
Philippine Agricultural Mechanization Bulletin, 8(2), 18-23.
Suministrado, D. C. and Gareza, A. G. (2007). “Comparison of methods of
determining liquid and plastic limits of selected Laguna soils”, Philippine
Agricultural Mechanization Bulletin, 14(1).
Wasti, Y. and Bezirci, M. H. (1986). “Determination of the consistency limits of soils
by the fall cone test,” Canadian Geotechnical Journal, 23(2), 241-246.
Weaver, T. B. and Hulugalale, N. R. (2001). “Evaluating plastic limit in vertisols with
a drop-cone penetrometer,” Communications in soil science and plant analysis,
32(9-10), 1457-1464.
Wood, D. M. (1990). “Soil behaviour and critical state soil mechanics,” Cambridge
University Press, Cambridge, NY, USA.
Wroth, C. P. and Wood, D. M. (1978). “The correlation of index properties with some
engineering properties of soils.” Canadian Geotechnical Journal, 15(2), 137-
145.
Yukeseleni, Y. and Kaya, A. (2006). “Prediction of cation exchange capacity from
soil index properties,” Clay Minerals, 41(4), 827-837.

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