Changes in penetration resistance of Ultisols fromsouthern China as affected by shearing

B. Zhanga,*, R. Hornb, T. Baumgartlb

aInstitute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, PR ChinabInstitute of Plant Nutrition and Soil Science, University Kiel, Olshausenstr. 40, 24118 Kiel, Germany

Received 14 March 2000; received in revised form 4 July 2000; accepted 10 August 2000

Abstract

Shear stresses and soil properties modi®ed due to stress play an important role during formation of seals in a series of

rainfall events and during tillage. The objectives of the study were to evaluate the effects of the penetrometer geometry on the

penetration resistance as affected by shearing under different initial soil conditions and to use the information on soil strength

to elucidate shearing process. Nine homogenous air-dried soils (<2 mm) were sprayed and stored so as to obtain equilibrium

soil water contents. The moist soils were sheared by horizontal displacement of layers of soil particles/aggregates in between

hands in one direction. The soil cores were prepared with comparable bulk density before the measurement of maximum

penetration resistance (Pmax) with a small ¯at tip and a cone tip penetrometers. At a wide range from 0.05 to 6.2 MPa, Pmax

was linearly correlated between the small ¯at tip and the cone tip penetrometers. The conversion ratio was higher under the

saturation condition irrespective of the shearing effect. The penetrometer with the small ¯at tip was more sensitive for the

weak soils. Shearing generally increased Pmax in most cases, but it decreased Pmax for some sandy soils under both saturated

and unsaturated conditions and for a clayey soil under the saturated condition. The soil consisting of swelling clay exerted a

decrease in Pmax. Rearrangement and/or sliding of particles/aggregates and increase in soil suction during shearing were

attributed to the increase in Pmax. Increase in porosity due to the aggregation during shearing was ascribed to the decrease in

Pmax. In addition, it was shown that agricultural cultivation resulted in a reduction in soil strength. # 2001 Elsevier Science

B.V. All rights reserved.

Keywords: Shearing process; Penetration resistance; Soil structure; Ultisols

1. Introduction

Surface sealing induced by rainfall results in trans-

formation of the surface of a structured soil into a layer

with higher bulk density, lower porosity and lower

hydraulic conductivity compared with the underlying

soil (Moore, 1981). The process results in an increase

of soil strength (Bradford and Huang, 1992). In addi-

tion to the deleterious effects of reduced water in®l-

tration and increased runoff and erosion, it can lead to

poor seedling emergence, drought stress, yield reduc-

tion due to poor in®ltration, off-site environmental

problems and even threaten the sustainability of agri-

culture (Sumner, 1995).

Shear stress is an important property in formation

of seals (Bryan et al., 1989). During rainfall the

Soil & Tillage Research 57 (2001) 193±202

* Corresponding author. Permanent address: PO Box 821,

Nanjing 210008, PR China. Tel.: �86-25-3369284;

fax: �86-25-3353590.

E-mail addresses: bzhang@ns.issas.ac.cn (B. Zhang),

rhorn@soils.uni-kiel.de (R. Horn), tbaumgartl@soils.uni-kiel.de

(T. Baumgartl).

0167-1987/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 7 - 1 9 8 7 ( 0 0 ) 0 0 1 4 8 - 3

dissipation of raindrop energy occurs in the surface

water ®lms as the soil surface ponds, resulting in high

shear stresses (Ghadiri and Payne, 1986). As a con-

sequence it can be assumed that soil compaction by

raindrops and soil shearing by water ®lms result in not

only an intensive local compaction in interrill erosion,

but also a compaction on rill wall or rill head cutting in

rill erosion (Rose et al., 1990). Shearing processes

are related to the structural issues such as compaction,

aggregate destruction, particle detachment, dispersion

and in-depth washing-in of the ®ne particles. Apart

from O'Sullivan et al. (1999) no reports are found

about the effects of shearing on soil structural proper-

ties, such as geometry, pore continuity and perme-

ability to water or air. In the past, soil shear strength

was only expressed as function of initial bulk density

and soil matric suction for modeling process of seal

formation (Cruse and Larson, 1977), implying that

shear strength may change in different pathways

due to different initial soil water content and bulk

density. Assouline and Mualem (1997) used this

function to model sealing processes, resulting in a

good agreement between in®ltration function and

measured data. Horn (1988) has summed up addi-

tional parameters such as soil texture, soil aggregate

stability, and soil organic matter and compression

history. Among the others, soil water content, bulk

density, and soil aggregate formation are time-

dependent (Roth, 1997) and may result in changes

in shear strength of soil (Le Bissonnais et al.,

1995).

There are several methods in the laboratory to

investigate the consequence of shearing deformation

on soil structure. In the triaixial test, the specimen is

not suitable to examine the behavior of thin layers of

soil and the testing is slow and laborious. In the normal

shear test a specimen is forced to fail along a single

plane. It is useful for determining the strength of the

soil but the nature of the deformation before failure

makes it less useful for investigating the effects of

shear deformation itself. O'Sullivan et al. (1999)

proposed an improved `̀ simple shear box'', which

caused a specimen to deform such that a cross-section

parallel to the direction of deformation was a paralle-

logram. The deformations within the specimens

were found fairly uniform when the normal stress

applied was not less than the maximum normal stress

that had been applied in the past. This, however,

made it dif®cult to obtain uniform specimens and

made it laborious to obtain a large number of

specimens.

Shear strength has been measured using fall cone

devices, vane testers and pocket penetrometers and

related to soil aggregate breakdown by a single rain-

drop (Al-Durrah and Bradford, 1981) to rain plash

(Bradford et al., 1992; Posen, 1981). Torri et al. (1987)

and Rauws and Govers (1988) related the soil shear

strength measured with a vane test apparatus to runoff

stress and erosion by ¯owing water. Truman and

Bradford (1990) measured soil penetration resistance

from a cone penetrometer before and after rainfall in

order to relate various measures of soil strength to soil

erodibility.

Since penetration resistance is essentially derived

from a point measurement, a set of measurements is

required to characterize this parameter for an overall

soil strength value. When tip and shaft are driven into a

soil, the soil re-expands into the cavity behind the tip

of the penetrometer. By pulling the penetrometer out

of the soil sample at the end of the measurement the

soil surrounding the penetrometer will be later loo-

sened and destroyed. When a soil column in a simula-

tion experiment on sealing processes is small, the

measurement area becomes a limitation if the in¯u-

ence is between measurement sites, and the destruc-

tion of the soil structure itself has to be avoided. Thus

care has to be taken not only to choose the measure-

ment site but also to select the diameter of the penet-

rometer.

To simulate the normal shearing under in situ

conditions, the moist soils in total 50 kg of materials

were sheared by horizontal displacement of layers of

soil particles/aggregates in between hands. This

method was not only very appropriate to the required

amount of soil, but also to enable us to obtain uni-

formly sheared materials. Although the way of pre-

paration does not allow us to quantify the normal shear

stress applied by hand, it was the only possibility to

prepare a sheared soil in one direction rather than a

homogenized soil, which results in a particle arrange-

ment at random.

The objectives of the study were to evaluate the

effects of the penetrometer geometry on the penetra-

tion resistance as affected by shearing under different

initial soil conditions and to use the information on

soil strength to elucidate shearing process.

194 B. Zhang et al. / Soil & Tillage Research 57 (2001) 193±202

2. Materials and methods

2.1. Soils

The soil samples were taken from tropical China,

with soil parent material covering quaternary red clay

(Q), sandstone (S), granite (G) and purple mudstone

(P). Soil samples were taken from the top layer (0±

15 cm). Soil properties determined by routine methods

(ISSAS, 1978) have been reported elsewhere (Zhang

and Horn, 2001) and selected physical properties are

given in Table 1. Particle size distribution was mea-

sured by the pipette method. The mean weight dia-

meter (MWD) was calculated by Eq. (1). Brie¯y, the

soils ranged from sandy loam, clayey loam, and silty

clay to silty loam. The subscript c, p and w represent

the soils for cultivation at least >5 years, parent

material and without cultivation, respectively. All soils

except the purple soils were strongly acidic. They are

very low in organic carbon content, ranging from 1.46

to 15.14 g kgÿ1. The purple soils had abundant smec-

tite and hydromica clay minerals, while others were

dominant in kaolinite and contain some additional

hydromica and vermiculite.

The sheared soils were sampled and dried in air for

determination of aggregate distribution by the sieving

method. The soil samples were sieved by hand on a

column of ®ve sieves: 2, 1, 0.63, 0.2, and 0.1 mm. The

mass percentage of each size fraction and the MWD

were calculated. The MWD was calculated as an index

of aggregation by

MWD �X �riÿ1 � ri�

2mi; i � 1 to n� 1 (1)

where MWD is the mean weight diameter in milli-

meter, ri the aperture of the ith mesh (millimeter) in

particle distribution analysis and aggregate distribu-

tion analysis, r0 � r1 and rn � rn�1, mi the mass

fraction of the soil remaining on the ith sieve, n the

number of the sieves.

2.2. Preparation of soil cores

The air-dried soils were ground and sieved through

2 mm mesh. Each soil was thereafter moistened by

adding distilled water up to a certain water content

(Table 2) in very ®ne droplets with a spray bottle. Soils

were remolded by hand, while about 10 g of the water

was sprayed on the soil. To obtain a homogenous soil

water content, the moist soil was kept in a plastic bag

at least for 2 weeks before use. Before the soils were

®lled into soil cylinders, they were remolded again and

a part of them was sheared by horizontal displacement

of layers of soil particles/aggregates in between hands

in one direction under constant speed. Only one person

completed shearing treatment for all soils, trying to

keep the force by hand the same, though it was

dif®cult to quantify the normal force by hand. The

sheared and unsheared soils were compacted without

layering in cylinders of 100 cm3 (40 mm high) to a

desired bulk density (Table 2). The ®lling techniques

gave reproducible results with small variations of bulk

density.

2.3. Measurements of soil penetration resistance

Soil cores were prepared in the above-mentioned

way. Each soil was packed to three levels of bulk

density: high (H-dB), moderate (M-dB) and low

(L-dB) (Table 2), repeated twice. Two tips, a cone

and a ¯at tip, were used as penetrometers. The cone of

the penetrometer had an apex angle of 608, base

diameter of 4.0 mm and a shaft diameter of

2.0 mm. The ¯at tip was a converse cone or taper

having an end diameter of 1.3 mm and a shaft diameter

of 1.0 mm. Penetration resistance was ®rst measured

immediately after the preparation of soil cores, which

was referred to as unsaturated condition. The second

measurement was conducted on the same soil core

after their saturation for 3 days, which was referred

to as saturated condition. Each time ®ve readings of

penetration force were recorded. Ten penetrometer

measurements from two soil cores with the same

bulk density and water content were then taken as

replicates.

The tips were mounted on a rack that allowed an

easy movement downward and upward. The soil core

was placed on an electronic digital balance of 1 g

resolution. The tip and shaft were driven manually

at a low speed. The maximum reading of the mass

component of a force M (g) was manually recorded

during the penetration distance of 10 mm into soil

core. Each value of M (g) was converted into a force F

(N). Maximum penetration resistance Pmax (MPa) was

then calculated dividing the force by the probe base

area.

B. Zhang et al. / Soil & Tillage Research 57 (2001) 193±202 195

Table 1

Soil parent material, soil taxonomic classi®cation, land use and selected physical properties

Soils Parent material Classification Land usage Gravel (%) Sand (%) Silt (%),

0.05±

0.002 mm

Clay (%),

<0.002 mm

MWD

(mm)a

SOC

(g kgÿ1)b

pH

(H2O)>2 mm 2±0.5

mm

0.5±

0.25 mm

0.25±

0.05 mm

Gc Granite Typic Paleudults Cultivated 21.0 18.2 11.2 21.3 30.5 18.8 0.877 5.24 5.53

Gw Granite Typic Paleudults Marginal 5.0 24.6 17.5 17.9 29.4 10.6 0.369 6.68 4.58

Pc Purple mud stone Haplaquepts Cultivated 1.0 0.6 1.0 1.5 48.7 48.2 0.053 15.14 7.21

Pp Purple mud stone Haplaquepts Eroded bare land 0.0 0.3 0.5 15.1 64.8 19.3 0.031 1.47 7.61

Qc Quaternary red clay Typic Plinthodults Cultivated 2.0 2.0 3.3 15.3 33.6 45.8 0.116 9.06 4.59

Qp Quaternary red clay Typic Plinthodults Eroded bare land 1.0 0.4 1.0 17.9 37.5 43.2 0.067 1.68 4.67

Qw Quaternary red clay Typic Plinthodults Marginal 0.2 0.6 4.1 14.8 40.0 40.5 0.057 3.94 4.62

Sc Sandy stone Typic Hapludults Cultivated 2.0 1.2 7.6 55.9 23.3 12.0 0.172 3.62 4.85

Sw Sandy stone Typic Hapludults Marginal 0.5 0.5 8.7 47.4 26.1 17.3 0.117 2.84 5.18

a Mean weight diameter, calculated from soil textural components.b Soil organic carbon.

19

6B

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Resea

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(2001)

193±202

2.4. Statistical analysis

An analysis of variance (ANOVA) on ranks was

performed using SigmaStat when normal ANOVA

failed to determine the signi®cance of effects of

shearing, soil moisture, bulk density or soil type on

maximum penetration resistance. If the signi®cance

was evident multiple comparison among means or

medians was then performed using Student±New-

man±Keuls test procedures.

3. Results

3.1. Prepared soils and soil cores

Soil water content ranged from 162 to 193 g kgÿ1

and from 201 to 225 g kgÿ1 for the moist sandy soils

and the moist clayey soils, respectively, and from 14 to

55 g kgÿ1 for the air-dried soils (Table 2). We chose

water contents at which the shearing effect by hand

appeared. During moistening and shearing the soils,

aggregation was visually observed. The values of

MWD of the aggregates before and after shearing

are also listed in Table 2. Shearing signi®cantly

increased the values of MWD except for the soils

Gw, Qc and Qp and the greatest difference between

MWDs due to shearing were found for Gc, Qw, Sc

and Sw.

In order to compare the effect of shearing data soil

cores were prepared at similar bulk densities. These

were 1.35±1.44, 1.24±1.28 and 1.08±1.13 Mg mÿ3 for

high, moderate and low bulk densities, respectively,

except the cultivated sandy soil (Sc). Sc had a bulk

density of 0.2±0.3 Mg mÿ3 higher than the value

mentioned above in order to obtain stable soil cores.

3.2. Comparison of Pmax from the two penetrometers

Fig. 1 shows the relationships of maximum pene-

tration resistance, Pmax, measured with the cone and

¯at tips for all nine soils. Each point in the ®gure

represents the Pmax value of a soil under a saturation

condition. It can be seen that the two methods gave

equivalent results. At a wide range from 0.05 to

6.2 MPa, Pmax measured with the two tips was linearly

related. Soil moisture affected the relationships. The

penetration resistance was 2.1 times higher for satu-

rated soils and 1.38 times higher for moist soils by the

¯at tip than that by the cone tip. The intercept of the

line was higher for moist soils. The relationship was

independent of soil type, bulk density and shearing,

indicating that both the tips were reliable for the

determination of the effect of shearing on soil strength.

Table 2

Initial soil water content and bulk density of the soil cores, mean weight diameter (MWD) of sheared (Sh) and unsheared (Unsh) soils

Soils y (g kgÿ1)a H-dB (b)

(Mg mÿ3)b

M-dB (m)

(Mg mÿ3)c

L-dB (s)

(Mg mÿ3)d

MWD

(mm)e

MWDSh±MWDUnsh

(mm)f

Unsh Sh Unsh Sh Unsh Sh Sh Unsh

Gc 193.4 1.41 1.42 1.27 1.26 1.11 1.11 1.74 b 1.63 a 0.11

Gw 167.0 1.42 1.42 1.28 1.28 1.12 1.13 1.18 a 1.17 a 0.01

Pc 225.3 1.38 1.38 1.25 1.24 1.08 1.10 1.54 b 1.45 a 0.09

Pp 205.2 1.42 1.42 1.28 1.28 1.12 1.13 1.90 b 1.84 a 0.06

Qc 203.1 1.41 1.37 1.26 1.27 1.11 1.11 0.94 a 0.87 a 0.07

Qp 219.3 1.35 1.35 1.29 1.25 1.10 1.10 0.79 a 0.80 a ÿ0.01

Qw 201.8 1.39 1.40 1.26 1.25 1.12 1.11 0.80 b 0.65 a 0.15

Sc 162.1 1.61 1.62 1.46 1.46 1.31 1.32 1.88 b 1.74 a 0.14

Sw 170.2 1.44 1.42 1.27 1.28 1.11 1.13 1.28 b 1.12 a 0.16

a Initial soil water content.b High bulk density (dB: bulk density).c Moderate bulk density.d Low bulk density.e Mean weight diameter of aggregates after being moist. Different small letters following the values in the columns indicate the signi®cant

difference (P � 0:05) in MWD affected by shearing.f MWDSh: MWD of sheared soil, MWDUnsh: MWD of unsheared soil.

B. Zhang et al. / Soil & Tillage Research 57 (2001) 193±202 197

3.3. Penetration resistance as affected by shearing,

initial soil condition and soil type

Figs. 2 and 3 sum up the results of Pmax measured

with ¯at and cone tips, which were affected by soil

type, bulk density, saturation condition and shearing.

Due to the great variation of the magnitude of Pmax

from soil to soil, soils with the same water content

were grouped in two diagrams. It can be seen that the

higher the bulk density, the higher the Pmax. Likewise,

Fig. 1. Relationships of maximum penetration resistance (Pmax) measured with the cone and ¯at tips under saturated and unsaturated

conditions.

Fig. 2. Maximum penetration resistance (Pmax) measured with the ¯at tip as affected by bulk density, saturation condition, shearing and soil

type. H, M and L represent high, moderate and low bulk density; Sh and Unsh stand for shearing treatment and unshearing treatment. The

asymmetric error bars indicated the medians with percentiles of 25 and 75%, while the symmetric error bars indicate the means with one S.D.

198 B. Zhang et al. / Soil & Tillage Research 57 (2001) 193±202

the Pmax measured with the same tip was lower for all

saturated soils than for unsaturated soils.

Pmax was signi®cantly different in the tested soils

(Figs. 2 and 3) and the results of statistics are tabulated

in Table 3. Under both unsaturated and saturated

conditions, Sc and Gw had the lowest Pmax, respec-

tively. Pmax was ranked by soil parent material as

follows: granite < sandstone < purple mudstone <quaternary red clay. The rankings of the soils varied

with Pmax. The cultivated soils either sheared or not

had lower Pmax values than the comparable unculti-

vated soils or parent materials under the unsaturated

condition. Under the saturated condition, this was true

for all soils except for Gc, which had higher Pmax

values than Gw.

Both tips produced agreeable results that shearing

increased Pmax in most cases (Figs. 2 and 3 and

Table 3). The signi®cant effect of shearing was

detected more in the saturated soils with ¯at tip, the

same being true for the cone tip in unsaturated soils.

Both penetrometers showed the increases in Pmax for

Qc under both saturated and unsaturated conditions

for Qp at all levels of bulk density, Sc at high bulk

density and Pp at small bulk density under the satu-

rated condition and for Gc at higher bulk densities

under the saturated condition. In addition, the ¯at

tip presented the increases in Pmax for Gc at higher

bulk densities under the saturated condition. The

cone tip presented the increases for Gw and Qw at

lower bulk densities under the unsaturated condition,

for Sw at moderate bulk density and for Qp at low

bulk density under the saturated condition. However,

shearing decreased the Pmax for Qw and Sc at high

bulk density levels and for Pc at moderate bulk

density under the saturated condition, which were

mostly detected by the ¯at tip. Under the unsaturated

conditions, shearing decreased the Pmax for Pc at

high bulk density, and Pc, Gc and Sw at moderate

bulk density, which were mostly detected with the

cone tip.

Fig. 3. Maximum penetration resistance (Pmax) measured with cone tip as affected by bulk density, saturation condition, shearing and soil

type. H, M and L represent high, moderate and low bulk density; Sh and Unsh stand for shearing treatment and unshearing treatment. The

asymmetric error bars indicated the medians with percentiles of 25 and 75%, while the symmetric error bars indicate the means with one S.D.

B. Zhang et al. / Soil & Tillage Research 57 (2001) 193±202 199

4. Discussion

The resistance to penetration of a tip into soil to a

given depth is a composite property. Depending on the

shape and type of instrument used, cutting or separa-

tion, shear failure, friction failure, compression failure

or even plastic failure or any combination of these may

occur as the instrument is pushed into the soil (Gill and

Vanden Berg, 1968). Diameters of the tip and aggre-

gate size determine the measured penetration resis-

tance related to the probe diameter (Whiteley and

Dexter, 1981). When the diameter d of a tip is larger

than the size of a soil structural unit s, the measured

force or penetration resistance is increased by

�d � x�2, where x represents a thin soil layer of a

shape similar to the tip. When d > s, it is expected that

x � 0. This leads to an apparent increase in tip dia-

meter d. The moist soil formed aggregates with MWD

larger than the diameters of the ¯at tip by 1.3 mm for

some soils. Therefore the Pmax with the ¯at tip was

higher than that with the big cone tip by 30±110%.

However, the small ¯at tip destroyed less the soil

surface and by conversion the results are comparable

and reliable. In addition more signi®cant relationships

were detected by the penetrometer with small tip

under the condition of saturation, indicating that the

penetrometer could be more effective to measure the

soil strength of weak soils, e.g. during saturation.

The moist soils were sheared by horizontal displa-

cement of layers of soil particles/aggregates in

between hands in one direction. The process implies

in shearing, compression and rolling. The original

aggregates may be destroyed and the aggregates

and particles may be rearranged. After shearing

Table 3

Multiple comparison of maximum penetration resistance (Pmax) of the soils affected by tip type, shearing, and saturation conditiona

Soil Flat tip Cone tip

H-dBb M-dBc L-dBd H-dB M-dB L-dB

Sh Unsh Sh Unsh Sh Unsh Sh Unsh Sh Unsh Sh Unsh

Unsaturated condition

Gc b b b bc b c b b Ab Bc c b

Gw c c bc b a b c b Bb Ab Bb Aa

Pc Ad Be Ae Bf d e Ae Bf e f e e

Pp c c d d c c d d d d d c

Qc Bf Ad Bg Ae Be Ad Bg Ae Bg Ae Bf Ad

Qp g g Bh Ag Bf Af Bh Ah Bh Ag Bg Ag

Qw e f f f e e f g f f Bf Af

Sc Ba Aa a a a a Ba Aa a a a a

Sw b b c c b c d c Ac Bd d c

Saturated condition

Gc Bb Ab Bb Ab b b a b b b a b

Gw a a a a a a a a a a a a

Pc Bc Ae Ac Bef d d c d e d d d

Pp c f c e Be Ad c e e d Bdc Ad

Qc Bd Ag Be Ae Be Ae Be Af Bf Ae Be Ae

Qp e i Bf Afg f f f h g f Bg Af

Qw Ad Bh d g Bf Ae Af Bg f f f f

Sc Aa Bc b c b b b c c c b c

Sw b d b d c c c d Bd Ad c d

a The small letters in column indicate the signi®cant difference at P � 0:05 in penetration resistance among soils. The values of penetration

resistance increased with the ascending order of the letters. The capital letters (A and B) indicate the signi®cant difference at P � 0:05 in

penetration resistance for a soil sheared or unsheared with a bulk density. Without the capital letters it is indicated that effect of shearing on

penetration resistance of soil at such soil condition was not detected. The value indicated by A is smaller than that by B for the soil.b High bulk density.c Moderate bulk density.d Low bulk density.

200 B. Zhang et al. / Soil & Tillage Research 57 (2001) 193±202

displacement more homogeneous media with high

porosity may change into a non-homogeneous media

with low porosity. The preferential sliding of those

grain contacts produced an anisotropic nature of the

particle arrangement in the samples, whose contact

strength is most easily mobilized on the chosen stress

(Feda, 1992). The formed aggregates increased

strength due to more particle contacts or interlocking

through surfaces of compact particles (Horn, 1976,

1994; Mitchell, 1993). The reduction in pore space in

aggregates during shearing and rolling wet soils

resulted in an equivalent drainage of excess soil water,

resulting in higher soil water suction. These processes

positively contributed to the increase in soil or aggre-

gate strength or Pmax under the unsaturated condition.

Mitchell (1993) con®rmed this process, which was

described by the effective stress theory, i.e. increasing

soil water suction increases effective stress and then

soil strength. The exception was found for Pc, Gc and

Sw, which had decreased Pmax under the unsaturated

condition. Pc consists predominantly of swelling clay.

The swollen clay may expand and block the pores, but

it is more plastic (Mitchell, 1993), keeping the ten-

dency of the soil to move when the soil is penetrated,

and a decrease of penetration resistance occurs.

During the preparation of the soils, larger aggre-

gates due to re-arrangement of particles/aggregates

were formed as indicated by the increased MWDs and

by the big difference in MWD due to shearing. The

bigger the newly formed aggregates due to shearing,

the more the macro-pores were formed at a given bulk

density. When soil penetrated the aggregates, the soil

aggregates tend to be moved and/or destroyed by the

applied penetration force, which results in an ®lling-in

of still existing macropores, resulting in a decrease of

penetration resistance. The strength of aggregates or

soils was weak for the sandy soils such as Gc, Sc and

Sw. Therefore the positive effect of shearing was

diminished by its negative effect, resulting in an

overall decrease in Pmax. The clayey soil Qw had

small MWD, but big difference in MWD before

and after shearing. Under saturation condition, its

cohesion decreases due to decrease of soil water

suction, resulting in a decrease in soil or aggregate

strength according to Mohr±Coulomb equation

t � c� s tanf (2)

where t is the shear stress, c the effective cohesion, s

the total stress, f the angle of internal friction. This

can explain why the overall Pmax for Qw decreased

under saturation condition.

5. Conclusion

At a wide range, maximum penetration resistance

Pmax from 0.05 to 6.2 MPa was linearly correlated

between the small ¯at tip and the cone tip penetrom-

eters. The conversion ratio was higher under the

saturated condition. Shearing did not affect these

ratios. However the penetrometer with the small ¯at

tip was more sensitive to weak soils.

Shearing generally increased Pmax in most cases,

but it decreased Pmax for the sandy soils Gc, Sc, and Sw

under both saturated and unsaturated conditions and

for clayey soil Qw under the saturated condition. The

soil Pc consisting of swelling clay minerals exerted a

decrease in Pmax. The results indicated that shearing

by hand increased aggregate strength due to compres-

sion and sliding of particles/aggregates and due to the

increase of soil water suction after drainage of excess

water. The process can result in the increase in soil or

aggregate strength or Pmax. On the other hand, shear-

ing by hand increased the aggregate size, resulting

probably in increasing inter-aggregate soil porosity at

a given bulk density. The process can reduce Pmax due

to the tendency that soil aggregates are moved and/or

destroyed by the applied penetration force, which

results in a ®lling-in of still existing macropores. In

addition, it was shown that agricultural cultivation

resulted in a reduction in soil strength.

Acknowledgements

We thank the Alexander von Humboldt Foundation

for the fellowship provided to Dr. B. Zhang and the

National Foundation of Sciences in China (NSFC)

(Grant No. 49701008 to B. Zhang) for the funded

research project. We thank Mr. J. Lohse for construct-

ing the penetrometers.

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