10th Asian Regional Conference of IAEG (2015)

Characteristics of Unloading Deep Soil Deformation in North China Plain
SUN Lin1,WANG Xiu-yang1(1)Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological
Sciences,China
E-mail:lin_sun166@163.com
Abstract
A compression test was applied to deep sand soil, silt and clay samples inthe depth of 199m~479m in North China Plain, to understand their deformation
characteristics of unloading after sampling. The result shows that under theloading conditions of 1200 kPa and 3200 kPa, the deformations of soil samples
taking from 199～439m depth were 0.43～1.386mm and 0.725～1.896mm,
respectively. The deformations of soils with different lithology are also different,
i.e. sand soil > clay > silt, in which clay is the most resilient. The analysis ofdeformation process reveals that the deformation of sand soil in the initial stage ismainly pore compression, and grain abrasion in the subsequent stages; thedeformation of clay is the interactive result of grain slippage and breakage; thedeformation of silt is pore compression.

Key words：Compression Test, Soil Deformation, Load, Pore space ratio, Unloading1.

respectively, to discuss the transformation
characteristics of the different soil masses ofthe different depth, in order to provide
instruction to reasonably control the deep andshallow groundwater exploitation in the North
China Plain.

Introduction

The groundwater has been seriously overextracted in China. The deep groundwater
extraction in the North China Plain has reached
the degree of 177.2%. Considering that theregional subsidence caused by deep
groundwater over-extraction may evolve intolarge scale environmental geological disaster,
the lowering bearing capacity of the overextracted layers attracts more attention.
However, due to climate, extraction, leakage
supplied deep aquifer, etc., there is serious
dewatering of the upper aquifer, which cause
loading decrease and then lead to soil mass
deformation.
If
thedeformationisunderestimated during the prediction of land
subsidence, the prediction result may beoverrated. Geotechnical test is an importanttask for acquisition of various soil mass indicesand effectively instructing groundwater
extraction object, and soil compression test is abasic method for obtaining compression
property, estimating regional subsidence. This
work takes the typical soil mass of the North
China Plain as research object, select deep soil
samples of clay, silt and sand soil for two sets,

2.

Material and method

2.1 Soil samples collection and physicaltesting
The samples are collected from
‘Groundwater science and project of Ministryof Land and Resources of the People’s
Republic of China – Zhengding Base, Heibei
Province’. The soil masses are from the same
core by applying double spiral drilling method,which disturbed little by boring process. The
diameter of the core is about 108mm. The
basic physical properties of the samples aremeasured according to ‘Specification of soil
test (DT-82)’, see Table 1. Because thegrouting drilling caused the disturbance of thewater content of the soil masses to some extent,
the water content data are different from that innatural condition.

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10th Asian Regional Conference of IAEG (2015)

Table 1 Physical property indexes of soil samples
lithologicalcharacters

Clay

Clay

Sand soil

Sand soil

Silt

Silt

depth (m)

199

269

321

479

312

439

water content (%)

22.29

20.28

20.53

16.08

20.28

16.34

initial void ratio

1.186

1.045

1.173

0.971

1.147

0.895

specific gravity

2.74

2.74

2.64

2.65

2.69

2.69

bulk density(g/cm3)

1.522

1.612

1.464

1.561

1.507

1.651

plasticity index

16.6

19.7

/

/

/

/

liquid index

-0.03

-0.29

/

/

/

/

liquid limit

39.4

45.7

/

/

/

/

plastic limit

22.8

26.0

/

/

/

/

resilience-compression process of the regularcompression-resilience test.

2.2 Testing design
The testing is divided into twocompression resilience processes, i.e. 200-400800-1200 kPa and 1200-2800-3200, themaximum loadings are both less than thehistorical loadings of the soil masses, thepurpose of which is to discuss thetransformation characteristics of compression
resilience after the unloading of the soil masses.
The KTG-800 automatic consolidometer isused to apply loading and unloading during thetest. Self-weights of the six sets of sample arerepeatedly loading and unloading in the rangeof P0+0.6 mPa, with loading stress not lessthan 400 kPa and repeated loading not lessthan twice. The unloading stresses of this set ofsamples are 1200-800 kPa, 1200-400 kPa and3200-2800 kPa, respectively. The criteria forstability judgment during tests is ≤0.005mm
per hour, above the precision of the national
standard for soil tests, which is sufficient forthe condition for studying deformation of thesamples. The samples are compressed under
the water-saturated condition.
According to the approximate formula, theoverlying self-weight is the product of thevolume weight and sampling depth. The
loadings of 400 kPa, 800 kPa, 1200 kPa aresignificantly less than the compressive stressbefore unloading of the testing soil mass, while
the loadings of 2800 kPa, 3200 kPa are closer
to that. The testing procedure equals to the

2.3 Method for data processing
The computation of the resilient
compression modulus, soil mass void ratio,
and correction of deformation under various
grades loading are applied according toSpecification of soil test (DT-82) .
3.

Results and analysis

3.1 Comparison of compression
deformations of the samples with the samelithology in different depth
The displacement is determined jointly
by historical loading and lithology of soil mass.
The samples are from the same bore hole, so
the historical loadings of the samples above
ground surface are deemed as the same. The
historical loading increases with the depth.
Under the condition of 1200 kPa and3200 kPa loadings, the deformations of the soil
mass samples are 0.43~1.386 and 0.725~1.896,
respectively. According to Figure 1 and Figure
2, in the whole compression-resilience
processes, there are common properties in thesoil masses with the same lithology: sand soil
(321 m) > sand soil (479m); clay (199 m) >clay (269m); silt (312 m) > silt (439 m),
indicating that the deformations of the shallow
soil masses are bigger than that from the deep2

10th Asian Regional Conference of IAEG (2015)

soil masses. E-log curves in Figure 3 indicatethat during the resilient process, the void ratios
in the same loading have the common
properties: clay (199 m) > silt (312 m) > sandsoil (321 m) > clay (269 m) > silt (439 m) >sand soil (479 m), the ratios of the soil masses
decrease with depth, i.e. the ratios decrease inrelation to the increase of the historicalloadings, the deformations decline incorrelation with the decrease of thecompression resilience ability.

grade loading compressions in the twocompression resilience processes of 200-400800-1200 kPa and 1200-2800-3200 kPa,
respectively. The figure shows that in the low
loading condition of 1200 kPa, there are
significant differences between sand and clay
soil masses in different depth, i.e. shallow sandsoil (321 m) > deep sand soil (479 m), shallow
clay (269 m) > deep clay (199 m);the
deformations of silt in different depth have no
obvious variance. In the high loading conditionof 3200 kPa, there are significant differences
between sand soil and silt masses in different
depth, i.e. shallow sand soil (321 m) > deepsand soil (479 m), shallow clay (199 m) > deep
clay (269 m). It can be seen that the
deformations of sand soil and clay are strongly
influenced by their depth, i.e. the deeper, themore historical loading and the lessdeformation, but influence of the depth on thesilt is relatively little.

Fig.1 Displacement under different loadings

Fig.4
Fig.2

Correction of displacement underComparison of the deformations ofdifferent soils under 1200 kPa

different loadings, this displacement means thesoil deformation is stabilization byconsolidation

Fig.5

Comparison of the deformations ofdifferent soils under 3200 kPa

Fig.3

e-log curve

3.2 Comparison of soil mass compressive
deformations with different lithology
It can been seen obviously that in thesame loading condition, the relationship

According to Figure 4 and Figure 5
denote the steady transformations of the last3

10th Asian Regional Conference of IAEG (2015)

between deformations of different lithologicalsoil masses is: sand soil > clay > silt, note thatthe historical variances caused by depth areneglected here. If the depth is considered, thevoid ratios in figure 3 show that therelationship are: clay (199 m) > silt (312 m) >sand soil (321 m) for shallow soils, clay (269m) > silt (439 m) > sand soil (479 m) for deep
soils, i.e. clay > silt > sand soil for void ratios
of soils with different lithology. The lithology
is an important factor that influences void ratio
and deformation.

modulus of soils under different loadings

3.3 Comparison of compression resilience
deformation characteristics of soil masses
with different lithology
Figure 6 shows that the compression
modulus of silt is apparently bigger than thoseof clay and sand soil. The compression
modulus of clay is obviously bigger than thatof sand soil under the condition of 200 kPa,
but they are gradually close to each other along
the increase of loading. Their relationship on
compressibility are: silt > clay > sand soil,
while on compression deformation are: sandsoil > clay > silt.
From figure 7, figure8 and figure 9 we
can see that the resilient modulus of clay islower than those of sand soil and silt inresilient processes of 1200~800 kPa and1200~400 kPa, reflecting that clay is relative
plastic and the resilient moduli of sand soil andsilt are close to each other. Under the conditionof 1200~800 kPa, the resilient moduli are
negative except clay (199 m) in the first
resilience. According to the soil mass
deformation stability criteria for the test, if weload pressure to 3200 kPa directly, the tested
soil masses is not yet in the stable state ofcompression deformation, therefore silt andsand soil have no resilience. The second
resilient process becomes normal, when theresilient moduli of silt and clay are close toeach other, the modulus of clay lower than thatof silt, the modulus of sand soil is apparently
bigger than those of clay and silt, the resilienceof sand soil is the lest and compression
deformation is the most.

Fig.6

Fig.7

Comparison of the resilient

modules of different soils in repeated springback process with 1.2～0.8 MPa

Fig.8Comparison of the resilient

modules of different soils in repeated springback process with 1.2～0.4MPa

Fig.9

Comparison of the resilient

modules of different soils in repeated springback process with 3.2～2.8MPa
3.4 Analysis of genesis of compression
deformation
3.4.1 Sand soil
Under the loading condition, sand soil
can be seen as skeleton construction formed byrigid grains, which is hard to generate grain
deformation. The historical loading of sandsoil is in positive, while the initial void ratio isin negative correlation with its depth (Table 1).
The compression moduli of the loading
conditions, except the 400 kPa condition, areComparison of the compression

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10th Asian Regional Conference of IAEG (2015)

increase with the depth, with which thecompression deformations are decrease,
however (Figure 5). Under the condition of200~1200 kPa, sand soil is mainly in theprocess of changing void ratio. The 2800
loading do not reach the high loading condition
before unloading, when the compression
modulus of sand soil is close to that of clay,
which possibly because the grain contact
points abrade each other, then make the sandsoil more compact.
3.4.2 ClayThe clay deformation include its
slippage and breakage, which entail continuous
media mechanics and micromechanics toanalyse. Clay grains are agglomerates
composed of numerous particles, which can beassumed as a rounded rigid-plastic body. In thecase of no grain damages, the compression
makes the rounded grains generate yieldsurfaces and form ellipses, the contact points
move against each other. After removing theloading, the yield surfaces try to recover their
original rounded forms. Therefore, clay ismore resilient than silt and sand soil, the more
loading, the more resilient. Clay may
experience resilience after it is sampled andbefore it is tested. Under the low loading
condition of 200~1200 kPa, historicalresilience is included in compression
deformation, so more historical loading of clay
from deeper site causes more historical loading
resilience, and then cause large deformation
under low loading condition. The deformationof clay is significantly more than silt withapproximate void ratio under the high loading
condition of 2800~3200 kPa, which is a
specific result for clay grains breakage.
Compression deformation of shallow clay isapparently greater than deep clay. Because thehistorical loading of the deep clay is high, so
are the grains, and the deep clay grains
vulnerable to breakage are less than shallow
clay grains, so the deformation is relative small.
At the same time, there is grain slippage, so thecompression of clay is the interactive result ofgrain slippage and breakage.
3.4.3 Silt
Silt grains aggregate tightly, withoutskeleton construction and plasticity. In this test,
the silt compression is merely a changing void
ratios process. Under the low loading conditionof 200~1200 kPa, in aspect of the historicalloading caused by lithology and depth, thevoid ratio of silt (312 m) is higher than silt
(439 m), and the deformation of silt (312) isslightly bigger than silt (439); under the highloading condition of 2800~3200 kPa, the two
sets of soil have no abrasion as sand soil has,nor breakage as clay has, so the deformation of

the two silt under the condition are close toeach other.
4.

Conclusions

From the analyses above mentioned, we
can see that the deep layer soil samples under
test have the characteristics as follow:(1)The deformation relationship of soils
with different properties is: sand soil > clay >silt.
(2)Sand soil and clay are more vulnerable
to deformation in shallower depth, and viceversa, while depth exerts little effect on silt.
(3)Deformation of sand soil includes porecompression in the initial stage and grain
abrasion in the subsequent stages; thedeformation of clay is the interactive result ofgrain slippage and breakage; the deformationof silt is pore compression.
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