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Web of Conferences 25,008 (2015 )
MATEC 3 0
DOI:10.1051/ 2 00
matecconf/2015 5 3 08
C Ownedbytheauthors, published by EDP Sciences, 2015
Mechanism Research on the Improvement of Bearing Capacity of Single
Pile by Pile-base Post Grouting Technique and Estimation of Bearing
Capacity
Feng Zhao
School of Construction Engineering, Xi'an University of Science and Technology, Xi’an, Shaanxi, China
Xi’an Research Institute of China Coal Technology and Engineering Group Corp, Xi’an, Shaanxi, China
ABSTRACT: The slurry wall bored grouting has some defects. The pile-base post grouting technique can effi-
ciently make up these defects and greatly improve the bearing capacity. Based on the pile-base post grouting
technique and its process analysis, this paper carries out a detailed analysis about the improvement of mechanism
of bearing capacity and mechanism of spherical expansion by the use of pile-base post grouting technique, as
well as the estimation method of the bearing capacity of pile-base post grouting pile, which has a certain refer-
ence value for theoretical research and technical application of the technique.
Keywords: pile-base post grouting; mechanism of action; calculation of bearing capacity
1 INTRODUCTION post grouting technique has been widely used in Eu-
rope. In recent years, such technique [10-12] is used to
The pile-base post grouting technique has been widely improve the bearing capacity of bored pile and reduce
used for bored pile, especially for large-diameter sedimentation at pile base of China’s high-rise build-
bored pile, and its effect has been verified by a lot of ings and bridge engineering, such as Yellow River
projects. The development of pile-base post grouting Bridge II, Donghai Bridge, Runyang Bridge, Sutong
technique has been mature. Currently, there are many Bridge and Hangzhou Bay Bridge and so on.
kinds of pile-base grouting devices. The principle of
construction control is reasonable, and the construc-
tion success rate is high. The pile-base post grouting 2 INFLUENCE FACTORS OF BEARING CA-
technique has been widely used in pile base engineer- PACITY OF BORED PILE
ing of industrial and civil buildings and major bridges
[1-3]
. The bearing capacity of pile-base post grouting The bored pile technique is a practical engineering
should be regarded as guidance for the construction technique. There are many influence factors of bearing
designers and pile-base testers. It is evident for post capacity, which are mainly in the following areas:
grouting to improve the calculation of the bearing 1. Strength, deformation nature and stress change
capacity of pile-base. The pile-base post grouting for pile body to access soil layer.
technique is a technique with the theory lagging be- 2. Strength and deformation nature of bearing
hind practice, which is established in the specific en- course at pile end.
[4-6]
gineering design and construction . 3. Geometrical characteristic of pile body and pile
The bored pile post grouting technique is a kind of base.
[7-9]
scientific and advanced technical method to 4. Material strength of pile body.
greatly improve the bearing capacity of the pile and 5. Pile-forming method.
reduce the settlement amount through elimination of In terms of the characteristics of bored pile, it be-
the defects on the traditional construction technique of longs to underground concealed project with more
cast-in-place pile, which solidifies soil within a certain construction processes and closely interrelated con-
range of sediment at the bottom of hole, mud cake at struction procedures and without pause for a long time.
the pile side, pile base and pile side through grouting The construction process of the main procedures is
pipes by the use of high-pressure grouting pump to underground or underwater, and it is inconvenient to
infuse grout with main agent of cement after the supervise. In addition, it is also restricted by air tem-
grouting pipes and grouting equipment are embedded perature, weather, traffic, water, electricity, environ-
in the pile base in the pile formation and the concrete ment, equipment, materials and other factors, and it
strength of the pile reaching 50% to 75%. fails to be accurately predicted. They are prone to
The technique is applicable to slurry wall drilling, have quality defects in pile necking, mud mixture, pile
digging, boring cast-in-place pile, drilling and digging breaking, over-thick sediment piles and other forms,
cast-in-place pile and other pile-base projects. The affecting the integrity of the pile body and the bearing
4
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Article available at http://www.matec-conferences.org or http://dx.doi.org/10.1051/matecconf/20152503008
MATEC Web of Conferences
capacity of single pile, especially the slurry wall fied grout (such as neat cement slurry, cement mortar,
cast-in-place pile. The over-large consistency may cement paste with admixture, chemical grout and so
form “mud cake” on the surface of pile side to greatly on) in penetration, filling, replacement, cleaving, con-
reduce the frictional resistance, while the over-thick solidation, solidification and other physical or chemi-
sediment at the bottom of hole will obviously decrease cal forms through the grouting pipe embedded in the
the end resistance [13]. pile body and pressing into the pile end with a certain
How to improve the side resistance and end re- pressure and an enlarged head at pile base after for-
sistance of bored pile is a research subject urgently mation of bored pile. Schematic diagram of pile-base
required to be addressed in the engineering practice. post grouting effect is shown in Figure 1.
The pile end and pile side post grouting technique is
one of effective measures to improve the bearing ca- 3.2 Technique classification
pacity of cast-in-place pile. This paper focuses on
illustration of the mechanism, technique and applica- Based on the grouting technique, pile end pressure
tion effect of bored pile-base post grouting at medi- grouting can be divided into two categories: the open
um-coarse sand layer of the bearing course at pile end grouting and the closed grouting.
combined with engineering practice. As we all know, The open grouting means that the grout is directly
in the bored pile construction at medium-coarse sand injected into pile end through embedded grouting pipe,
layer, there are difficulties in cleaning slag and reduc- and respectively mixed with sediment at pile base and
tion of bearing capacity at pile end by disturbance of surrounding soil at pile end, so as to form an effect of
medium-coarse sand layer. The pile-base post grouting the composite foundation at pile end; The closed
technique effectively reinforces the medium-coarse grouting means that the grout is injected into a prefab-
sand layer, thereby improving the bearing capacity of ricated cavity with a good elasticity, and the elastic
pile. Meanwhile, it also greatly reduces the defor- cavity will gradually swell and expand with the in-
mation of single pile and the differential settlement of crease of grouting pressure and grouting amount,
pile groups due to reinforcement of pile end and sur- forming grout bubble in a lower soil layer at pile end,
rounding soil. and the generation and gradual expansion of the grout
This paper proposes a principle of design calcula- bubble will be bound to produce a compaction effect
tion through analysis about influence factors of bear- on the soil layer at pile end [14-16].
ing capacity of bored pile and the mechanism of im- Obviously, the growing rate of bearing capacity of
proving the bearing capacity of single pile by bored single pile is relatively stable to use the closed grout-
pile post grouting. ing. Especially when the pile end is a sand layer, the
effect is better. However, the construction process of
closed grouting is more complicated, while the im-
3 OVERVIEW OF PILE-BASE POST GROUTING provement of bearing capacity of open grouting is
TECHNIQUE AND TECHNIQUE CLASSIFI- inferior to that of the former, but its construction pro-
CATION cess is relatively simple, so it is widely-used.
The bored pile-base post grouting technique herein
3.1 Overview of pile-base post grouting technique is an open grouting technique of the embedded grout-
ing pipe on pile body.
4 MECHANISM OF IMPROVING BEARING
CAPACITY BY PILE-BASE POST GROUTING
1) The grout infiltration rate of grouting is high in
bearing course at pile end of the coarse-grained soils
(medium sand with a greater porosity, coarse sand,
pebble, gravel). The grout can greatly improve the
disturbance of the bearing course and strength and
deformation modulus of the bearing course that main-
ly through penetration, part of compaction, filling and
Figure 1. Schematic diagram of pile-base post grouting effect consolidation effects, and form an enlarged head to
increase the stressed area of pile end and improve the
resistance of pile end.
The pile-base post grouting refers to a project’s tech- In order to implement the permeability grouting, we
nical measure that can improve the resistance at pile assume that the stratum structure is substantially free
end and pile side to different degrees, reduce pile of disturbance and destruction, and can overcome
sedimentation and significantly improve the bearing various obstacles of grout flow and penetrate into
capacity of pile through physical and mechanical holes or fractures in the stratum under grouting pres-
properties’ changes of soil at pile end and surrounding sure. The holes should be filled by penetration and
pile side by individual and combined action of solidi- filling methods. After the solidification of grout, soil
03008-p.2
EMME 2015
particles are bonded together to form cement-soil particles, so that the mud filling among sediments
stone body, which can greatly improve the disturbance produces dehydration and consolidation; in case of
of the bearing course and strength and deformation enough grouting pressure, it will produce fracture
modulus of the bearing course, and form an enlarged grouting and compaction effects.
head to increase stressed area of pile end. Thus the 6) When the grouting pressure increases and the
growing rate of bearing capacity is large. grouting amount continuously increases, the grout
Influence factors of grout diffusion range are holes injected into the pile end may penetrate under pressure
or fractures in the stratum (or permeability coefficient), along the mud cake among pile soil within a certain
grout viscosity, grouting pressure and grouting time height range above the pile end, reinforce the mud
and so on. cake, fill in gap of the pile body and soil body around
2) The grout infiltration rate of grouting is low in pile, and penetrate into a certain width of soil layer
bearing course at pile end of the fine-grained soils around pile. After solidification of grout, a wider
(cohesive soil, silty soil, silt, fine sand and so on), thus range of soil body around pile is mobilized to partici-
achieving the fracture grouting. The so-called fracture pate in the pile bearing, so as to increase the pile side
grouting means that under the grouting pressure, the friction.
grout overcomes the initial stress and tensile strength 7) For pressure grouting at pile end, when the infil-
of the stratum, causing destruction and disturbance of tration capacity at pile end is restricted, the grout
soil structure, and making it fracture along the plane pressure of sploid will increase continuously. Under
perpendicular to the stress, so that the original frac- the effect of high-pressure liquid, it may impose up-
tures or holes in the stratum open to form new frac- ward reverse prestress on the pile end face, and
tures or holes, and the grout permeates into the stra- slightly lift up the pile body. When the slurry wall
tum along the fracture vein, so the groutability and bored pile bears downward vertical load, the reverse
diffusion length of the grout increase. Un- prestress will bear partial load, thereby increasing the
der the state of fracture grouting, the failure in the bearing capacity of pile end.
increase of bearing capacity of grouting pile at pile 8) Under the effect of grouting pressure, the com-
end is due to the existence of fracture grout vein. The pressive deformation at pile end is partially completed
single medium of soil body is segmented and rein- ahead of schedule during the construction period, so as
forced by mesh stone body as composite soil body, so to reduce the vertical compressive deformation in the
as to improve the density of pile end soil body and later period of use.
effectively transfer and share the load, thereby im-
proving the resistance at pile end [17-18].
With the comparison of grout infiltration rate, the 5 CALCULATION OF SPHERICAL EXPANSION
rate of fracture grouting mode is smaller than that of THEORY OF PILE END POST GROUTING
permeability grouting. Therefore, the growing rate of
ultimate bearing capacity of pile end grouting pile of The spherical expansion theory of pile end post grout-
the former is much smaller than that of the latter. ing is used for interpretation of representative theory
3) The pile end sediment and injected grout has of permeability grouting in the foundation of above
physical and chemical reaction and solidification, the medium sand. The calculation model of spherical
condensing into a stone body with a new structure, expansion theory is shown in Figure 2. The calculation
high strength, and stable chemical property, so as to is used to assume:
enhance the resistance at pile end.
4) With the increase of grouting amount and grout-
ing pressure, the cement grout continuously penetrates
into the bearing course at pile end due to immersion
by mud and becoming soft, thus forming aploid at pile
end. When the aploid enlarges continuously, the infil-
tration capacity is restricted by surrounding dense soil,
so the pressure continues to rise. The pressure in-
creases play a role of compaction in the bearing course
at pile end, thus improving the bearing capacity of pile
end soil body. Meanwhile, the aploid is formed at pile
end, so the bearing area at pile end increases, and it is
equivalent to expanding base for the bored pile,
thereby increasing the resistance of slurry wall bored
pile at pile end.
5) When grouting in the bearing course at pile end
of weathered bedrock with the medium or above im-
permeability, in case of not enough grouting pressure,
due to the constraints of surrounding rock, the pres- Figure 2. Schematic diagram of grout spherical expansion at
sure grout can only be penetrated or filled in sediment, the bottom of grouting pipe
forming grout bubble to extrude surrounding sediment
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MATEC Web of Conferences
(1) The grouting sand is homogeneous and homo- Q is the grouting amount;
dromous. kg is the permeability coefficient of grout in the
(2) The grout is Newtonian fluid, which is generally stratum, kg = k / β;
in line with Newton’s law of friction, that is, the linear β is the ratio of grout viscosity to water viscosity.
relationship between the shear stress and the shear Assume that the grouting process is constant;
deformation speed is called Newtonian fluid. A is the permeability area;
(3) The grout is injected into foundation soil from rˍis the radius of grout diffuser;
the bottom of grouting pipe. h1 is the water pressure head at the edge of grout
(4) The grout presents a spherical expansion in the diffuser;
stratum. h0is the grout pressure head at the grouting nozzle;
The grout expansion radius at pile end can be cal- H is a sum of underground water head at the grout-
culated by such assumptions. In the process of grout- ing nozzle and grouting pressure;
ing, assuming that the grout infiltration is in line with r is the radius of grouting pipe;
0
Darcy’s Law: t is the grouting time;
3 n is the soil porosity.
QkgiAt4kgir t (1) As can be seen from the Formula (7), the solid size
increases with the increase of the permeability coeffi-
dh cient of soil (k) and the grouting head difference at the
i exit (Δϴ), and it decreases with the increase of filling
dr rate (α), soil porosity (n) and grout viscosity. In addi-
tion, the permeability coefficient of soil decreases
k 2 dh with the increase of filling rate (α). The compaction
Substitute into (1):Q 4 r t dr and fracture grouting should also involve in soil
strength, stress-strain relationship and changes in the
[19-25]
grouting process .
Through the above integration, Q hC(2) Therefore, the reinforcement range could not ex-
4ktr pand with the increase of time, which only indicates
that the achievement of a certain reinforcement range
When r=r0, h=h0 requires adequate grouting time.
r=r1, h=h1
Substitute into (2) Formula, 6 ESTIMATION METHOD OF BEARING CA-
Q 1 1 PACITY OF PILE END GROUTING PILE
h -h = ( ) (3)
0 1 4kt r r 6.1 Problems in specifications
0 1
Assuming that the filling rate of grout is , then According to the specifications of Article 5.2.8 and
Article5.2.9 in Technical Specifications for Pile
Foundation (JGJ94-94), the standard value of vertical
4 3 ultimate bearing capacity of single pile of slurry wall
Q dr n (4) bored pile can be calculated by the Formula (5.2.8).
3 1 Where: q takes the value according to Table 5.2.8-1
sik
Substitute Formula (4) into Formula (3): in the specification; qpk takes the value according to
Table 5.2.8-1 in the specification. When the pile di-
r 3n 1 1 ameter d≥800mm, there is a need to introduce two
h -h = 1 ( ) (5) large-diameter piles side resistance and coefficient of
0 1 pile end size effect of ψ and ψ , and take value ac-
3kt r r si p
0 1 cording to 5.2.9-2 in the specification.
3 Q Q Q u q l q A
r n 1 1 uk sk pk
si sik si p pk p
So: h= 1 ( ) (6)
3kt r r This method mainly considers the improvement of
0 1 vertical bearing capacity of single pile, which is
mainly reflected in the end bearing. The first is the
3ktr h increase of intensity of pile end soil and elimination of
When r »r , so r = 0 (7)
0 1 1 sediment, and the second is expansion of stressed area
at pile end without considering the increase of side
Where: resistance. Therefore, this method considers that the
k is the permeability coefficient of soil and assume pile diameter after grouting should be calculated by
that the grouting process is constant; the pile diameter designed originally. qsik takes value
according to Table 5.2.8-1 in JGJ94-94, while the end
03008-p.4
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