application of lime soil pile in collapsible loess ground treatment

5
Application of lime soil pile in collapsible loess ground treatment CHEN Pei 1, a , CUI Qiang 1, b 1 China Electric Power Research Institute, China, 102401 a [email protected], b [email protected] Keywords: lime soil pile; collapsible loess; ground treatment Abstract. When the bearing capacity of natural foundation cannot meet load requirements, generally the ground treatment usually can be taken to strengthen it. In this paper, from the practical engineering, the design, construction, and detection of the lime soil pile were introduced. The results show that the lime soil pile in collapsible loess area reinforcement technology foundation is feasible. The bearing capacity and stability of the treated foundation can meet the requirements. Introduction Soil pile in collapsible loess foundation were invented by the Soviet Union. Experimental study and application of the lime soil pile has been carried out since the late 50’s in our country. Shaanxi province took the lead in the preparation of the “Lime soil pile and soil pile of design construction and acceptance specification (DBJ24-2-85)” in 1985, then, in “Code for building construction in collapsible loess regions (GB 50025)”, compaction method was put into one of the main processing methods in collapsible loess foundation. The effect and process equipment of lime soil pile and soil pile are basically the same, but the effect of the lime soil pile is more significant in reducing the compressibility of soil, enhancing the stability of water and reducing the deformation. In this paper, discussion of the application of lime soil pile in collapsible loess foundation treatment were introduced from the aspects of design, construction, quality detection of a desulfurization system in power plant collapsible loess foundation treatment engineering. Reinforcement mechanism of the lime soil pile [1] Principle and characteristic of soil compaction: Pile of soil compaction process, the existing research, can be described using the cylindrical cavity expansion theory. Professor Gong Xiaonan, according to the Vesic cavity expansion theory, think that when it is in the limit equilibrium state, the hole periphery will be the emergence of a plastic zone, outside of the plastic zone for the elastic zone, and soil keeps elastic state. Plastic zone of soil density increased, the porosity of the soil by squeezing, outside of the plastic zone remains in its natural state. If the pile hole radius is r u , the radius of plastic zone r p can be calculated by: cos sin p u G r r c q ϕ ϕ = + (1) Where: C, φ are soil cohesion and internal friction angle; G is the shear modulus; q is the original consolidation pressure of soil. From (1) shows that, the radius of plastic zone and the pile hole radius is proportional to, and related to the shear modulus G of soil and shear strength index. The soil between the piles lateral compaction is different from the general rolled or compacted soil. Lime soil pile interaction: Under the vertical load, the pile in a certain depth which produces compressive deformation and lateral expansion. Lime soil pile bearing capacity mainly depends on the strength of pile body, especially the upper soil quality and strength. The load test results show that, only accounts for about 20% of the lime soil pile plate area assumed a total load around 50%. In addition, because of the soil between piles can play the role of lateral restraint of lime soil pile, lateral move limit soil and the strength is increased. So the soil between the piles produce only deformation Advanced Materials Research Vols. 1049-1050 (2014) pp 256-259 Submitted: 31.08.2014 Online available since 2014/Oct/10 at www.scientific.net Accepted: 01.09.2014 © (2014) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.1049-1050.256 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 130.207.50.37, Georgia Tech Library, Atlanta, USA-09/12/14,16:17:57)

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Page 1: Application of Lime Soil Pile in Collapsible Loess Ground Treatment

Application of lime soil pile in collapsible loess ground treatment

CHEN Pei1, a, CUI Qiang1, b 1 China Electric Power Research Institute, China, 102401

[email protected],

[email protected]

Keywords: lime soil pile; collapsible loess; ground treatment

Abstract. When the bearing capacity of natural foundation cannot meet load requirements, generally

the ground treatment usually can be taken to strengthen it. In this paper, from the practical

engineering, the design, construction, and detection of the lime soil pile were introduced. The results

show that the lime soil pile in collapsible loess area reinforcement technology foundation is feasible.

The bearing capacity and stability of the treated foundation can meet the requirements.

Introduction

Soil pile in collapsible loess foundation were invented by the Soviet Union. Experimental study and

application of the lime soil pile has been carried out since the late 50’s in our country. Shaanxi

province took the lead in the preparation of the “Lime soil pile and soil pile of design construction and

acceptance specification (DBJ24-2-85)” in 1985, then, in “Code for building construction in

collapsible loess regions (GB 50025)”, compaction method was put into one of the main processing

methods in collapsible loess foundation. The effect and process equipment of lime soil pile and soil

pile are basically the same, but the effect of the lime soil pile is more significant in reducing the

compressibility of soil, enhancing the stability of water and reducing the deformation. In this paper,

discussion of the application of lime soil pile in collapsible loess foundation treatment were

introduced from the aspects of design, construction, quality detection of a desulfurization system in

power plant collapsible loess foundation treatment engineering.

Reinforcement mechanism of the lime soil pile [1]

Principle and characteristic of soil compaction: Pile of soil compaction process, the existing

research, can be described using the cylindrical cavity expansion theory. Professor Gong Xiaonan,

according to the Vesic cavity expansion theory, think that when it is in the limit equilibrium state, the

hole periphery will be the emergence of a plastic zone, outside of the plastic zone for the elastic zone,

and soil keeps elastic state. Plastic zone of soil density increased, the porosity of the soil by squeezing,

outside of the plastic zone remains in its natural state. If the pile hole radius is ru, the radius of plastic

zone rp can be calculated by:

cos sinp u

Gr r

c qϕ ϕ=

+ (1)

Where: C, φ are soil cohesion and internal friction angle; G is the shear modulus; q is the original

consolidation pressure of soil. From (1) shows that, the radius of plastic zone and the pile hole radius

is proportional to, and related to the shear modulus G of soil and shear strength index. The soil

between the piles lateral compaction is different from the general rolled or compacted soil.

Lime soil pile interaction: Under the vertical load, the pile in a certain depth which produces

compressive deformation and lateral expansion. Lime soil pile bearing capacity mainly depends on

the strength of pile body, especially the upper soil quality and strength. The load test results show that,

only accounts for about 20% of the lime soil pile plate area assumed a total load around 50%. In

addition, because of the soil between piles can play the role of lateral restraint of lime soil pile, lateral

move limit soil and the strength is increased. So the soil between the piles produce only deformation

Advanced Materials Research Vols. 1049-1050 (2014) pp 256-259 Submitted: 31.08.2014Online available since 2014/Oct/10 at www.scientific.net Accepted: 01.09.2014© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.1049-1050.256

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 130.207.50.37, Georgia Tech Library, Atlanta, USA-09/12/14,16:17:57)

Page 2: Application of Lime Soil Pile in Collapsible Loess Ground Treatment

pressure vertical, and the relationship between pressure and settlement is linear. It improves the

bearing capacity and deformation modulus of foundation.

General engineering situation and geological conditions of the site

A power plant site is located in collapsible loess area, installation of 2 sets of 600MW supercritical

coal-fired units. In the desulfurization system construction, the natural foundation bearing capacity

cannot meet the requirements, so it needs the treatment for foundation. After schemes comparison and

previous experience in similar works, design of foundation treatment with lime soil pile.

The surface is about 0.3~0.4m of miscellaneous fill or farming figure and the layer is not a separate

layer. The upper (2~3layer) for the Holocene (Q4l~2) silty clay;(4~9 middle layer) upper Pleistocene

(Q3) silty clay and gravel layer; lower (the layer and the exposed strata exploration) to the middle

Pleistocene (Q2) silty clay, gravel and powder clay mixed gravel layer, middle Pleistocene strata are

mutually staggered distribution.

Design of lime soil pile [2, 3]

Lime soil pile in collapsible loess foundation treatment design includes space, depth, and the number

of pile hole.

(1) Determination of pile spacing

Lime soil pile hole diameter of the majority is 0.3m ~ 0.45m and the pile spacing is generally 2~

2.5 times of the pile hole diameter. Determine the pile hole spacing should ensure the soil between the

piles of average compaction coefficient not less than 0.93. In order to make the soil between piles

uniform compaction, pile holes should regular triangle arrangement, and the distance can be

calculated by:

max

max

0.95 c d

c d d

s dη ρ

η ρ ρ=

− (2)

Where: s is the distance between the center of the pile hole (m); d is the pile hole diameter (m);

maxdρis the maximum dry density of soil between piles (t/m3); dρ

is the average dry density of soil

before foundation treatment(t/m3); cηis the soil between the piles by the average coefficient of

compaction after the hole forming. The center distance between the pile holes in the power plant

desulfurization system regional is 1.15m.

(2) Determination of the depth of the pile holes

In the non weight collapsible loess site,after the building foundation soaked by water, collapsible

deformation occur in the foundation soil depth below the surface of the foundation layer, and is

related to the foundation type, basal area and its pressure .The pile foundation depth eliminate all the

collapsibility volume can be calculated by:

Pz +Pcz ≥ Psh (3)

Where: Pz is the lime soil pile composite foundation’s additional pressure on top of the under

layer(kPa);Pcz is the lime soil pile composite foundation’s weight pressure under a top layer overlying

soil (kPa); Psh is the lime soil pile composite foundation’s initial collapse pressure under the top layer

soil(kPa). In the self weight collapsible loess site, after the building foundation soaked by water,

external load collapsible and self weight collapsible often happen at the same time. According to the

survey data, the power plant desulfurization region of collapsible soil is gravity collapsible, so the pile

hole depth is from the ground to the top of the gravel layer in the ninth layer.

(3) The number of pile holes

Pile hole number, diameter and spacing are related to the foundation treatment area, and can be

calculated by:

n=A/Ae (4)

Advanced Materials Research Vols. 1049-1050 257

Page 3: Application of Lime Soil Pile in Collapsible Loess Ground Treatment

Where: n is the number of pile holes; A is the foundation treatment area(m2); Ae is the treatment

foundation area of one lime soil pile. Put into the formula calculation of related parameters, the

number of power plant desulfurization region of lime soil pile is 3000 and the pile hole is regular

triangle arrangement.

The construction of lime soil pile

(1) Pile Lofting

Firstly put the control axis to working surface and fix the stakes to prevent the offset, and then

according to the design requirements of distribution of pile spacing.

(2) Hole creating

With the immersed tube method and each pile hole must ensure that the pile machine smooth and

firm. Immersed vertical and with red paint mark the pore depth.

(3) Preparation and control of filler

Before the mixing of soil and lime should be screened, which meet the requirements according to

the volume ratio of the mixed system.

(4) Tamping machine in place

Tamping machine in the installation and debugging, entered the site in stable place, must ensure

that the hammer and pile hole coaxial line.

(5) Tamp and fill the hole

The hammer weight is about 260 kg. When the tamping machine is in place, put the hammer tip

alignment the hole center. Ensure the hammer fall free into the bottom of the hole.

Quality detection of lime soil pile [4, 5]

Static load test of single pile composite foundation: The static loading test of composite foundation

with bearing plate for 1.07m plate, plate at the end of the laying of 50mm ~ 150mm in the coarse sand

layer and test pit excavation to the basement level. The jack as loading device, force system for weight

platform counterforce device. The steel erection as loading platform and sandbag as reaction force.

The desulfurization system engineering made a total of 20 single pile loading test of composite

foundation, of which 18 passed, 2 failed. The 6 single pile composite foundation loading tests on

unqualified area reinforcement to continue to do so, the results of all qualified.

Fig.1 Static load test of single pile composite foundation

Moisture content, density test: Detection of lime soil pile’s soil water content and dry density.

Through calculation of minimum coefficient of dry density of soil between piles, to determine the

compaction effect and construction quality of piles.

The minimum coefficient of compaction between the 3 holes, can be calculated by:

ηdmin=ρd0/ρdmax (5)

Where: ηdmin is the minimum coefficient of compaction soil; ρd0 is dry density of centroid position

of 3 holes; ρdmax is the maximum dry density of compaction of experimentally determined.

258 Modern Technologies in Materials, Mechanics and Intelligent Systems

Page 4: Application of Lime Soil Pile in Collapsible Loess Ground Treatment

The desulfurization system engineering made a total of 100 hole minimum coefficient of

compaction tests, of which 92 passed, 8 failed. The unqualified area reinforcing continued to do the

20 holes of the minimum coefficient of compaction test, the results were all qualified.

Table1 Minimum compaction coefficient of part of the soil between piles Serial number Hole number(hole of three piles ) The minimum coefficient of compaction

1 (A2-8,A3-11,A3-12) 0.892

2 (A3-20,A4-22,A4-23) 0.887

3 (A5-13,A6-17,A6-18) 0.893

4 (A7-21,A8-25,A8-26) 0.884

5 (A81-10,A82-9,A82-10) 0.891

6 (A81-22,A82-36,A82-37) 0.888

Collapsibility test: The collapsibility test to determine whether to eliminate the collapsibility of the

site. Collapsibility coefficient can be calculated by:

δs=(h1-h2)/h0 (6)

Where: δs is the coefficient of collapsibility; h1 is in a certain pressure, the specimen deformation

stability height (mm); h0 is the initial specimen height (mm); h2 is in a certain level pressure, sample

collapse height after deformation stability. The desulfurization system engineering made a total of 9

hole collapse tests, the results of all qualified. The results show that the area of collapsibility

eliminated.

Conclusion

(1) Using the lime soil method in collapsible loess foundation treatment is feasible and can eliminate

all or part of the foundation soil collapsibility.

(2) The two years use of the desulfurization system was good. Since the main reinforcement technique

used can obtain raw material locally, thus shortening the construction period and saving the

investment. Therefore, strengthening the large foundation of lime soil pile have special requirements

on the site, which has good application value.

References

[1] Gong Xiaonan. The third edition foundation treatment manual[M], China Building Industry Press,

2008.

[2] Industry standard of the People's Republic of China. JGJ79-2002. Building foundation treatment

[S]. Beijing: China Building Industry Press, 2002.

[3] The national standard of the People's Republic of China.GB50025-2004. Code for building

construction in collapsible loess regions[S]. Beijing: Chinese Building Industry Press, 1989.

[4] Industry standard of the People's Republic of China. JGJ106-2003. Technical Code for Testing of

Building Foundation Piles[S]. Beijing: China Building Industry Press, 2003.

[5] Yang Honggui, Liu Cunli, Wan Xisheng. The soil compaction characteristics and effects test of

the composite foundation of compaction pile and soil[J].Shaanxi construction and building materials,

2004 (6): 20-25

Advanced Materials Research Vols. 1049-1050 259

Page 5: Application of Lime Soil Pile in Collapsible Loess Ground Treatment

Modern Technologies in Materials, Mechanics and Intelligent Systems 10.4028/www.scientific.net/AMR.1049-1050 Application of Lime Soil Pile in Collapsible Loess Ground Treatment 10.4028/www.scientific.net/AMR.1049-1050.256