Application of K Chart in Supporting Axial Force Analysis

of Subway Deep Foundation Pit

Guoqiang Zhao1 and Wei Qi2,3

1 Construction Engineering, Jilin University; No.6, West

Minzhu Street, Changchun 130026; TEL: 15943077401；Email:

tianlangxingx@163.com

2 Construction Engineering, Jilin University; No.6, West

Minzhu Street, Changchun 130026; TEL: 13843160203

3 The City College of Jilin Architectural and Civil

Engineering Institute; No.1666, Yingbin Street,

Changchun;

ABSTRACT

Previous researchers processed monitoring data through

some math means mainly, which make the result reflect

better the character of building monitoring data. But

data expression is single. However you process the data

in various methods, the result is a line chart or a

smooth curve fitting. It is necessary to develop more

data expression and processing methods. It will help us

find the character of monitoring objects that is

difficult to find in a single line chart. This paper

introduces the K chart which is used in the stock market

commonly. And the author did some trials in this new

method to analyze axial force of deep silt foundation pit

supporting.

KEYWORDS

K chart; foundation pit monitoring; deep silt; axialforce

INTRODUCTION

With the exploiting of cities, the numbers of deep

foundation pits are increasing. Due to the big individual

differences between foundation pit projects, it usually

contains some uncertain factors for engineers to make the

plan of design and construction only according to

geological exploration information and indoor experiment

data. For this reason, engineers pay great attention to

the monitoring of foundation pit. And predecessors had

done a lot of study(An & Gao, 2005 and Liu et al., 2007).

However, at present, there are some common defects that

are lack of processing monitoring data and utilization

rate of monitoring data is low. Sometimes, the data

processing is too complex to use it in engineering

project(Zhao & Yuan, 2008 and Liu et al., 2010). The

value of the monitoring data is difficult to play a role,

which is a block in the way of improving design and

construction level. This paper focuses on the process of

monitoring data and makes use of K chart developing a new

method to process monitoring data. It is expected to work

on deep foundation engineering even civil engineering.

BRIEF INTRODUCTION OF K CHART

K chart is also called candlestick chart, which can be

traced back to the rice market trade in Tokugawa Bakufu

in the eighteenth century. Originally it is used to

calculate daily fluctuation of rice prices. Because this

method is convenient, comprehensive, intuitive and

unique, people use it to analyze the stock price trend.

After over 300 years of development, it is widely used in

securities market, such as stock, futures and etc. From

the current international and domestic literature, this

is the first time that K line is used in processing the

monitoring data of deep foundation pit.

APPLICATION OF K LINE CHART IN ENGINEERING

From the form, K line chart is divided into negative

(black) line, positive (red/white) line and the

equivalent price line. Each line is composed of three

parts, namely, the upper shadow, the lower shadow and the

real body. As shown in Figure 2.1, Figure 2.1 (a) is the

K line expressing in the stock market, and Figure 2.1 (b)

is the data expressing that this paper introduced to the

foundation pit monitoring. In the stock market, taking a

period of time's share price to draw on a coordinate

paper as negative and positive line in figure 2-1 (a),

can get K chart of share price variation for a period of

time with share price as ordinate against time as

abscissa. According to the time, K chart can be divided

into 60 minutes K line, day K line, weeks K line, etc,

while in monitoring of foundation pit by monitoring time

and frequency, it can be divided into day K line, weeks K

line, months K line, etc.

Considering the characteristic of foundation pit

engineering, K chart can also be plotted according to

time span of construction nodes. It can highlight

condition change on the influence of the foundation pitFigure 2.1 (b) K line

expressing in theFigure 2.1 (a) K line

expressing in the

supporting. This paper mainly with weeks K line as an

example introduces the application of K chart to process

monitoring data of axial force of foundation pit. As

shown in figure 2.1 (b), in weeks K line, the initial

value and stop value of positive line indicate

respectively the measured data value on the first day and

the last day in a week. The highest and lowest values

indicate respectively the maximum and minimum values of

axial force change in this week.

APPLICATION OF ENGINEERING PRACTICE

General situation of the engineering. The project issubway station for leaving and arriving in Ningbo. Thestarting point is R5K0+800 and terminal point isR5K1+152. Its total length is 352m, and width is 17m to22.2m. The construction method is open excavation andcut-bottom up. Floor buried depth is about 0m to 13m.

Geological and hydrological conditions. According to thedesign scheme, open excavation section is related to thelayers as 1-2 ① plain fill, 2 grayish yellow clay, 3① ①gray silt clay, 2-1 silt, 2-2 gray silt clay. Among② ②them, 1-2 plain fill structure is loose. Table 3.1①shows parameters of main layers.Phreatic water mainly exists in the shallow cohesivesoil, silty soil. And on the basis of regional geologicalinformation, a change of ground water level is little,generally in the range of 0.5m ~ 1.0m. In accordance withthe survey information, the confined water level in the

deep part ⑥4 grey sandy silt and ⑧1 grey silty sand isrelatively stable and has no effect to engineering.According to the environmental conditions of the transfertrack, the deformation protection class of foundation pit(excavation depth > 6 m) is controlled in accordance withthe requirements of class II protection; for theexcavation depth ＜ 6 m, it is according to protectionclass III.

Table 3.1. Physical and mechanical parameters of soils

Lay

er

No.

Lay

er

nam

e

Layer

avera

ge

depth

(m)

Layer

unit

weight

Void

rati

o

Modulus

of

compress

ion

Cohesi

on

Internal

friction

angle

①2

grayi

sh

yello

w

clay

1.1 18.8 0.85 5.14 26.5 6.2

①3

gray

silt

clay

3.5 17.7 1.14 2.04 17.8 6.3

②2-1 silt 6.8 16.4 1.58 1.96 12.3 6.1

②2-2

gray

silt

clay

2.7 16.9 1.42 2.15 14.0 6.5

Bracing structure. The work well of the project

foundation pit sets 4 vertical supports, as shown in

Figure 3.1, the first is reinforced concrete support, the

second to the fourth is steel supports of φ609; 2 to 4

axes sets three vertical supports, the first is

reinforced concrete support, the second and the third are

steel pipe supports of φ609; 4 to 7 axes set three

vertical supports or two. The places where set the fourth

and the third supports, due to its large depth of

excavation and strict requirements of construction, are

given emphasis to analyze in this paper. 2~4 Axes

supporting sectional drawing is shown as the Figure3.2.

Figure 3.1 Work well supporting sectional drawing

Figure 3.2 2~4 Axes supporting sectional drawing

Layout of monitoring sites. In accordance with monitoring

specification and design requirements (GB50308-2008,2008

and GB50497-2009,2009), Layout of axial force monitoring

sites for the foundation pit project 1 to 4 axes is shown

as Figure 3.3.

Figure 3.3 Layout of axial force monitoring site

This supporting axial force monitoring uses reinforcement

tensiometer and axial force meter. The working principles

are: (1) axial force monitoring of concrete supporting

use steel string-typed reinforcement tensiometer combined

with frequency identifier to make measurement. While

binding supporting reinforcing steel bar, cut off the

main reinforcement on the middle place supported the four

sides, and weld the reinforcement stress meter on the

cut-off part, draw the wire on the stressometer out to

proper position for future testing when pouring

supporting concrete. (2) Before installing the supporting

axial force meter, test on technical indexes and

calibrated coefficient must be made. When installing, as

is shown in Figure 3.4.

Figure

3.4

Installing reinforcement tensiometer sectional drawing

one piece of steel plate(400 mm×400mm×20mm) is welded

with one end of the round steel cylinder(φ15cm), and then

welded on the fixed head at one end of the steel support;

one end of the axial force meter is placed inside the

steel cylinder, and supported on the enclosing purlins of

the enclosure wall with the installation of the steel

support. The data measured by instrument is recorded;

test several times repeatedly to observe whether the data

is stable or not. If the variation of the testing data is

within 1Hz for several times, it is considered the

testing data is stable, and make its average value as the

testing value.

Because the axial force monitoring is greatly affected by

the temperature, to get stable monitoring data, the

monitoring times are all in the same time period.

Monitoring analysis. In accordance with the supporting

load conditions of foundation pit, the paper selects the

supporting axial force as the object of discussion, which

can reflect the characteristic of the deformation of

foundation pit and also the key monitoring parts for the

construction. According to the K line expressing in the

foundation pit monitoring, draw the K line chart of the

selected monitoring sites. It is required to excavate on

the principles of layering, blocking and balancing, when

reaching 0.5 m below the specified supporting elevation,

set up supports in time.

Zh1-1 is a concrete diagonal bracing, from the Figure

3.5(a), we can see clearly, one week to two weeks before

concrete bracing putting into use, the axial force is

increasing continuously, later the variation scope of it

is decreasing gradually and tends to a stable state. The

jump of the upper and lower shadow and real body shows

the abrupt change of axial force within a certain time

scope. The upper shadow in the first week is due to the

soil body excavating around it and the lower part, stress

releases, making the axial force increase, within 24 h

after excavating in place, the lower support erected,

Zh1-1 axial force is recovered. The K line chart reflects

visually the increasing and recovery of the supporting

axial force, as well as the change reduces gradually and

tends to a stable state.

Zh1-1

0100200300400500600700800900

1 2 3 4 5 6 7 8 9 10 11Weeks

KN

Fig. 3.5(b), (c) are the K line chart of axial force for

two supports of standard sections H5 and H8. From the

Figure 3.5（a）Zh1-1 K line chart of

Figure, we can see, except the one or two weeks before

the supports starting to work, the change of axial force

is severe, in other time periods, the variation

amplitudes are relatively small. Analyzing from the point

of layer, this place is silt soft soil, with high

sensitivity; its bearing capacity is reducing obviously

after being disturbed. When excavating, the axial force

change greatly. The method to avoid this case is that the

working procedure of the construction shall be compactly

arranged to reduce the non-supporting exposure time, the

feedback from the construction site proved this method is

effective. From the above two Figures, we can also find

that, no matter how great the variation of upper and

lower shadow is, the axial force is always oriented to

the variation of the value of the middle part of the real

body, and this value can be compared with the calculation

value as the reference value for the follow-up project

design.

Zh3-1

0200

400600

8001000

12001400

1 2 3 4 5 6 7 8 9 10 11 12Weeks

KN

Figure 3.5（b）Zh3-1 K line chart ofaxial force

Figure 3.5（c）Zh4-1 K linechart of

Zh4-1

020040060080010001200140016001800

1 2 3 4 5 6 7 8 9 10 11Weeks

KN

The common characteristic of Fig. 3.5(d) and (e) is in

the first two weeks, the axial force is increasing

continuously. After the third week, the variation

amplitude is reducing obviously.

It shows that in the standard section construction of thefoundation pit, the first two weeks for supporting workare the key period to control the supporting axial force.Through the observation on other monitoring sites, it isdiscovered that the variation of stress strain is obviouswithin 10 days after the silt soft soil is disturbed, andafter that, if construction conditions are normal, thereis no severe change generally. This has certain referencevalue for studying the influence of the silt soilsolidification characteristic, and void water pressuredissipation law on the project.Figure 3.5 (c), (d) and (e) form variation drawing ofsectional supporting axial force, the variation of thefirst and second supports fluctuated greatly, and thethird support becomes stable when reaching the peakvalue. When reaching the stable state, the load conditionfor the first and the second supports is basically the

Zg4-2

0100200300400500600700800

1 2 3 4 5 6 7 8 9 Weeks

KN

Zg4-3

020040060080010001200140016001800

1 2 3 4 5 6 Weeks

KN

Figure 3.5（d）Zh4-2 K line chartof axial force

Figure 3.5（e）Zh4-3 K linechart of

same, the third support’s load condition is about the sumof the above two supports. This is not fully inconformity with the result calculated according to thesoil pressure, which deserves the designers to take itinto consideration, and increase the designing value ofthe first and the third supports properly.

CONCLUSIONS

Through analyzing the above K line chart, it shows that

the chart can reflect the variation law on another aspect

when excavating the foundation pit, and visually reflects

the variation amplitude of the foundation pit in a

certain time period, as well as the relationship between

the stable value and variation value, with the updating

of the monitoring equipments, it is possible to get the

real time data of monitoring sites, or soil pressure,

water level and other monitoring values can also be added

on the lower part of the figure to make comprehensive

analysis. This will give full play of K line chart.

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