Real time thermal field analysis on Wudongde super high arch dam during construction

1 Introduction

Temperature monitoring is very important in concrete dam construction to control cracking and the overall stability of the dam [1-5].At present,there are two main methods for obtaining the thermal field of dam concrete.The first is by restructuring the thermal field base of the temperature data measured by the thermometers [6-9].However,due to the discrete and sporadic temperature measurement data,the restructured thermal field and the actual thermal field have dominating errors.The second is to calculate the thermal field by numerical analysis [10,11].The calculation model generally assumes that the concrete is an isotropic heat conduction material and the influence of the cement hydration degree on the temperature duration is generally not considered during the construction period.Therefore,there is also a certain difference between the simulated thermal field and the real thermal field.

Distributed optical fiber temperature sensing (DTS)technology has the advantages of integrating sensing and transmission,many measuring points (line temperature measurement),high precision,high real-time performance,high reliability,and remote distance monitoring [12].It is widely used in hydropower engineering [13],in projects such as the Three Gorges,Xiaowan,Xiluodu,Laxiwa,Baihetan,and Wudongde dams.Compared with the restructuring thermal field by point thermometers,temperature monitoring or simulation calculation,thermal field restructuring based on distributed fiber temperature measurement data is undoubtedly a more accurate and effective method.

In this study,the 2D thermal field restructuring method based on site monitoring data is proposed.Based on site distributed optical fiber temperature monitoring,the results of restructuring on the Wudongde (WDD) dam is elucidated.

2 Dam site temperature monitoring

2.1 WDD project

The WDD Hydropower Station is currently under construction located at the Jinsha River,in Luquan county,Southwest China in a dry and hot river valley,with large temperature differences in the mornings and evenings with a peak temperature of 14 °C.The project includes a doublecurvature super-high arch dam (height 270 m),a spillway tunnel,and an underground power generation system [14].The installed electrical capacity is 10.2 million kilowatts.The dam is divided into 15 dam monoliths and the designed total volume is 2.8×106 m3.The pouring temperature control is challenging,and requires strict control of the concrete properties to avoid dam cracking over time.In order to obtain the law of temperature evolution and prevent temperature cracks,distributed optical fibers will be embedded in typical concrete dam monoliths,which are 7#,12# (Fig.1).Among them,7# is the river bed dam monolith,12# is the bank slope dam monolith.

2.2 DTS system

The DTS system comprises the host computer,data network,distributed optical fiber,human-computer interface,and cloud platform.The host computer of the DTS is an optical instrument that measures the temperature through the optical fiber.The optical fiber is similar to a sensor that monitors the temperature.The host computer provides the operation interface and it can not only measure the temperature change intermittently,but also collects temperature data continuously in real time.Furthermore,it acts as an interface for data file and real-time protocol of data exchange for networking.Data networks can use wired or wireless means.

The basic principle (Fig.2) of a DTS system is to make use of the principle of an optical time domain reflectometer(OTDR) [15]and the temperature effect of Raman backscattering [16]of the fiber.The thermal expansion coefficient of optical fiber can modulate the phase of the laser to reflect the temperature information.

Fig.1 Typical monoliths embedded with distributed optical fiber at WDD dam

Fig.2 Schematic diagram of DTS system

Compared with traditional thermometers,distributed optical fibers have many advantages in mass concrete temperature monitoring:(1) It can realize distributed temperature monitoring and overcome the spatial discontinuity of point monitoring; (2) The fiber is delicate,which does not affect the performance of the surrounding concrete,or the representativeness of the observation temperature; (3) The optical fiber has anti-electromagnetic interference,high sensitivity,is reliable and durable,and easy to integrate with optical fiber transmission.(4) The cable has high strength and can adapt to the complex and variable dam concrete construction environment.

2.3 Temperature monitoring design

Nowadays,for dam concrete temperature monitoring,the thermometers and DTS technology are always combined to monitor the internal temperature of pouring concrete blocks.

(1) The arrangement principle of thermometers

For concrete blocks without special structure,the thermometers are usually arranged along the center line of the concrete block in the direction of the water flow.In addition,the distance between the two thermometers or the distance between the thermometer and the upstream and downstream surfaces is usually 10 m to 12 m.If the block contains a gallery structure,the overall principle of the thermometer is unchanged,but the distance from the outer boundary of the gallery is greater than 0.5 m.

(2) The layout principle of distributed optical fiber

1) The embedded line must be at least 2 m from the left and right transverse joints in order to obtain the internal temperature of concrete.

2) The distributed optical fiber should monitor the temperature change rate of a dam concrete along the transverse river direction,the river direction,and the vertical direction.

3) The turning radius of the optical fiber is larger than the turning radius required by the cable itself to reduce the fiber loss.

4) The optical fiber is kept away from the cooling water pipe as much as possible to prevent the influence of the cooling water on the temperature monitoring data.

5) In the 200 m fiber embedding range,the optical fiber should pass at least one thermometer so as to compare and analyze the temperature data measured by the thermometers and the optical fiber.

3 Real time thermal field analysis

3.1 Restructuring method of thermal field

In order to restructure the internal 2D thermal field of a concrete block,an interpolation method based on measured data of installed thermometers should be determined first.In this study,the Kriging interpolation method is employed to restructure a 2D thermal field of concrete block.

The Kriging interpolation method [17,18]adopts the concept of “regionalization variable”.By studying the spatial variability of an element and making full use of the information of each known monitoring point in a region,the value of the unknown spatial point can be estimated.Assuming the study area of concrete block is A,the regionalization variable is Z(x) and Z(x)∈A.x represents the spatial 2D coordinates,and Z(x) is the attribute value at the sampling point xi (i=1,2,3,…,n).According to the principle of ordinary Kriging interpolation,the estimated attribute value Z(x0) at the unknown spatial point x0 is the weighted sum of the n attribute values of known monitoring sample points,which is as follows:

λi (i=1,2,3,…,n) is the weight coefficient to be determined.

Assuming that Z(x) satisfies the second-order stationary hypothesis throughout the study area,or Z(x) satisfies the eigen assumption.According to unbiased requirements:Z*(x0) = E[Z(x0)],Z*(x0) is the true attribute value at x0,it can be calculated as Eq.2:

Then,the equations for solving the weight coefficient λi(i=1,2,3,…, n) are available in Eq.3:

where:μ is the Lagrangian multiplier.

By obtaining each weight coefficient λi (i=1,2,3,…,n),the attribute value Z*(x0) at the unknown spatial point x0 can be obtained.

3.2 Evolution law of thermal field in early age

A 2D thermal field of each typical concrete block at different ages was restructured based on the monitoring data (obtained with the embedded fibers) and the Kriging temperature interpolation method.

Duo to the large amount of heat released by cement hydration reaction in early ages,the internal temperature of the concrete changes rapidly.Therefore,a case study of the 7#-0046 dam block,the thermal field evolution law during the 28 d age,is studied.Thermal field restructured results of dam block 7#-0046 in ages 1 d,3 d,5 d,7 d,14 d,and 28 d are shown in Fig.3.

It can be seen from Fig.3 that the concrete temperatures near the downstream and upstream faces were higher than those in the middle of the dam block at the age of 1 d.The courses were that the pouring temperature and cement hydration heat of concrete were low at the age of 1 d,and the air temperature was about 18-21 °C at this moment and it mainly increased the concrete temperature near the surface.At 3 d,much heat was released because of the cement hydration of concrete,which caused a sharp rise in concrete internal temperature and the average increase temperature was about 5 °C.A temperature peak reached close to 23 °C at this age.At 5 d and 7 d,the internal temperature of the concrete block decreased compared with the 3 d,but the decrease was small.When the concrete age was more than 14 d,the concrete temperature of the dam block stabilized between 20 °C and 22 °C,and the thermal field distribution grew more uniform.At this time,the concrete temperature in the center of the dam block was slightly higher than the temperature near the downstream and upstream surfaces.

Fig.3 The 2D thermal fields of the 7#-0046 concrete block during 28 d ages

3.3 Temperature difference control based on the restructured thermal field

The temperature difference in the concrete is the focus during the dam concrete pouring process.If the temperature difference in the block is too large,concreate cracking occurs and the quality of dam is adversely affected.According to the grouping of cooling water pipes,the concrete block area is divided into three zones.The first group of cooling water pipes is zone #1,the second group of cooling water pipes is zone #2,and the third group of cooling water pipes is zone #3 (Fig.4).The calculated temperature difference by the restructured thermal field and the thermometers temperature monitoring data from 7#11 to 7#40 at 15:00 on August 13,2018 is shown in Fig.5.

Fig.4 The zones of concrete block and thermometers distribution

Fig.5 The temperature difference in the concrete block calculated based on the thermal field and thermometer data

It can be seen from Fig.5 that the temperature difference in the concrete block calculated based on the thermal field data is lower than the temperature difference monitored by the thermometers,and the relative average difference between the two methods is 29.6%.At present,the maximum temperature difference based on the thermometer data in the concrete block of the WDD dam is controlled at 3 °C.If the restructured thermal field result is used,the maximum temperature difference in the concrete block can be increased to 3.5 °C.

4 Conclusions

A 2D thermal field restructuring method is proposed to determine the real thermal field of a concrete block based on the temperature monitoring data and the Kriging space temperature interpolation method.The restructuring method was then applied to the WDD dam site to restructure the 2D temperature distribution and ensure the effectiveness of temperature data of the concrete block.From the results of this study,the following conclusions can be drawn:

(1) The evolution law of 2D temperature of concrete dam in early age obtained by the restructuring method and is consistent with the actual engineering situation at the WDD dam site.

(2) The temperature difference calculated based on the thermal field data is lower than the temperature difference monitored by the thermometers; therefore,the maximum temperature difference control standard of a concrete block can be increased to 3.5 °C.