渦北煤礦1.5Mta新井設(shè)計(jì)含5張CAD圖-采礦工程.zip
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英文原文
The performance of pressure cells for sprayed concrete tunnel linings
C . R . I . C L AY TO N,J. P. VA N D E R B E R G , G . H E Y M A N N, A . V. D. B I C A a n d V. S . H O P E
Abstract:The paper examines the factors that affect the performance of tangential cells embedded in shotcrete tunnel linings. New data, derived from field monitoring, numerical modelling,and calibration tests carried out to simulate the embedment and crimping processes, are presented. These suggest that although well-designed embedded total pressure cells will have cell action factors close to unity, they cannot be assumed to provide reasonable estimates of the stresses within sprayed concrete linings, unless the influences of installation effects, temperature changes, shrinkage and subsequent crimping can be taken into account.
Keywords: field instrumentation; tunnels.
Introduction
The pressure cells used for measuring the compressive stresses in shotcrete tunnel linings generally consist of two stainless steel plates with a thin fluid- filled cavity between them. The cavity is connected either to a membrane-type bypass valve or to a vibrating-wire pressure transducer. The use of other direct-stress instruments has been reported in the literature, although infrequently. Pressure cells are typically installed in one of two orientations: radial, to record the stress between the sprayed concrete and the ground surrounding the tunnel; and tangential, to record the hoop stress within the tunnel lining itself. This paper considers only tangential cells.
Despite their widespread use in practice, there has been very little research reported in the literature on the use and behaviour of shotcrete pressure cells. Many practitioners remain doubtful of the ability of embedded pressure cells to measure the actual stresses in concrete tunnel linings. In a previous paper reviewing instrumentation for sprayed concrete lined tunnels the present authors noted some of the potential difficulties, stating that it was `extremely unlikely that embedded cells be used for monitoring the actual stress in a tunnel lining'. Yet, potentially, pressure cells are a valuable source of information that might be used to assess whether tunnel design assumptions are justified, and this paper therefore reports the findings of our further research into this important topic.
Factors affecting the pressures recorded by tangential pressure cells in tunnel linings
Direct stress measurement within any medium is made difficult by the many factors that can affect the results. In the case of tangential pressure cells embedded in shotcrete our recent experiences during tunnel monitoring suggest that these are as follows.
Cell properties
The cell should be constructed so that the stresses in the shotcrete are not significantly modified by its presence. Since the compressibility of the fluid in the cell is less than the surrounding materials it will under-read, but this can largely be compensated for by making the cell wide and thin. The use of cell fluids such as mercury or oil will affect not only the compressibility of cells but also their temperature sensitivity. Changes in temperature will expand the fluid against the surrounding, relatively rigid cell metal and surrounding concrete, and will produce a change in measured stress.
Installation effects
The inadvertent formation of cavities around the cell during shotcreting will lead to a soft measurement system, which will subsequently under-read. Incorrect positioning of the cell within the lining, rotating it towards the radial direction, can also cause it to under-read somewhat, because radial stresses are typically less than 10% of tangential. Indeed the actual thickness of the lining at the point of installation will also affect the interpretation of the stress measurements.
Post-installation factors
As noted above, temperature changes can be expected to lead to changes in measured stresses. Shrinkage during the early life of the shotcrete will result in changes in the recorded stress that are not due to external stress changes. Crimping, which is often undertaken to ensure that pressure cells are properly bedded within the shotcrete , can provide a significant offset to the measured pressures.
Numerical and physical experiments, and results from monitoring
Numerical modelling and physical simulation have been carried out to assess the actual performance of some stress cells used in practice, and to place their performance in the context of other cell designs.
Numerical modelling to assess the effects of cell fluid
To examine the effect of cell fluid on cell performance two idealised circular cells embedded in a block of concrete were modelled under axisymmetric conditions using the finite element package LUSAS. The geometry of the cells and the material properties modelled are shown in Fig. 1. The 160 mm diameter cell is somewhat larger than many of the cells currently in use, whereas the 80 mm cell is smaller, and was considered by the authors to be likely to have an excessive T/D ratio. In the first numerical experiment the effect of the bulk modulus of the cell fluid was investigated, by applying a constant external axial stress and varying the cell cavity pressure . The bulk modulus equivalent to each cell action factor was calculated by integrating the displacements along the surface of the cell cavity. Fig. 2 shows the considerable influence of bulk modulus on cell action factor, but it also shows that when reasonable cell geometries are used cell action factors remain tolerably close to unity when oil is substituted for mercury.
Fig. 1. Geometry of cells and properties of materials used during numerical modelling: (a) idealised pressure cell; (b) geometry modelled; (c) material properties
Fig. 2. Effect of bulk modulus of cell fluid on cell action factor
Physical simulation
Calibration tests were conducted to evaluate the performance of the vibrating-wire mercury-filled pressure cells, in three phases:
(a) During the first phase the manufacturer's calibration of the pressure cells was checked by conducting an air pressure calibration on all the cells used.This was done in a 1 m diameter chamber, in the laboratory.
(b) The second phase of the experimental work was conducted to investigate whether tangential cells installed under ideal and controlled conditions could produce reliable results. This was done by installing two pressure cells in a precast concrete slab, constructed in the laboratory.
(c) The final phase of the experimental work was designed to investigate the performance of the tangential cells under 'working conditions', as well as to investigate ways of installing the cells to improve their performance. This was done by installing tangential pressure cells in shotcrete slabs, formed in a tunnel under working conditions.
The cells used for the experimental work were mercury-filled vibrating-wire cells supplied by Geokon, and with a full-scale range of 20 MPa (Fig. 3). All the pressure measurements were calculated using the temperature correction supplied by the manufacturer.
Fig. 3. Vibrating-wire mercury-filled concrete stress cell
The second phase of the calibration testing was carried out to investigate the performance of the tangential pressure cells under ideal and controlled conditions. For this experiment two cells were embedded in a 25 MPa ready-mix concrete slab, 1.0 m high, 1.0 m wide and 0.3 m thick. The two cells were tied to a cage constructed from reinforcing bar meshes, which were identical to those in use in the Heathrow Terminal 4 station tunnels. The experimental set-up is shown in Fig. 4, except that in the first set of experiments two bare cells were used. One bare cell and one precast cell were used in a subsequent experiment, described later in this paper.
Fig. 4. Layout of cells embedded in concrete panel
During the curing period the slab temperature increased to about 32°C, and afterwards decreased slowly over several days. Monitoring was carried out until the cell temperatures reached equilibrium with the laboratory environment (Fig. 5).
Fig. 5. Temperatures measured after casting cells in ready-mixed concrete
At each load increment the cell readings were observed to stabilise rapidly: creep effects were not apparent. The value of cell action factor was subsequently calculated from pressure cell readings and average applied vertical stresses .
The final phase of the experimental work consisted of the evaluation of the tangential pressure cells installed under `working conditions'. This was done by placing two pressure cells in each of two slabs, similar to the above but constructed using shotcrete in a tunnel at Heathrow Terminal 4. Because the results of the experiment on the cells installed in the concrete slab showed that under ideal conditions the cells seem to perform satisfactorily, it was argued that failure under working conditions might result from installation effects. A major difficulty with the installation of tangential pressure cells is ensuring that no voids are formed in `shadow zones' around the cell during shotcreting. It was therefore decided to precast one of the cells in each of the two slabs in a tapered concrete block, designed to prevent the formation of shadow zones (Fig. 6).
Fig. 6. Detail of cell cast in precast concrete
Discussion
Theoretical considerations suggest that a well-designed embedded cell, with high stiffness and a low aspect ratio (T/D), should have a cell action factor close to unity. Our experiments on commercially available cells support this view. The ability of a cell to measure the applied pressure correctly is dependent upon additional factors, however. Offsets due to temperature, crimping and shrinkage must be properly taken into account. The quality of cell installation must be assessed.
Although manufacturers routinely supply temperature correction factors for vibrating-wire cells, these correct for the sensitivity of the transducer alone. For a 160 mm radial cell filled with mercury the authors' numerical modelling results suggest that a temperature change of 20°C will produce a pressure change of the order of 2 MPa. Oil-filled cells can be expected to be more temperature sensitive. A 20°C temperature change might produce about a 3 MPa pressure increase, which is of the same order as the tangential stress found in many completed shotcrete tunnel linings.
To the authors' knowledge, no estimates of the increase in measured stress induced by shrinkage have ever been reported in the literature. In order to make an initial estimate the data from the authors' laboratory experiments were reprocessed, using only those data obtained when the concrete slabs were unloaded.
If crimping is carried out then the zero offset of the cell is permanently altered. In a real installation the absolute pressure can be recovered only if the pressure change during crimping is carefully recorded. It is suggested that, although crimping is unnecessary if cells are well-designed and installation is good, the initial gradient of the crimping curve should provide a good guide to the cell action factor of the installed cell, with high crimping gradients indicating satisfactory cells. The pressure increases caused by crimping can be eliminated by subtracting them from the values subsequently recorded.
Conclusions
Using numerical and physical experiments, coupled with field observations, this paper has for the first time attempted a rational assessment of the many factors that may lead to embedded shotcrete pressure cells misreading.
The data suggest that, unless installation defects are present, the cell action factors of well-designed shotcrete pressure cells are likely to be near to 1. However, other factors need to be taken into account before embedded pressure cell data can be used to determine the true stress in a tunnel lining.
Temperature changes immediately after cell placement will be large, and, coupled with the high rate of shrinkage that occurs during the early life of shotcrete, will prevent satisfactory stress measurement during this period. Seasonal temperature changes will cause further changes in pressure cell readings. Strains due to shrinkage of the shotcrete may also significantly increase the measured stresses. Our data suggest that it is possible to predict the temperature sensitivity of the shell/shotcrete system using numerical modelling. Field data, laboratory measurement and estimates based upon an analytical approach are in good agreement.
After temperature changes due to cement hydration have ceased, the shape of the crimping curve can be used to assess the quality of cell installation. However, the crimping procedure will generate offset pressures that may be of the same order of magnitude as the actual pressure to be measured in most shotcrete linings. Measurements made after the crimping procedure is completed must be corrected by subtracting the pressure increase observed during crimping. If shadow zones cannot be prevented during installation then the use of precast cells may be advantageous, although the cell action factor of the installation will be modified.
The various factors influencing the measurement of the absolute stress in a shotcrete lining cannot be taken account of when routinely interpreting pressure cell data, without the benefit of careful calibration, numerical estimates of thermal sensitivity, and experimental determinations of the effects of shrinkage. We have shown that the effects of temperature, shrinkage and crimping will probably be large, and of the order of the stresses to be measured. However, the cell action factors of well-designed and well-installed pressure cells will be close to unity and, as we have shown, it should be possible to take account of the effects of temperature changes and shrinkage, to estimate the quality of the installation, and to correct forcrimping offset. Despite the potential difficulties the authors believe that tangential pressure cells can still be useful in many tunnelling applications, but only provided great care is taken in the interpretation of their measurements.
Acknowledgements
The authors gratefully acknowledge the support of Heathrow Express, Mott MacDonald Consulting Engineers, and the Engineering and Physical Sciences Research Council of the UK, and the help of Mr J. Barrie Sellers, President of Geokon, Inc.,USA, in reviewing the manuscript. The work described in this paper forms part of a wider research programme now being carried out by the University of Southampton, UK, into the behaviour of SCL tunnels.
中文譯文
噴射混凝土巷道應(yīng)力測(cè)量?jī)x的性能
C.R.I.克萊頓，J.P.范德伯格，G.霍曼，A.V.D.哈曼和V.S.霍普
摘要：本文研究了影響噴射混凝土巷道應(yīng)力測(cè)量?jī)x性能的因素。新數(shù)據(jù)通過(guò)實(shí)地檢測(cè)、數(shù)字模擬、模擬埋設(shè)標(biāo)定試驗(yàn)和褶曲變化進(jìn)程等進(jìn)行派生的表達(dá)。這些數(shù)據(jù)表明，雖然精心設(shè)計(jì)的應(yīng)力測(cè)量?jī)x測(cè)出的數(shù)據(jù)很接近圍巖整體運(yùn)動(dòng)規(guī)律，但是它們不能被假定為對(duì)噴射混凝土巷道圍巖應(yīng)力提供了合理的估計(jì)，除非將安裝影響、溫度變化、圍巖收縮及后續(xù)的褶曲變化等因素考慮在內(nèi)。
關(guān)鍵詞：實(shí)地檢測(cè)；巷道
1前言
用來(lái)測(cè)量噴射混凝土巷道壓縮應(yīng)力的應(yīng)力測(cè)量?jī)x（以后簡(jiǎn)稱(chēng)測(cè)力儀）通常由兩個(gè)不銹鋼板組成，兩個(gè)板之間有一個(gè)充滿液體的管。這個(gè)管一般和一個(gè)膜式旁路閥門(mén)或一個(gè)弦式壓力傳感器連接。在文獻(xiàn)中，也提到了一些其他測(cè)量應(yīng)力的器材的作用，雖然不常見(jiàn)。測(cè)力儀通常安裝在兩個(gè)方向之一：徑向，記錄噴射的混凝土與巷道圍巖之間的應(yīng)力；切向，記錄巷道內(nèi)的切應(yīng)力。本文認(rèn)為只有切向應(yīng)力。
雖然它們?cè)趯?shí)踐中廣泛使用，但在研究報(bào)告文獻(xiàn)中很少有介紹噴射混凝土巷道測(cè)力儀的使用和作用。很多學(xué)者仍然保留著對(duì)嵌入式測(cè)力儀能夠測(cè)量噴射混凝土巷道實(shí)際應(yīng)力的性能的懷疑。在前一篇審查噴射混凝土巷道測(cè)力儀性能的文章中，作者指出了一些潛在困難，特別說(shuō)明嵌入式測(cè)力儀用于監(jiān)測(cè)巷道實(shí)際應(yīng)力是極不可能的。然而，潛在的測(cè)力儀是一種寶貴的信息，可能被用來(lái)評(píng)估巷道設(shè)計(jì)假設(shè)是否合理，因此，這篇文章在這個(gè)重要的項(xiàng)目中報(bào)告了我們進(jìn)一步研究的成果。
2切向測(cè)力儀測(cè)量巷道應(yīng)力的影響因素
由于很多因素可以影響結(jié)果，所以在任何媒體中直接測(cè)量都很困難。在切向測(cè)力儀嵌入混凝土的情況下，在巷道監(jiān)測(cè)過(guò)程中，我們最近的經(jīng)驗(yàn)表明如下所示。
2.1測(cè)力儀特性
測(cè)力儀應(yīng)該被改進(jìn)，以便巷道中的壓力不會(huì)被它的存在而受到明顯改變。當(dāng)測(cè)力儀中的流體的可壓縮性小于周?chē)牧希鼤?huì)讀不出數(shù)據(jù)，但這可以在很大程度上通過(guò)將測(cè)力儀變寬變長(zhǎng)的方法來(lái)彌補(bǔ)。測(cè)力儀中的流體（如汞或油）的使用不僅會(huì)影響測(cè)力儀的可壓縮性，而且會(huì)影響他們的溫度敏感性。溫度的變化將促使流體對(duì)抗環(huán)境、相對(duì)剛性的測(cè)力儀金屬和周?chē)幕炷?，并且?huì)使測(cè)量的應(yīng)力產(chǎn)生變化。
2.2安裝影響
噴漿過(guò)程中，在測(cè)力儀周?chē)鸁o(wú)意形成的空隙會(huì)導(dǎo)致一個(gè)松軟的測(cè)量環(huán)境，這就會(huì)使測(cè)量?jī)x讀不出數(shù)據(jù)。不正確安裝的測(cè)量?jī)x，旋轉(zhuǎn)它往徑向，有時(shí)也會(huì)導(dǎo)致讀不出數(shù)據(jù)，這是因?yàn)閺较驊?yīng)力一般比切向應(yīng)力小10%。事實(shí)上，實(shí)際安裝時(shí)的襯砌厚度也會(huì)影響應(yīng)力測(cè)量的結(jié)果。
2.3安裝后的影響因素
如上所述，溫度的變化將會(huì)導(dǎo)致實(shí)測(cè)應(yīng)力變化。早期噴漿中的收縮過(guò)程導(dǎo)致應(yīng)力記錄的變化不取決于外部應(yīng)力的變化。經(jīng)常進(jìn)行褶曲以確保測(cè)力儀正常嵌入巷道內(nèi)，可以提供一個(gè)明顯的偏移測(cè)量應(yīng)力。
3數(shù)字模擬與物理實(shí)驗(yàn)和檢測(cè)結(jié)果
數(shù)字和物理模擬實(shí)驗(yàn)已進(jìn)行，用來(lái)評(píng)估一些測(cè)力儀在實(shí)踐中的性能，并且將這些性能用于一些其它測(cè)力儀的設(shè)計(jì)中去。
3.1數(shù)字模擬實(shí)驗(yàn)評(píng)估壓力計(jì)流體的影響
為了檢驗(yàn)測(cè)力儀中流體對(duì)測(cè)力儀性能的影響，參照實(shí)驗(yàn)——將兩個(gè)圓形測(cè)力儀以軸對(duì)稱(chēng)方式嵌入混凝土塊中，使用有限元包LUSAS進(jìn)行測(cè)定。測(cè)力儀的幾何結(jié)構(gòu)和材料屬性參照?qǐng)D如圖1所示。160毫米直徑的測(cè)力儀比許多目前正使用的測(cè)力儀要大一點(diǎn)，而80毫米直徑的要小一點(diǎn)，作者認(rèn)為可能是T/D比較大。在第一個(gè)數(shù)字模擬實(shí)驗(yàn)中，通過(guò)應(yīng)用一個(gè)恒定的外部軸向應(yīng)力和不同的測(cè)力儀管內(nèi)應(yīng)力，調(diào)查了測(cè)力儀流體體積彈性模量的影響。體積彈性模量相當(dāng)于通過(guò)整體的測(cè)力儀管曲面位移計(jì)算每一個(gè)應(yīng)力單元。圖2顯示了應(yīng)力變化因素體積彈性模量的相當(dāng)大的影響力，但同時(shí)還顯示了當(dāng)應(yīng)用合理的幾何形狀時(shí)，在用汞替代油的情況下，應(yīng)力單元變化情況仍和整體一致。
圖1 測(cè)力儀的幾何結(jié)構(gòu)和用于實(shí)驗(yàn)的材料屬性
圖2 測(cè)力儀流體體積彈性模量的影響因素
3.2物理模擬實(shí)驗(yàn)
用來(lái)評(píng)估弦式汞應(yīng)力測(cè)力儀性能的校準(zhǔn)測(cè)試分為三個(gè)階段：
1）在第一階段，通過(guò)對(duì)所有使用的測(cè)力儀進(jìn)行一個(gè)空氣壓力試驗(yàn)來(lái)檢查制造商校準(zhǔn)過(guò)的測(cè)力儀。這個(gè)實(shí)驗(yàn)是在實(shí)驗(yàn)室中一個(gè)1米直徑的空間中進(jìn)行的。
2）實(shí)驗(yàn)工作的第二階段是研究測(cè)力儀在理想和能控制的條件下安裝，到底哪個(gè)能得出可靠的結(jié)果。這通過(guò)在實(shí)驗(yàn)室中構(gòu)建的預(yù)制混凝土板中安裝兩個(gè)測(cè)力儀來(lái)進(jìn)行。
3）最后一個(gè)階段是調(diào)查切向測(cè)力儀在工作條件下的性能，以及研究如何通過(guò)改變安裝方法來(lái)改善測(cè)力儀的性能。這通過(guò)在處于工作條件下的巷道中形成的混凝土板中安裝切向測(cè)力儀來(lái)實(shí)現(xiàn)。
用于實(shí)驗(yàn)的弦式汞測(cè)力儀都質(zhì)優(yōu)價(jià)廉，而且測(cè)量范圍達(dá)到20兆帕（如圖3）。所有的應(yīng)力測(cè)量都通過(guò)制造商提供的溫度修正來(lái)進(jìn)行計(jì)算。
圖3 弦式汞應(yīng)力測(cè)量?jī)x
校準(zhǔn)實(shí)驗(yàn)的第二階段是研究切向測(cè)力儀在理想和現(xiàn)實(shí)條件下的性能。在這個(gè)實(shí)驗(yàn)中，將兩個(gè)測(cè)力儀嵌入拌好的混凝土板中，這個(gè)板高1.0米，寬1.0米，厚0.3米。兩個(gè)測(cè)力儀被綁定到一個(gè)用鋼筋加固的網(wǎng)格籠中，這個(gè)籠和希思羅機(jī)場(chǎng)4號(hào)站隧道中所使用的是完全相同的。實(shí)驗(yàn)設(shè)置如圖4所示，只是在第一組實(shí)驗(yàn)中使用了兩個(gè)原裝的測(cè)力儀。本文稍后會(huì)介紹，在隨后的實(shí)驗(yàn)中使用了一個(gè)原裝測(cè)力儀和一個(gè)預(yù)制的測(cè)力儀。
在固化階段，鋼坯溫度增長(zhǎng)到32°C，在隨后的幾天，溫度緩緩下降。監(jiān)測(cè)一直進(jìn)行到測(cè)力儀溫度和實(shí)驗(yàn)室環(huán)境平衡為止（如圖5）。
圖4 嵌入混凝土板中測(cè)力儀的布局
圖5 測(cè)力儀嵌入混凝土后的溫度變化
隨著每個(gè)增量的加載，測(cè)力儀的讀數(shù)迅速穩(wěn)定：蠕變影響不明顯。隨后通過(guò)測(cè)力儀讀數(shù)和提供的垂直應(yīng)力的平均值來(lái)計(jì)算測(cè)力儀單位應(yīng)力值。實(shí)驗(yàn)的最后階段包括對(duì)處于工作條件下的切向測(cè)力儀性能的評(píng)估。將兩個(gè)測(cè)力儀分別放在兩個(gè)板中，和上述的相似，但使用的是希思羅機(jī)場(chǎng)4號(hào)站隧道的混凝土板。因?yàn)檫@個(gè)實(shí)驗(yàn)的結(jié)果表明在理想條件下，測(cè)力儀似乎表現(xiàn)得更讓人滿意，有人認(rèn)為在工作條件下會(huì)失敗可能是安裝影響所導(dǎo)致的。安裝切向測(cè)力儀的一大困難是確保在處于混凝土中的測(cè)力儀周?chē)鷽](méi)有空隙所形成的“陰影區(qū)”。因此，當(dāng)局決定在一個(gè)圓錐形的混過(guò)凝土塊中預(yù)制兩個(gè)測(cè)力儀中的一個(gè)，旨在防止形成“陰影區(qū)”（如圖6）。
圖6 嵌入混凝土中的測(cè)力儀的詳細(xì)信息
4討論
理論思考表明一個(gè)精心設(shè)計(jì)的具有高剛性和低長(zhǎng)寬比（T/D）的嵌入式測(cè)力儀應(yīng)該有一個(gè)單元接近整體。我們對(duì)商業(yè)上可用的測(cè)力儀的實(shí)驗(yàn)支持這一觀點(diǎn)。然而，一個(gè)測(cè)力儀能正確測(cè)量所施加的應(yīng)力的能力取決于其它因素。除了溫度的影響，褶曲和壓縮的偏移量也必須適當(dāng)?shù)乜紤]在內(nèi)。必須評(píng)估測(cè)力儀的安裝質(zhì)量。
雖然制造商經(jīng)常為弦式測(cè)力儀提供溫度修正，但他們僅僅是對(duì)傳感器靈敏度進(jìn)行了校正。對(duì)于160毫米直徑的汞測(cè)力儀，作者的數(shù)字模擬實(shí)驗(yàn)結(jié)果表明溫度每變化20°C，測(cè)出的結(jié)果會(huì)有2兆帕的變化。油測(cè)力儀有望有更高的溫度敏感性。20°C的溫度變化可能產(chǎn)生約3兆帕應(yīng)力的增加，這是在很多已完成的噴射混凝土巷道中發(fā)現(xiàn)的切應(yīng)力變化的共同規(guī)律。
據(jù)作者所知，在已報(bào)告的文獻(xiàn)中，沒(méi)有因收縮導(dǎo)致已測(cè)應(yīng)力增加的評(píng)估。為了使作者從實(shí)驗(yàn)室實(shí)驗(yàn)中得到的初步估計(jì)數(shù)據(jù)得到進(jìn)一步的處理，當(dāng)混凝土板卸載時(shí)僅用這些提到的數(shù)據(jù)。
如果進(jìn)行褶曲，則測(cè)力儀的零偏移量會(huì)永久改變。在實(shí)際安裝中，只有仔細(xì)記錄褶曲過(guò)程中的應(yīng)力變化，才能使絕對(duì)應(yīng)力恢復(fù)。有人建議，雖然當(dāng)測(cè)力儀經(jīng)過(guò)精心設(shè)計(jì)，且安裝不出錯(cuò)時(shí)，褶曲是不必要的，但是褶曲曲線的起始梯度應(yīng)該為安裝好的測(cè)力儀提供一個(gè)很好的指南，并且褶曲高梯度標(biāo)示令人滿意的測(cè)力儀。褶曲產(chǎn)生的應(yīng)力增量可以通過(guò)隨后的記錄值減去增量來(lái)消除。
5結(jié)論
通過(guò)數(shù)字和物理模擬實(shí)驗(yàn)，再加上實(shí)地觀測(cè)，本文第一次嘗試對(duì)眾多影響因素進(jìn)行理性的評(píng)估，這些因素可能會(huì)導(dǎo)致對(duì)噴射混凝土巷道測(cè)力儀做出錯(cuò)誤判斷。
數(shù)據(jù)表明，除非安裝缺陷存在，精心設(shè)計(jì)的測(cè)力儀的應(yīng)力單元值才可能和1接近。然而，只有將其它因素考慮在內(nèi)之后，嵌入式測(cè)力儀得到的數(shù)據(jù)才能用于確定巷道中的真實(shí)應(yīng)力。
在測(cè)力儀布局增大后，溫度會(huì)馬上發(fā)生變化，再加上早期發(fā)生在混凝土中的高頻率收縮運(yùn)動(dòng)，溫度變化將會(huì)給這個(gè)時(shí)期令人滿意的測(cè)量造成困擾。季節(jié)溫度變化將會(huì)對(duì)測(cè)力儀讀數(shù)造成進(jìn)一步的影響。由于混凝土收縮產(chǎn)生的應(yīng)力也可能使測(cè)量的應(yīng)力大大增加。我們的數(shù)據(jù)表明，預(yù)測(cè)使用數(shù)字模擬的噴混凝土系統(tǒng)的溫度敏感性是可能的。實(shí)測(cè)數(shù)據(jù)、實(shí)驗(yàn)室測(cè)量和基于一種解析方法的估算達(dá)成了一致。
在水泥水化引起的溫度變化停止后，褶曲曲線的形狀可以用來(lái)評(píng)估測(cè)力儀的安裝質(zhì)量。然而，褶曲過(guò)程將會(huì)產(chǎn)生可能和多數(shù)巷道中實(shí)測(cè)應(yīng)力同一個(gè)數(shù)量級(jí)的應(yīng)力偏移量。褶曲過(guò)程結(jié)束后，測(cè)量的數(shù)據(jù)必須通過(guò)減去觀測(cè)褶曲過(guò)程中產(chǎn)生的應(yīng)力增量進(jìn)行修正。如果測(cè)力儀安裝過(guò)程中不能阻止陰影區(qū)的形成，那么使用預(yù)制測(cè)力儀會(huì)更有利，雖然這會(huì)使測(cè)力儀的安裝因素改變。
當(dāng)按慣例解釋測(cè)力儀數(shù)據(jù)時(shí)，影響噴射混凝土巷道絕對(duì)應(yīng)力測(cè)量的各種數(shù)據(jù)可以不用考慮在內(nèi)，不包括小心校準(zhǔn)、熱靈敏度的數(shù)據(jù)估算及收縮影響的實(shí)驗(yàn)測(cè)定的好處。我們已經(jīng)說(shuō)明了溫度、收縮和褶曲的影響可能會(huì)很大，包括要測(cè)量的應(yīng)力的順序。不過(guò)，精心設(shè)計(jì)和安裝合理的測(cè)力儀應(yīng)力單元會(huì)接近整體，如我們所示，應(yīng)該盡可能將溫度和收縮的影響考慮在內(nèi)，評(píng)估安裝質(zhì)量，修正褶曲偏移量。盡管有很多的潛在困難，作者扔堅(jiān)信切向測(cè)力儀在很多巷道的應(yīng)用程序中是有作用的，但只有在測(cè)量中非常認(rèn)真才能讓它實(shí)用。
6鳴謝
作者衷心感謝希思羅機(jī)場(chǎng)快遞、莫特 · 麥克唐納咨詢(xún)工程師、英國(guó)工程與自然科學(xué)研究委員會(huì)的支持，以及J.巴里 · 塞勒先生的幫助，感謝他們對(duì)本文的大力支持。本文描述的實(shí)驗(yàn)形成了一個(gè)更廣泛的研究項(xiàng)目，目前被英國(guó)南安普頓大學(xué)提出，用于研究SCL巷道的表現(xiàn)形式。
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