Study On The Ultra-weak FBG Technology For Deep Excavation Deformation

Pit monitoring,as an important means to ensure safety and guide construction,has developed towards long-distance,distributed and remote monitoring in recent years.The traditional methods are always scattered,resistive or vibrating-wire type,which has a comparably small amount of data and sometimes the monitoring measurements are hard to survive.Also,it is difficult to monitor subsurface oil or the retaining structure deformation process because of the limited construction conditions or excavation sequence.However,the ultra-weak FBG technology(UWFBG)can not only make the monitoring of the retaining structure become accessible,but also can realize precisely structure and soil monitoring.In this paper,by ultra-weak FBG techniques,the state of the retaining wall build-in zone and excavation heave have been studied from lab tests to engineering project,which helps to proposes a way to determine the retaining wall build-in zone’s stability through continuous monitoring data and a method to calculate the retaining wall horizontal displacement in a unstable condition,meanwhile,this paper also establish a complete set of optical cable installation for excavation heave monitoring and how to calculate the upheaval.Last but not least,this paper also designs an ultra-weak FBG online monitoring system based on 4G wireless transmission technology,which has already achieved a good effect in engineering.The main work and conclusion of this paper are as follows:(1)Proposes to use the positive and negative change of the strain-time curve’s slope as the criterion to evaluate the deep build-in zone’s stability: when the strength of the soil in deep foundation pit is low,the retaining wall build-in zone’s stability can be divided into two periods during excavation,which called stress accumulating period and stress release period,at the beginning of the excavation,the excavation depth is shallow,and the internal and external earth pressure of the pit can keep balance,its moment and strain increases synchronously;when the deep soil’s strength reaches its limited value,it becomes partly damaged,which means the build-in zone comes to a unstable condition,at this time,the moment of the retaining wall’s bottom decreases,its strain gradually decreases after its peak occurs.Therefore,the positive and negative slope change point of the strain-time curve can be used to evaluate the retaining wall build-in zone stability.(2)Calculate the unstable-bottom retaining wall displacement with the help of FBG inclinometer successfully.In the indoor test,when the 16 m inclinometer is applied by when 0 ～ 45 cm displacement,percentage error of its displacement is reduced by12.15%;When it comes to 35 m diaphragm wall in actual project,the maximum horizontal displacement error is reduced from 22 mm ～ 74 mm to 3mm,and the deep displacement monitoring is realized,which error comes to 5mm ～ 15 mm.(3)Proposes a kind of cable laying methods to sense excavation heave.In the field monitoring,we not only get the vertical deformation characteristics of the deep excavation soil,but the shear deformation is also found in the long-term strain monitoring,which can help us to scope the potential sliding surface roughly.(4)Establishes a set of ultra-weak FBG pit remote monitoring system which is composed of optical cable(sensors),optical series connection,demodulation equipment,power supply circuit,4G network,automatic processing terminal and etc.The system can realize data remote sampling,automatic transmission,automatic processing and early warning.All in all,this paper improves the deficiencies of optical fiber monitoring in retaining wall displacement and excavation heave,and it further expands the scope of optical fiber sensing technology in foundation pit engineering,which will also lead to a wide application in low-strength soil areas.