JP3594843B2 - Ground liquefaction detection method, liquefaction detection device and system - Google Patents

Description

【００３８】
既に述べたように、地震波検出手段１０、周期最大加速度検出手段１２およびＳＩ値検出手段１３は、それぞれこの地震波検出センサ１０２により実現することができる。
【００３９】
液状化判定部１０３は、この地震波検出センサ１０２の出力（加速度波形、周期最大加速度Ａ_ｐｍａｘおよびＳＩ値）を基に、加速度波形の周期Ｐおよび地盤変位Ｄを求め、これら周期Ｐ、地盤変位Ｄ、周期最大加速度Ａ_ｐｍａｘおよびＳＩ値が共に閾値（ Ａ_０ ，Ｐ_０ ，ＳＩ_０ ，Ｄ_０ ）より大きいか否かを判断し、いずれもが閾値よりも大きい場合に当該液状化検知装置１０１が設置された地盤に液状化現象が発生したと判定し、通信網を介して監視室３００に液状化発生信号１０２１を送信するものである。この液状化判定部１０３は、具体的にはマイクロコンピュータにより構成され、図示しないＲＯＭ（Ｒｅａｄ Ｏｎｌｙ Ｍｅｍｏｒｙ）等に格納されたプログラムを実行することによって後述のように動作するようになっている。すなわち、この液状化判定部１０３により上記周期検出手段１１、地盤変位検出手段１４および液状化判定手段１５がそれぞれ実現される。
【００４０】
本実施の形態では、加速度波形の周期Ｐは、図３に示したように加速度波形が基線Ｚを横切る時間（ゼロクロス時間）間隔を基にして推定されるゼロクロス周期を用いる。ここでは、基線Ｚを横切った地震波が更に基線Ｚを２回横切るまでの時間を１ゼロクロス周期とするが、この周期の定義は任意であり、例えば、加速度波形が基線と交わる点と点との間隔１つ分を計測したり、点と点との間隔３つ分を計測してそれを３で割り、間隔の平均値を求めて、それを半周期と定義することもできる。
【００４１】
ところで、地震動による加速度波形の特徴として、図４に示したように、Ｐ波，Ｓ波等の実体波の後から周期の長い表面波が到来することが挙げられる。中規模以上の地震の際には、この表面波の到来時においてはじめて４つのパラメータ全ての条件を満足することがあり、このような場合には、液状化が発生していないにもかかわらず、発生していると誤判定を行うこととなる。従って、表面波による波形と実体波による液状化波形とを識別し、表面波による影響を排除する必要がある。ここで、表面波は、前述のように液状化波形と同様に、長周期成分を多く含むが、その加速度は液状化波形の加速度よりも小さくなる特徴を有する。
【００４２】
本実施の形態では、このような表面波による影響を排除するために、判定に用いる最大加速度の値（絶対値）を、地震波全体（Ｐ波，Ｓ波等の実体波および表面波）での最大値（地震波最大加速度Ａ_ｍａｘ ）ではなく、周期内での最大値（周期最大加速度Ａ_ｐｍａｘ）とし、各周期毎にこれら周期最大加速度Ａ_ｐｍａｘと周期Ｐが共に所定の閾値Ａ_０ ，Ｐ_０ 以上であるか否かを判断し、これらが同時に閾値以上であり、かつ他のパラメータが閾値以上である場合に、液状化が発生したと判定するものである。
【００４３】
また、液状化現象が発生する場合の地震波の他の特徴として、加速度波形にもかかわらず変位が大きく、波形がサイン（ｓｉｎ）カーブに類似していることが挙げられる。上述のＴＯＷＨＡＴＡ ｅｔ ａｌ．は、地震波形がサインカーブに類似している場合には、地盤変位Ｄの最大値Ｄ_ｍａｘ は下記に示す式（３）によって近似的に算出することができることを示している。本実施の形態では、ＳＩ値およびＡ_ｍａｘ として検出センサ１０２により出力される周期最大加速度Ａ_ｐｍａｘを用いてこの
近似式（式２）から地盤変位Ｄを算出する。
【００４４】
【数２】
Ｄ_ｍａｘ ＝２×（ＳＩ）^２ ／Ａ_ｍａｘ …（２）
【００４５】
地震波検出センサ１０２は、常時_、 周期最大加速度Ａ_ｐｍａｘおよびＳＩ値を出力しているため、式（２）を用いて地盤変位Ｄを算出する方法は本システムに好適な方法である。これにより、地震波の長周期のノイズが拡大されて不安定である地盤の変位を直接測定する必要がなくなり、容易にかつ精度よく変位を算出することができる。
【００４６】
液状化判定部１０３においては、次のような判定動作が行われるようになっている。すなわち、地震波の加速度の周期Ｐおよび地盤変位Ｄが求められると、液状化判定手段１５では、周期最大加速度Ａ_ｐｍａｘ、周期Ｐ、地盤変位ＤおよびＳＩ値 が共に閾値（Ａ_０ ，Ｐ_０ ，ＳＩ_０ ，Ｄ_０ ）より大きいか否かの判断がなされる。この判断は、まず、ゆるい砂質土層で液状化現象が発生するのに十分なＳＩ値ＳＩ_０ を例えば２０ｋｉｎｅ（＝ｃｍ／ｓ）と設定し、算出されたＳＩ値が設定した閾値ＳＩ_０ より大きい地震波を抽出する。次に、抽出された地震波のうちで、上述のように一般的に液状化現象が発生した場合には大きい値を示す地盤変位Ｄが設定した閾値Ｄ_０ 以上であるものを更に抽出する。なお、地盤変位Ｄの閾値Ｄ_０ は例えば１０ｃｍとする。
【００４７】
更に、各周期毎に、周期最大加速度Ａ_ｐｍａｘと周期Ｐが共にそれぞれ設定した閾値（Ａ_０ ，Ｐ_０ ）以上であるか否かを判定し、共に閾値以上である場合を抽出する。本実施の形態では、液状化現象が発生するのに十分な最大加速度Ａ_０ を例えば１００ｇａｌ（＝ｃｍ／ｓ^２ ）と設定し、周期Ｐの閾値Ｐ_０ は例えば２．０秒とする。なお、この周期最大加速度の閾値Ａ_ｏ は、表面波より先に到来する実体波における最大加速度（すなわち、前述の地震波最大加速度Ａ_ｍａｘ ）に１．０未満の所定の値（例えば０．２）を乗じた値にすることも可能である。このようにして、周期毎に検知された周期最大加速度Ａ_ｐｍａｘ、周期Ｐ、および算出された地盤変位Ｄ、ＳＩ値が全て閾値（Ａ_０ ，Ｐ_０ ，ＳＩ_０ ，Ｄ_０ ）より大きい地震波が抽出された検知地点においては、地盤に液状化現象が発生したと判定される。
【００４８】
非防爆域にある制御盤２００は、機械式地震センサ２０１と、判定回路２０２および警報報知部２０３を含んで構成されている。液状化判定部１０３において地盤に液状化現象が発生したと判定された場合、液状化判定部１０３から判定回路２０２へ例えばリレー信号などの遮断信号が出力されるようになっている。一方、機械式地震センサ２０１の信号も判定回路２０２へ出力されるようになっている。判定回路２０２は、これら液状化判定部１０３からの出力と機械式地震センサの出力が共に検知される場合を論理積により判定し、その場合には遮断信号を出力して遮断弁１０４ａを駆動しガバナ１０４を遮断するようになっている。同時に、警報報知部２０３にも警報信号を出力し、信号に応答した警報報知部は警報（例えば表示ランプの点灯または点滅、ブザー警報）を報知するようになっている。
【００４９】
監視室３００は、判定回路２０２において地盤に液状化現象が発生したと判定された場合に、判定回路２０２から出力される液状化発生信号１２０２に対応して、ガバナ４０２と緊急遮断弁５０２の両方またはいずれか一方に対して緊急遮断指示を出力するようになっている。この緊急遮断指示の出力先は、当該液状化検知装置１０１の属する液状化検知地点におけるガス配管の埋設状況等を考慮して決定される。
【００５０】
次に、以上のような構成の地盤の液状化検知装置１００および液状化検知システムの動作について、図５および先の図１，図２を参照しながら説明する。
【００５１】
まず、地震波検出センサ１０２により地震波形（加速度波形）が所定の時間間隔で検知される（ステップＳ１００）。
【００５２】
液状化判定部１０３では、地震波検出センサ１０２により算出されるＳＩ値がその閾値ＳＩ_０ より大きいか否かを判断し（ステップＳ１０１）、閾値ＳＩ_０ より大きい場合（ステップＳ１０１；Ｙ）には、続いて液状化判定部１０３において式１を用いて求めた地盤の変位Ｄがその閾値Ｄ_０ より大きいか否かを判断する（ステップＳ１０２）。地盤の変位Ｄがその閾値Ｄ_０ より大きい場合（ステップＳ１０２；Ｙ）には、地震波検出センサ１０２から出力される周期最大加速度Ａ_ｐｍａｘがその閾値Ａ_０ より大きく、かつ液状化判定部１０２において求めた周期Ｐがその閾値Ｐ_０ より大きいか否かを判断する（ステップＳ１０３）。
【００５３】
周期最大加速度Ａ_ｐｍａｘがその閾値Ａ_０ より大きく、かつそのときの周期Ｐがその閾値Ｐ_０ より大きい場合（ステップＳ１０３；Ｙ）には、液状化判定部１０３は、地盤に液状化現象が発生したと判定し（ステップＳ１０４）、判定回路２０２を介して中央の監視室３００に対して液状化発生信号１２０２を出力すると共に警報報知部２０３に警報信号を出力する（ステップＳ１０５）。また、同時にガバナ１０４に緊急遮断指示を出力することによりガバナ１０４の遮断弁１０４ａを駆動し、ガバナ１０４以降のガス流路を遮断する。
【００５４】
監視室３００では液状化発生信号１２０２に応答してガス製造工場４００のガバナ４０２とガバナステーション５００における緊急遮断弁５０２の両方またはいずれか一方に対して緊急遮断指示を出力する。これによりガバナ４０２の遮断弁４０２ａと緊急遮断弁５０２の両方またはいずれか一方が駆動され、ガス流路が遮断される。
【００５５】
一方、得られたＳＩ値がその閾値ＳＩ_０ 未満であった場合（ステップＳ１０１；Ｎ）、得られた地盤の変位Ｄがその閾値Ｄ_０ 未満であった場合（ステップＳ１０２；Ｎ）、あるいは周期最大加速度Ａ_ｐｍａｘと周期Ｐの一方もしくは両方が閾値（Ａ_０ ，Ｐ_０ ）未満であった場合（ステップＳ１０３；Ｎ）には、いずれの場合においても液状化判定部１０３は、地盤に液状化現象が発生していないと判定する（ステップＳ１０６）。その後、所定の時間間隔で次の加速度波形が入力されると、動作はステップＳ１００に戻り、以下、同様の判定が周期的に繰り返し実行される。
【００５６】
以上のように本実施の形態の地盤の液状化検知システムでは、地表の地震波形のみを用いて地盤の液状化が発生したか否かの判定を行うようにしたので、各液状化検知地点の地表に地震波検出センサ１０２を設置するだけでよく、その他の演算用の電子機器等も全て地表に存在するので、測定機器を地下に埋設するものと比較して安価なシステムとなる。また、容易に設置することができるため、より多くの地震波検出センサ１０２を設置することができ、高密度の液状化検知網を実現して面的に液状化発生状況を把握することが可能である。更には、このシステムは、液状化の有無について加速度が計測可能な一般的な地震計のみからなるシステムにおいて構成することも可能である。従って、液状化現象を直接観測するための特殊な装置を要せず、安価に実現できる。
【００５７】
また、本実施の形態では周期Ｐをゼロクロス周期とし、地盤の変位Ｄを近似式により求めるようにしたので、演算速度が著しく向上する。また、本実施例では所定間隔で加速度値が入力される毎に判定がなされるため、条件を満足した時点で直ちに判定結果が出力される。従って、地盤の液状化現象が生じた場合に、液状化判定部１０２により迅速に検知できると共に、その後の対応措置を迅速に採ることができる。
【００５８】
更に、本実施の形態では、液状化発生の有無を判定するためのパラメータとして、周期Ｐ，地盤の変位Ｄ，周期最大加速度Ａ_ｐｍａｘおよびＳＩ値の４つを用いるようにしたので、信頼性の高い判定結果を得ることができる。
【００５９】
加えて、本実施の形態では、主要なパラメータとしての最大加速度（絶対値）を、地震波の加速度波形全体から抽出したＡ_ｍａｘ ではなく、周期（ゼロクロス周期）毎にリアルタイムに連続して抽出し、各周期毎に、周期および周期最大加速度が同時に各々閾値以上であるか否かを判断し、これらが同時に閾値以上である場合に、液状化発生と判断するようにしたので、中規模地震等が発生し、加速度は小さいが長周期成分を含む表面波が到来した場合において、液状化と誤って判断するようなことがなくなり、精度よく液状化を判定することができる。
【００６０】
〔第２の実施の形態〕
次に、本発明の第２の実施の形態について説明する。
【００６１】
本実施の形態に係る液状化検知システムは、地震動の加速度を水平面の直交２軸方向およびそれ以外の方向それぞれについて求めることを除き、その他の構成および作用効果は第１の実施の形態と同様である。以下、第１の実施の形態と異なる点についてのみ説明し、その他の構成および作用効果の説明は省略する。
【００６２】
本実施の形態においては、図１に示した地震波検出センサ１０２に含まれる地震波検出手段１０は、水平面の直交２軸方向の地震波の加速度波形を検出し、その加速度から射影演算法により他の方向の加速度波形を求め、これら複数方向の加速度波形データを周期最大加速度検出手段１２、ＳＩ値検出手段１３および周期検出手段１１にそれぞれ出力するようになっている。
【００６３】
本実施の形態における地震波検出センサ１０２は、所定の一方向に向けて設置されるものであり、地震動により生じた加速度波形を水平面の直交する２軸（Ｘ，Ｙ）方向それぞれについて保存し、これら２軸方向の加速度波形を射影投影法により他の方向へ射影（ベクトル射影）し、その射影成分の合成から水平面内の所定の方向についての加速度波形を求め、各方向について加速度波形を出力できると共に、各方向それぞれについて最大加速度Ａ_ｍａｘ およびＳＩ値を出力できるようになっている。
【００６４】
図６は、この射影演算法によって、直交する２軸方向以外の方向の加速度成分を求める場合の具体例を説明するためのものである。同一水平面の直交する２軸（Ｘ，Ｙ）のうち、Ｘ軸を０゜、Ｙ軸を９０゜とし、Ｘ軸からの角度をθとしたとき、θ方向への射影加速度αθはＸ軸方向の加速度αＸとＹ軸方向の加速度αＹを用いて以下の式（３）により求めることができる。
【００６５】
【数３】
αθ＝αＸｃｏｓθ＋αＹｓｉｎ（θ−９０゜）…（３）
（０°≦θ≦１８０°）
【００６６】
具体的には、例えば射影方向の軸数を８軸、射影角度θを２２．５゜刻みに、０゜、２２．５゜，４５゜，６７．５゜，９０゜，１１２．５゜，１３５゜，１５７．５゜とする。このうち、ある時刻で、地震波検出センサ１０２のＸ軸，Ｙ軸の出力がαＸ_１ ，αＹ_１ であるとき、射影角度θ_１ ＝２２．５°の方向の加速度は、図のＸ_１ ｃｏｓ２２．５°と、Ｙ_１ ｓｉｎ（２２．５°−９０゜）で求められる加速度の和となる。なお、θが１８０°以降は各角度の加速度の負の値となるため、射影演算は不要となる。
【００６７】
本実施の形態の地震波検出手段１０では、このような射影演算によって求めた多方向（３以上）の加速度波形を用いて液状化判定を行うものである。その他の構成は、第１の実施の形態と同様である。
【００６８】
次に、本実施の形態における地盤の液状化検知システムの動作について、図７および先の図１，図２を参照しつつ説明する。
【００６９】
まず、地震波検出センサ１０２において、２軸方向の地震動による加速度が検知される（ステップＳ２００）と共に、前述のような加速度射影演算が行われ、例えば８軸方向の加速度（射影成分の合成ベクトル）が算出される（ステップＳ２０１）。以下、８軸方向それぞれにおいて、周期最大加速度Ａ_ｐｍａｘ、周期Ｐ、地盤変位ＤおよびＳＩ値が共に閾値（Ａ_０ ，Ｐ_０ ，ＳＩ_０ ，Ｄ_０ ）より大きいか否かの判断がなされる。
【００７０】
すなわち、液状化判定部１０３では、まず、地震波検出センサ１０２により算出されるＳＩ値がその閾値ＳＩ_０ より大きいか否かを判断し（ステップＳ２０２）、閾値ＳＩ_０ より大きい場合（ステップＳ２０２；Ｙ）には、続いて液状化判定部１０３において式１を用いて求めた地盤の変位Ｄがその閾値Ｄ_０ より大きいか否かを判断する（ステップＳ２０３）。地盤の変位Ｄがその閾値Ｄ_０ より大きい場合（ステップＳ２０３；Ｙ）には、地震波検出センサ１０２から出力される周期最大加速度Ａ_ｐｍａｘがその閾値Ａ_０ より大きく、かつ液状化判定部１０２において求めた周期Ｐが、その閾値Ｐ_０ より大きいか否かを判断する（ステップＳ２０４）。
【００７１】
周期最大加速度Ａ_ｐｍａｘがその閾値Ａ_０ より大きく、かつそのときの周期Ｐがその閾値Ｐ_０ より大きい場合（ステップＳ２０４；Ｙ）には、液状化判定部１０３は、地盤に液状化現象が発生したと判定する（ステップＳ２０５）。その後は、第１の実施の形態と同様に、判定回路２０２を介して中央の監視室３００に対して液状化発生信号１２０２を出力すると共に警報報知部２０３に警報信号を出力する（ステップＳ２０６）。
【００７２】
一方、得られたＳＩ値がその閾値ＳＩ_０ 未満であった場合（ステップＳ２０２；Ｎ）、得られた地盤の変位Ｄがその閾値Ｄ_０ 未満であった場合（ステップＳ２０３；Ｎ）、得られた周期最大加速度Ａ_ｐｍａｘと周期Ｐの一方もしくは両方が閾値（Ａ_０ ，Ｐ_０ ）未満であった場合（ステップＳ２０４；Ｎ）には、いずれの場合においても液状化判定部１０３は、地盤に液状化現象が発生していないと判定する（ステップＳ２０７）。その後、所定の周期で次の加速度波形が入力される毎に、動作はステップＳ２００に戻り、以下、同様の判定を繰り返す。
【００７３】
本実施の形態では、水平面内の直交２軸方向の加速度波形に加え、他の方向の加速度波形をも求め、これら多方向の加速度波形に基づいて液状化の判定を行うようにしたので、地震波検出センサの取り付け角度によって、液状化が発生しているにもかかわらず液状化なしと判定するような誤判定がなくなり、第１の実施の形態よりも判定精度が向上する。
【００７４】
更に、本実施の形態では、直交２軸方向の実測データを基に、射影投影法によって他の方向の加速度データを演算により求めるようにしたので、１つの地震波検出センサで、複数のセンサを互いに方向（取り付け角度）を変えて設置した場合と同等の効果を得ることができる。勿論、本発明では、実際に複数の地震波検出センサを互いに取り付け角度を変えて設置し、これら複数のセンサによって地震波形を全て実計測にて検知するようにしてもよい。
【００７５】
次に、過去に地震が発生した際に記録された地震波形を分析して、周期最大加速度Ａ_ｐｍａｘ，ゼロクロス周期Ｐ，地盤の変位Ｄ，およびＳＩ値を算出し、上記第２の実施の形態に係る地盤の液状化検知システムの判定方法に当てはめた結果を説明する。
【００７６】
表１は、過去に発生した地震の際に記録された各地の地震波形について、第３の実施の形態の方法によるシミュレーションを行って得た液状化判定結果と、加震実験を行い実機にて地震波を検知し判定して得た液状化判定結果を、各観測地点で実際に調査した結果と比較して表したものである。加震実験については、加震台の能力限界により全ての地震波を実験することはできなかった。表中、○は液状化したと判定されたことを示し、×は液状化は起こらなかったと判定したもの、―は、加震実験について実施できなかったものを示している。
【００７７】
【表１】

【００７８】
実際に液状化を生じた地点で記録された地震波形については、全て液状化が発生したと判定することができた。また、液状化を生じなかった地点で記録された地震波形を基に液状化が発生したとする誤った判定は存在せず、過去の地震で生じた液状化現象をほぼ１００％の精度で判定することができた。実際に液状化検知システムを駆動させるときには、誤った判定によりガス配管が遮断された場合、その後の復旧作業に相当の時間を要するが、本システムにおいてはそのようなおそれは殆どないといえる。
【００７９】
このように過去の地震波形を基に行ったシミュレーション結果および加震実験の結果から、周期最大加速度Ａ_ｐｍａｘおよび周期Ｐが共に閾値を越え、さらにＳＩ値および地盤変位Ｄについても共に閾値を越える地震については、ほぼ１００％液状化現象が発生することが分かった。
【００８０】
以上実施の形態を挙げて本発明を説明したが、本発明はこれらの実施の形態に限定されるものではなく、種々変形が可能である。例えば、第２の実施の形態では、８軸方向についてそれぞれ加速度データを得るようにしたが、その数は限定するものではない。
【００８１】
また、上記実施の形態では、加速度波形に基づいて得られるＳＩ値、地盤の変位、加速度波形の周期、および周期毎の最大加速度（周期最大加速度）の４つのパラメータのいずれもが所定の閾値を越えた場合に液状化現象が発生したと判定するようにしたが、パラメータの数はこれに限定するものではなく、これらパラメータのうち少なくとも２つのパラメータを用いるようにすれば、従来技術よりも判定精度が向上する。更に、上記実施の形態では、最大加速度を、周期毎の最大加速度（すなわち周期最大加速度）として表面波の影響を排除するようにしたが、周期毎ではなく、加速度波形全体のなかでの最大の加速度（最大加速度）を加えて、５つのパラメータのうち少なくとも２つのパラメータを用いるような構成としてもよい。
【００８２】
【発明の効果】
以上説明したように本発明の地盤の液状化検知方法または液状化検知装置によれば、地震動による加速度波形を同一水平面内の複数の方向について検出し、この加速度波形に基づいて、ＳＩ値、最大加速度、地盤の変位、加速度波形の周期およびこの周期毎の最大加速度（周期最大加速度）のうちの少なくとも２つを検出すると共に、それぞれが所定の閾値以上であるか否かを判断することによって液状化現象の発生を検知するようにしたので、簡易かつ迅速に地盤の液状化を精度良く検知することができる。
【００８３】
また、特に、判定に際し、加速度波形の周期を検出すると共に、この周期毎の最大加速度（周期最大加速度）を検出し、各周期毎にこれら周期と最大加速度とが共に所定の閾値以上であるか否かを判断することにより、長周期であっても加速度が液状化を起こすほど大きくない表面波の影響を排除でき、より信頼性の高い判定結果を得ることができる。
【００８４】
更に、地震動の加速度波形を、同一水平面内の互いに直交する２軸方向およびこれら２軸方向とは異なる他の方向それぞれにおいて求め、これら複数の加速度波形に基づいて各パラメータが閾値以上であるかを判断することにより、センサの設置方向に影響されることなく、より信頼性の高い液状化判定を行うことができる。
【００８５】
また、本発明の液状化検知システムによれば、複数の地点に本発明の液状化検知装置を備えるようにしたので、簡易な構成で、精度よく液状化の有無を判断し、複数の検知地点における液状化発生状況を容易に監視することができると共に迅速な対応措置をとることが可能になる。更に、この液状化検知システムは、液状化の有無について加速度が計測可能な一般的な地震計のみからなるシステムにおいて構成することもできるので、液状化現象を直接観測するための特殊な装置を要せず、安価に実現することができる。
【図面の簡単な説明】
【図１】本発明の第１の実施の形態に係る液状化検知装置の機能構成を表すブロック図である。
【図２】図１に示した液状化検知装置を備えた液状化検知システムの構成を表す概略図である。
【図３】地震動による加速度波形の周期（ゼロクロス周期）を説明するための波形図である。
【図４】液状化判定に及ぼす表面波の影響を説明するための波形図である。
【図５】図２に示した液状化検知システムの動作を説明するための流れ図である。
【図６】射影演算法を説明するためのベクトル図である。
【図７】本発明の第２の実施の形態に係る液状化検知システムの動作を説明するための流れ図である。
【符号の説明】
１０…地震波検出手段、１１…周期検出手段、１２…周期最大加速度検出手段、１３…ＳＩ値検出手段、１４…地盤変化検出手段、１５…液状化判定手段、１０１…液状化検知装置、１０２…地震波検出センサ、１０３…液状化判定部、２０３…警報報知部（警報手段）、３００…監視室（監視手段）[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a ground liquefaction detection method, a liquefaction detection device, and a system for detecting a liquefaction phenomenon caused by an earthquake motion.
[0002]
[Prior art]
The liquefaction of the ground is caused by a decrease in the shear strength of the sandy soil as the pore water pressure of the sandy soil layer increases during an earthquake, and the ground suddenly becomes unstable. This is a phenomenon that occurs. The liquefaction phenomenon is likely to occur in a loose sandy soil layer located deeper than the groundwater level, and is one of the factors that expand the damage caused to buried pipes in buildings such as lifelines. Therefore, it is important to detect the occurrence of liquefaction immediately after the earthquake and to know the extent of liquefaction when it occurs, in order to reduce secondary disasters.
[0003]
For example, it is said that the damage rate of water lines and gas pipes, which are lifelines, will increase if the ground liquefies. If liquefaction occurs and the pipes are severely damaged, supply will be promptly made. I need to stop. Therefore, development of a rapid detection method for liquefaction is desired.
[0004]
Conventionally, as a method for detecting such liquefaction of the ground, in addition to a method for measuring the pore water pressure by installing a pore water pressure meter directly in the basement, for example, There is a liquefaction detection method. In this liquefaction detection method, a hollow tube with an opening at the tip is installed in a loose sandy soil layer where liquefaction is expected, and the groundwater flow caused by excess pore water pressure in the ground that occurs during an earthquake or the like is opened. The degree of liquefaction is estimated by guiding the amount of water level increase in the hollow tube with a water pressure gauge installed in the hollow tube. As another detection method, there is a ground liquefaction detection method disclosed in JP-A-6-17413. In this liquefaction detection method, the upper end of the above-mentioned hollow tube is closed, a large number of openings are provided on the lower side surface, and a pressure gauge for measuring air pressure is installed inside the hollow tube, for example, to raise the water level in the hollow tube. The increase in the air pressure in the hollow tube is measured, and the degree of liquefaction is estimated based on the sum of the water level rise in the hollow tube and the air pressure at the upper end. Furthermore, as another detection method, there is a ground liquefaction early detection method disclosed in JP-A-7-109725. In this liquefaction detection method, the scale and distance range and acceleration level of the earthquake causing liquefaction are set in advance, the magnitude and distance of the earthquake are estimated from the information of the initial motion of the earthquake, and the earthquake acceleration is monitored. Based on these results, it is determined whether or not to cause liquefaction of the ground, and prevention of liquefaction is achieved by pumping water based on the determination.
[0005]
[Problems to be solved by the invention]
As described above, various liquefaction detection methods have been conventionally proposed. However, in the method of measuring pore water pressure by installing a pore water pressure gauge directly in the basement, in addition to the cost associated with the borehole construction when installing the pore water pressure gauge, the water pressure to be buried directly underground. The meter is prone to failure, and if it fails, there is a problem that it is necessary to perform a boring work again, which is more expensive.
[0006]
Further, in the liquefaction detection method disclosed in Japanese Patent Application Laid-Open No. 6-10335, compared with the method of directly measuring the fluctuation of the pore water pressure by burying the pore water pressure meter underground, Labor can be reduced. In addition, all the measurement electronics are on the surface, have few failures and excellent durability, and can be easily replaced even if they fail, making maintenance and management easy It has the advantage of being. Moreover, in the liquefaction detection method disclosed in Japanese Patent Laid-Open No. 6-17413, in addition to the above-mentioned advantages, a wide range of pore water pressure can be measured by closing the upper end and suppressing the rise in water level. , And good reactivity. However, these methods still have a problem that a boring work is still required to install a detection device such as a hollow tube, which is large and expensive.
[0007]
Furthermore, the liquefaction detection method disclosed in Japanese Patent Application Laid-Open No. 7-109725 has the advantage of being able to detect liquefaction at an early stage, but basically it is determined whether or not liquefaction has occurred based only on the earthquake acceleration level. Therefore, there has been a problem in terms of reliability regarding the determination of occurrence of liquefaction.
[0008]
The present invention has been made in view of such problems, and its purpose is inexpensive, easy to install, and capable of quickly and accurately determining whether or not a liquefaction phenomenon has occurred. An object of the present invention is to provide a liquefaction detection method and a liquefaction detection device and system.
[0009]
[Means for Solving the Problems]
The ground liquefaction detection method according to claim 1 detects an acceleration waveform due to earthquake motion, and based on the acceleration waveform, the SI value, maximum acceleration, displacement of the ground, period of the acceleration waveform, and maximum acceleration for each period ( That is, at least two of the maximum cyclic acceleration) are detected, and it is determined whether each is equal to or greater than a predetermined threshold.In addition, the acceleration waveform is obtained in a biaxial direction perpendicular to each other in the same horizontal plane and in other directions different from the two axes, and the occurrence of the liquefaction phenomenon is detected based on the acceleration waveforms in the plurality of directions. It is.
[0010]
In the present specification, the maximum acceleration means a value having the largest absolute value among positive and negative amplitude values with the baseline of the acceleration waveform as a boundary.
[0011]
In this ground liquefaction detection method,Multiple directions in the same horizontal planeTwo or more parameters of the SI value detected based on the acceleration waveform, the maximum acceleration, the displacement of the ground, the period of the acceleration waveform, and the maximum acceleration for each period (period maximum acceleration) are detected, and each has a predetermined threshold value. It is determined whether or not this is the case. Based on this determination, it is determined whether or not a liquefaction phenomenon has occurred in the ground. Here, when detecting the SI value, the maximum periodic acceleration, etc., the detected acceleration waveform is preliminarily corrected to the baseline, that is, the correction for subtracting the drift amount of the output during normal times when no earthquake is detected from the waveform output. The
[0012]
The ground liquefaction detection method according to claim 2 detects an acceleration waveform due to earthquake motion, detects a period of the acceleration waveform, detects a maximum acceleration for each period (period maximum acceleration), and detects each period. It is determined whether or not both the cycle and the cycle maximum acceleration are equal to or greater than a predetermined threshold.In addition, the acceleration waveform is obtained in a biaxial direction perpendicular to each other in the same horizontal plane and in other directions different from the two axes, and the occurrence of the liquefaction phenomenon is detected based on the acceleration waveforms in the plurality of directions. It is.
[0014]
Claim3The ground liquefaction detection method described in claim 1 or 2, wherein the acceleration waveform is detected in two orthogonal directions in the same horizontal plane,The acceleration waveform in a direction different from the two axes is obtained by calculation based on the detected acceleration waveform.
[0015]
Claim4The ground liquefaction detection device described above includes an earthquake wave detecting means for detecting an acceleration waveform of seismic motion, an SI value detecting means for detecting an SI value, a maximum acceleration detecting means for detecting a maximum acceleration, and a ground displacement detecting a displacement of the ground. At least two detection means among detection means, period detection means for detecting the period of the acceleration waveform, and period maximum acceleration detection means for detecting the maximum acceleration for each period (period maximum acceleration), and detection value output of each of these detection means And liquefaction determining means for determining whether or not a liquefaction phenomenon has occurred by determining whether or not is equal to or greater than a predetermined threshold.And the seismic wave detecting means obtains the acceleration waveform in each of a biaxial direction orthogonal to each other in the same horizontal plane and a direction different from the two axes, and the acceleration waveforms in a plurality of directions detected by the seismic wave detecting means are obtained. Based on this, the occurrence of the liquefaction phenomenon is detected.
[0016]
In this ground liquefaction detection device, it was detected by the seismic wave detection means.Multiple in the same horizontal planeAt least two of various parameters (SI value, maximum acceleration, ground displacement, period, period maximum acceleration) based on the acceleration waveform are detected, and it is determined whether or not the detected parameter is equal to or greater than a threshold value. When these parameter outputs are equal to or greater than the threshold value, it is determined that a liquefaction phenomenon has occurred in the ground.
[0017]
Claim5The ground liquefaction detection device described above includes an earthquake wave detecting means for detecting an acceleration waveform due to earthquake motion, a period detecting means for detecting a period of an acceleration waveform based on the acceleration waveform, and a maximum acceleration (period maximum acceleration) for each period. Detecting the occurrence of a liquefaction phenomenon by determining whether to detect the maximum cyclic acceleration detection means and whether the detection value output of each of the cycle detection means and the cyclic maximum acceleration detection means is equal to or greater than a predetermined threshold value. And determining means, andThe seismic wave detecting means obtains the acceleration waveform in two orthogonal directions and different directions from the two axes in the same horizontal plane, and based on the acceleration waveforms in a plurality of directions detected by the seismic wave detecting means, It detects the occurrence of the liquefaction phenomenon.
[0018]
In this ground liquefaction detection device, it was detected by the seismic wave detection means.Multiple in the same horizontal planeFrom the acceleration waveform, the cycle and the maximum acceleration for each cycle (cycle maximum acceleration) are detected, and it is determined whether the cycle and the maximum acceleration for each cycle (cycle maximum acceleration) are each equal to or greater than a threshold value. When the period and the maximum acceleration for each period (period maximum acceleration) are each equal to or greater than the threshold value, it is determined that a liquefaction phenomenon has occurred in the ground.
[0021]
Claim6The ground liquefaction detection device according to claim4 or 5In the description, the seismic wave detecting means detects an acceleration waveform in each of two axial directions orthogonal to each other in the same horizontal plane, and obtains an acceleration waveform in a direction different from these two axes by calculation based on the acceleration waveform in the two axial directions. StuffThe
[0022]
Claim7The described ground liquefaction detection systems were installed at multiple locations.Claims 4, 5 and 6The liquefaction detection device according to any one of the above, communication means for transmitting detection results from the plurality of liquefaction detection devices, and monitoring means for monitoring the liquefaction occurrence status at the plurality of points based on the detection results And.
[0023]
In this ground liquefaction detection system, the determination result of the liquefaction detection device at each liquefaction detection point is transmitted to the monitoring unit by the communication unit, and liquefaction occurrence at a plurality of liquefaction detection points is performed by this monitoring unit. The situation is monitored.
[0024]
Claim8The ground liquefaction detection system described in claim7In the description, a plurality of liquefaction detection points are provided in a gas supply area in which a large number of gas pipes are embedded.
[0025]
Claim9The ground liquefaction detection system described in claim8In addition, when the occurrence of the liquefaction phenomenon is confirmed by the monitoring means, the shutoff instruction is given to the gas pipe in consideration of the burial condition of the gas pipe at the liquefaction detection point.
[0026]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0027]
[First Embodiment]
FIG. 1 is a block diagram showing a functional configuration of a ground liquefaction detection device 101 according to a first embodiment of the present invention. The ground liquefaction detection method according to the present invention is embodied in the operation of the liquefaction detection device 101, and the description thereof is omitted.
[0028]
The liquefaction detection device 101 includes a seismic wave detection sensor 102 and a liquefaction determination unit 103. The seismic wave detection sensor 102 is a seismic wave detecting means 10 for detecting an acceleration waveform of the seismic wave in each of two orthogonal axes on the horizontal plane, and the maximum acceleration (period maximum acceleration, A_pmax) And a SI value detection means 13 for detecting an SI value (SI) based on the acceleration waveform detected by the seismic wave detection means 10. The liquefaction determining unit 103 includes a period detecting unit 11 that detects a vibration period (P) of the acceleration waveform detected by the seismic wave detecting unit 10 and a ground displacement detecting unit 14 that detects a ground displacement (D). Yes.
[0029]
The liquefaction determination unit 103 further includes liquefaction determination means 15. The liquefaction determining means 15 is provided with a periodic maximum acceleration A detected by the periodic maximum acceleration detecting means 12._pmaxThe period P detected by the period detection means 11, the SI value detected by the SI value detection means 13, and the ground displacement D detected by the ground displacement detection means 14 are respectively set to predetermined threshold values (A₀, P₀, SI₀, D₀) To determine whether it is greater than the maximum period acceleration A_pmaxWhen the period P, the SI value, and the ground displacement D are both larger than the respective threshold values, it is determined that the liquefaction phenomenon has occurred in the ground. In the ground displacement detection means 14, the ground displacement D may be directly detected by a displacement sensor. However, as will be described later, the periodic maximum acceleration A_pmaxAnd SI values were used to determine the TOWHATA et al. A simple configuration can be obtained by obtaining the approximate expression shown in (SOILS AND FOUND ATIONS, Vol. 36, 29-44, 1996).
[0030]
FIG. 2 shows a configuration example in which the ground liquefaction detection system including the liquefaction detection device 101 shown in FIG. 1 is applied to a gas supply line.
[0031]
This gas supply line supplies gas from the gas manufacturing factory 400 to the home 600, factory 601, etc. of each district, and a governor station 500 and a governor chamber 100 for adjusting the gas pressure are arranged in the middle. Has been. The liquefaction detection device 101 installed in the governor chamber 100 is connected to the central monitoring chamber 300 via the control panel 200. The monitoring room 300 is connected to liquefaction detection devices in other districts in the same manner, and monitors the occurrence of liquefaction based on information from each district and shuts off the gas supply. The monitoring room 300 corresponds to the monitoring means of the present invention.
[0032]
In this gas supply line, the high-pressure gas produced in the gas production plant 400 is normally stored in the gas tank 401, and the pressure is adjusted by a governor (gas pressure regulator) 402 having a shut-off valve 402a that is shut off in an emergency. Thereafter, the gas passes through the high-pressure conduit 403 and is sent to a governor station 500 installed in each area of the gas supply area, for example, 3600 locations. The governor station 500 includes a governor 501 and an emergency shutoff valve 502, and the gas sent to the governor station 500 is adjusted to an intermediate pressure by the governor 501 and then passes through the intermediate pressure conduit 503 to be further liquefied. The pressure is adjusted to a low pressure by a governor 104 having a shut-off valve 104a that is shut off in an emergency included in the detection device 101, passes through the low-pressure conduit 105, and is supplied to each home 600, factory 601 and the like.
[0033]
The governor room (explosion-proof area) 100 is installed between the governor station 500 and each household 600, factory 601, etc., and includes the liquefaction detector 101 and the governor 104.
[0034]
The liquefaction detection device 101 includes an earthquake detection sensor 102 and a liquefaction determination unit 103 inside an explosion-proof case made of a metal such as aluminum so as to satisfy a predetermined pressure resistance.
[0035]
The seismic wave detection sensor 102 in this embodiment is a so-called SI (Spectrum Intensity) sensor, and detects an acceleration waveform (earthquake waveform) on the ground surface in two orthogonal (X, Y) directions on a horizontal plane. This can be stored and output, and the period maximum acceleration (A_pmax) Can be measured.
[0036]
The SI value obtained by the seismic wave detection sensor 102 is a numerical value of the degree of general building shaking caused by an earthquake, and in a specific natural frequency range obtained by performing a response analysis using the acceleration of the earthquake as an input. Average value. Specifically, the SI value can be obtained by the following equation (1), and it is known from the past observation results that the correlation between the seismic intensity and the presence or absence of earthquake damage is higher than the maximum acceleration. . The seismic wave detection sensor 102 includes a capacitance type, piezoresistive type, or piezoelectric type semiconductor acceleration sensor.
[0037]
[Expression 1]

[0038]
As already described, the seismic wave detecting means 10, the periodic maximum acceleration detecting means 12 and the SI value detecting means 13 can be realized by the seismic wave detecting sensor 102, respectively.
[0039]
The liquefaction determining unit 103 outputs the output of the seismic wave detection sensor 102 (acceleration waveform, period maximum acceleration A_pmaxAnd the SI value), the period P and the ground displacement D of the acceleration waveform are obtained, and the period P, the ground displacement D, the period maximum acceleration A_pmaxAnd SI value are both threshold values (A₀, P₀, SI₀, D₀) And if both are larger than the threshold value, it is determined that a liquefaction phenomenon has occurred in the ground where the liquefaction detection device 101 is installed, and the monitoring room 300 is connected via the communication network. A liquefaction generation signal 1021 is transmitted. The liquefaction determining unit 103 is specifically composed of a microcomputer, and operates as described later by executing a program stored in a ROM (Read Only Memory) (not shown) or the like. That is, the liquefaction determining unit 103 implements the period detecting unit 11, the ground displacement detecting unit 14, and the liquefaction determining unit 15, respectively.
[0040]
In the present embodiment, the period P of the acceleration waveform uses a zero cross period estimated on the basis of a time interval (zero cross time) in which the acceleration waveform crosses the base line Z as shown in FIG. Here, the time until the seismic wave crossing the base line Z further crosses the base line Z twice is defined as one zero-crossing period. However, the definition of this period is arbitrary. For example, the point where the acceleration waveform intersects the base line It is also possible to measure one interval or measure three intervals between points and divide them by 3 to obtain an average value of the intervals and define it as a half cycle.
[0041]
By the way, as shown in FIG. 4, the characteristic of the acceleration waveform due to the earthquake motion is that a surface wave having a long period arrives after the body wave such as P wave and S wave. When an earthquake of medium or larger scale occurs, the conditions of all four parameters may be satisfied for the first time when the surface wave arrives. In such a case, although liquefaction has not occurred, If it occurs, an erroneous determination is made. Therefore, it is necessary to discriminate between the waveform caused by the surface wave and the liquefaction waveform caused by the body wave and eliminate the influence of the surface wave. Here, as described above, the surface wave contains a lot of long-period components as in the case of the liquefaction waveform, but the acceleration is smaller than the acceleration of the liquefaction waveform.
[0042]
In the present embodiment, in order to eliminate the influence of such a surface wave, the maximum acceleration value (absolute value) used for the determination is set to the whole seismic wave (body wave and surface wave such as P wave and S wave). Maximum value (maximum acceleration of seismic wave A_max), Not the maximum value within the cycle (cycle maximum acceleration A_pmax), And the maximum acceleration A for each period_pmaxAnd period P are both predetermined threshold values A₀, P₀It is determined whether or not the above is the case, and when these are simultaneously equal to or greater than the threshold and other parameters are equal to or greater than the threshold, it is determined that liquefaction has occurred.
[0043]
Another characteristic of the seismic wave when the liquefaction phenomenon occurs is that the displacement is large despite the acceleration waveform, and the waveform is similar to a sine curve. The above-mentioned TOWHATA et al. If the seismic waveform is similar to a sine curve, the maximum value D of the ground displacement D_maxIndicates that it can be approximately calculated by the following equation (3). In this embodiment, the SI value and A_maxAs the periodic maximum acceleration A output by the detection sensor 102_pmaxUsing this
The ground displacement D is calculated from the approximate expression (Expression 2).
[0044]
[Expression 2]
D_max= 2 x (SI)²/ A_max... (2)
[0045]
The seismic wave detection sensor 102 is always_,Periodic maximum acceleration A_pmaxSince the SI value is output, the method of calculating the ground displacement D using the equation (2) is a suitable method for this system. As a result, it is not necessary to directly measure the displacement of the ground that is unstable due to the expansion of the long-period noise of the seismic wave, and the displacement can be calculated easily and accurately.
[0046]
In the liquefaction determination unit 103, the following determination operation is performed. That is, when the acceleration period P and the ground displacement D of the seismic wave are obtained, the liquefaction determining means 15 determines the maximum period acceleration A._pmax, Period P, ground displacement D and SI value are both threshold values (A₀, P₀, SI₀, D₀) Is determined whether or not it is larger. This determination is based on the SI value SI which is sufficient to cause the liquefaction phenomenon in the loose sandy soil layer.₀Is set to, for example, 20 kine (= cm / s), and the calculated SI value is a set threshold value SI.₀Extract larger seismic waves. Next, among the extracted seismic waves, when the liquefaction phenomenon generally occurs as described above, the threshold value D set by the ground displacement D indicating a large value is set.₀What is more is further extracted. In addition, threshold D of ground displacement D₀Is, for example, 10 cm.
[0047]
Further, for each period, the period maximum acceleration A_pmaxAnd the threshold value (A₀, P₀) It is determined whether or not it is greater than or equal to, and cases where both are equal to or greater than the threshold are extracted. In the present embodiment, the maximum acceleration A sufficient to cause the liquefaction phenomenon₀For example, 100 gal (= cm / s²) And the threshold P of the period P₀Is, for example, 2.0 seconds. In addition, the threshold A of this period maximum acceleration_oIs the maximum acceleration in the body wave that arrives before the surface wave (that is, the above-mentioned maximum seismic wave acceleration A_max) Multiplied by a predetermined value less than 1.0 (for example, 0.2). In this way, the cycle maximum acceleration A detected for each cycle._pmax, Period P, and calculated ground displacement D, SI values are all threshold values (A₀, P₀, SI₀, D₀) It is determined that a liquefaction phenomenon has occurred in the ground at a detection point where a larger seismic wave is extracted.
[0048]
The control panel 200 in the non-explosion-proof area includes a mechanical earthquake sensor 201, a determination circuit 202, and an alarm notification unit 203. When the liquefaction determination unit 103 determines that a liquefaction phenomenon has occurred in the ground, a blocking signal such as a relay signal is output from the liquefaction determination unit 103 to the determination circuit 202. On the other hand, a signal from the mechanical earthquake sensor 201 is also output to the determination circuit 202. The determination circuit 202 determines the case where both the output from the liquefaction determination unit 103 and the output of the mechanical seismic sensor are detected by logical product, and in that case, outputs a cutoff signal to drive the cutoff valve 104a. The governor 104 is cut off. At the same time, an alarm signal is output to the alarm notification unit 203, and the alarm notification unit responding to the signal notifies an alarm (for example, lighting or blinking of a display lamp, buzzer alarm).
[0049]
When the determination circuit 202 determines that a liquefaction phenomenon has occurred in the ground, the monitoring room 300 responds to the liquefaction generation signal 1202 output from the determination circuit 202 to both the governor 402 and the emergency shut-off valve 502. Alternatively, an emergency cutoff instruction is output to either one. The output destination of this emergency shut-off instruction is determined in consideration of the burial condition of the gas pipe at the liquefaction detection point to which the liquefaction detection device 101 belongs.
[0050]
Next, operations of the ground liquefaction detection device 100 and the liquefaction detection system having the above-described configuration will be described with reference to FIG. 5 and FIGS.
[0051]
First, an earthquake waveform (acceleration waveform) is detected at predetermined time intervals by the earthquake wave detection sensor 102 (step S100).
[0052]
In the liquefaction determination unit 103, the SI value calculated by the seismic wave detection sensor 102 is used as the threshold SI.₀It is determined whether it is larger (step S101), and the threshold SI₀If it is larger (step S101; Y), the ground displacement D obtained by using the formula 1 in the liquefaction determining unit 103 is the threshold value D.₀It is determined whether it is larger (step S102). The displacement D of the ground is the threshold D₀If larger (step S102; Y), the periodic maximum acceleration A output from the seismic wave detection sensor 102 is obtained._pmaxIs the threshold A₀The period P obtained by the liquefaction determination unit 102 is larger than the threshold value P.₀It is determined whether it is larger (step S103).
[0053]
Periodic maximum acceleration A_pmaxIs the threshold A₀Larger and the period P at that time is the threshold value P₀If larger (step S103; Y), the liquefaction determination unit 103 determines that a liquefaction phenomenon has occurred in the ground (step S104), and liquefies the central monitoring room 300 via the determination circuit 202. The alarm signal 1202 is output and an alarm signal is output to the alarm notification unit 203 (step S105). At the same time, an emergency shutoff instruction is output to the governor 104 to drive the shutoff valve 104a of the governor 104 and shut off the gas flow path after the governor 104.
[0054]
In the monitoring room 300, in response to the liquefaction generation signal 1202, an emergency shut-off instruction is output to either or both of the governor 402 in the gas manufacturing plant 400 and the emergency shut-off valve 502 in the governor station 500. As a result, the shutoff valve 402a of the governor 402 and / or the emergency shutoff valve 502 are driven to shut off the gas flow path.
[0055]
On the other hand, the obtained SI value is the threshold SI.₀When it is less than (step S101; N), the obtained ground displacement D is the threshold value D.₀If it is less than (step S102; N), or the cycle maximum acceleration A_pmaxAnd one or both of period P and threshold value (A₀, P₀) (Step S103; N), in any case, the liquefaction determining unit 103 determines that the liquefaction phenomenon has not occurred in the ground (step S106). Thereafter, when the next acceleration waveform is input at a predetermined time interval, the operation returns to step S100, and the same determination is repeated periodically thereafter.
[0056]
As described above, in the ground liquefaction detection system according to the present embodiment, it is determined whether or not ground liquefaction has occurred using only the seismic waveform on the ground surface. It is only necessary to install the seismic wave detection sensor 102 on the ground surface, and all other electronic devices for computation are present on the ground surface. Therefore, the system is cheaper than that in which the measuring device is buried underground. Moreover, since it can be installed easily, more seismic wave detection sensors 102 can be installed, and it is possible to realize a high-density liquefaction detection network and grasp the liquefaction occurrence situation on a surface. is there. Furthermore, this system can also be configured in a system consisting only of a general seismometer capable of measuring acceleration with or without liquefaction. Therefore, a special apparatus for directly observing the liquefaction phenomenon is not required and can be realized at a low cost.
[0057]
In the present embodiment, since the period P is set to the zero cross period and the ground displacement D is obtained by an approximate expression, the calculation speed is remarkably improved. Further, in this embodiment, since the determination is made every time the acceleration value is input at a predetermined interval, the determination result is output immediately when the condition is satisfied. Therefore, when the liquefaction phenomenon of the ground occurs, the liquefaction determining unit 102 can quickly detect it, and the subsequent countermeasures can be taken quickly.
[0058]
Furthermore, in the present embodiment, as parameters for determining whether or not liquefaction has occurred, period P, ground displacement D, period maximum acceleration A_pmaxSince four SI values are used, a highly reliable determination result can be obtained.
[0059]
In addition, in the present embodiment, the maximum acceleration (absolute value) as a main parameter is extracted from the entire acceleration waveform of the seismic wave._maxInstead, it is extracted continuously in real time for each period (zero cross period), and for each period, it is judged whether or not the period and the maximum acceleration of the period are both equal to or greater than the threshold value at the same time. In addition, since it was judged that liquefaction occurred, when a medium-scale earthquake or the like occurred and a surface wave arrived with a small acceleration but containing a long period component, it would not be mistakenly judged as liquefaction. The liquefaction can be accurately determined.
[0060]
[Second Embodiment]
Next, a second embodiment of the present invention will be described.
[0061]
The liquefaction detection system according to the present embodiment is the same as that of the first embodiment except that the acceleration of seismic motion is obtained for each of the two orthogonal axes on the horizontal plane and the other directions. is there. Hereinafter, only differences from the first embodiment will be described, and descriptions of other configurations and effects will be omitted.
[0062]
In the present embodiment, the seismic wave detection means 10 included in the seismic wave detection sensor 102 shown in FIG. 1 detects the acceleration waveform of the seismic wave in the two orthogonal axes on the horizontal plane, and from the acceleration to the other direction by the projection calculation method. The acceleration waveform data in a plurality of directions are obtained and output to the period maximum acceleration detecting means 12, the SI value detecting means 13, and the period detecting means 11, respectively.
[0063]
The seismic wave detection sensor 102 according to the present embodiment is installed in a predetermined direction, and stores acceleration waveforms generated by seismic motion in each of two orthogonal (X, Y) directions perpendicular to the horizontal plane. The biaxial acceleration waveform is projected to the other direction (vector projection) by the projection projection method, the acceleration waveform in a predetermined direction in the horizontal plane is obtained from the synthesis of the projection components, and the acceleration waveform can be output in each direction. , Maximum acceleration A for each direction_maxAnd SI value can be output.
[0064]
FIG. 6 is a diagram for explaining a specific example in a case where an acceleration component in a direction other than the orthogonal two-axis directions is obtained by the projection calculation method. Of the two orthogonal axes (X, Y) on the same horizontal plane, when the X axis is 0 °, the Y axis is 90 °, and the angle from the X axis is θ, the projected acceleration αθ in the θ direction is the X axis direction The following equation (3) can be obtained using the acceleration αX and the acceleration αY in the Y-axis direction.
[0065]
[Equation 3]
αθ = αX cos θ + αY sin (θ−90 °) (3)
(0 ° ≦ θ ≦ 180 °)
[0066]
Specifically, for example, the number of axes in the projection direction is 8 and the projection angle θ is 22.5 ° in increments of 0 °, 22.5 °, 45 °, 67.5 °, 90 °, 112.5 °, Set to 135 ° and 157.5 °. Among these, at a certain time, the output of the X-axis and Y-axis of the seismic wave detection sensor 102 is αX₁, ΑY₁Is the projection angle θ₁= Acceleration in the direction of 22.5 ° is X in the figure₁cos 22.5 ° and Y₁This is the sum of accelerations determined by sin (22.5 ° -90 °). Since θ becomes a negative value of acceleration at each angle after θ is 180 °, projection calculation is not necessary.
[0067]
The seismic wave detection means 10 of the present embodiment performs liquefaction determination using multi-directional (three or more) acceleration waveforms obtained by such a projection calculation. Other configurations are the same as those of the first embodiment.
[0068]
Next, the operation of the ground liquefaction detection system according to the present embodiment will be described with reference to FIG. 7 and FIGS.
[0069]
First, in the seismic wave detection sensor 102, acceleration due to seismic motion in the biaxial direction is detected (step S200), and the above-described acceleration projection calculation is performed. Calculated (step S201). Hereinafter, in each of the eight axial directions, the periodic maximum acceleration A_pmax, Period P, ground displacement D and SI value are both threshold values (A₀, P₀, SI₀, D₀) Is determined whether or not it is larger.
[0070]
That is, in the liquefaction determination unit 103, first, the SI value calculated by the seismic wave detection sensor 102 is the threshold SI.₀It is determined whether it is larger (step S202), and the threshold value SI₀If it is larger (step S202; Y), the ground displacement D obtained by using the equation 1 in the liquefaction determination unit 103 is the threshold D.₀It is determined whether it is larger (step S203). The displacement D of the ground is the threshold D₀If larger (step S203; Y), the periodic maximum acceleration A output from the seismic wave detection sensor 102 is obtained._pmaxIs the threshold A₀The period P obtained by the liquefaction determination unit 102 is larger than the threshold value P.₀It is determined whether it is larger (step S204).
[0071]
Periodic maximum acceleration A_pmaxIs the threshold A₀Larger and the period P at that time is the threshold value P₀If larger (step S204; Y), the liquefaction determining unit 103 determines that a liquefaction phenomenon has occurred in the ground (step S205). Thereafter, as in the first embodiment, a liquefaction generation signal 1202 is output to the central monitoring room 300 via the determination circuit 202 and an alarm signal is output to the alarm notification unit 203 (step S206). .
[0072]
On the other hand, the obtained SI value is the threshold SI.₀If it is less than (step S202; N), the obtained ground displacement D is the threshold value D.₀When it is less than (step S203; N), the obtained periodic maximum acceleration A_pmaxAnd one or both of the period P and the threshold P₀, P₀) (Step S204; N), in any case, the liquefaction determining unit 103 determines that the liquefaction phenomenon has not occurred in the ground (step S207). Thereafter, every time the next acceleration waveform is input at a predetermined cycle, the operation returns to step S200, and the same determination is repeated thereafter.
[0073]
In the present embodiment, in addition to the acceleration waveforms in the two orthogonal directions in the horizontal plane, acceleration waveforms in other directions are also obtained, and the determination of liquefaction is performed based on these multidirectional acceleration waveforms. Depending on the mounting angle of the detection sensor, there is no erroneous determination that it is determined that there is no liquefaction despite the occurrence of liquefaction, and the determination accuracy is improved as compared with the first embodiment.
[0074]
Furthermore, in this embodiment, since acceleration data in other directions is obtained by calculation based on the measured data in the orthogonal two-axis directions by the projective projection method, a plurality of sensors are connected to each other with one seismic wave detection sensor. The effect equivalent to the case where it installs by changing direction (attachment angle) can be acquired. Of course, in the present invention, a plurality of seismic wave detection sensors may actually be installed at different mounting angles, and all of the seismic waveforms may be detected by actual measurement using the plurality of sensors.
[0075]
Next, the earthquake waveform recorded when an earthquake occurred in the past is analyzed, and the period maximum acceleration A_pmax, Zero cross period P, ground displacement D, and SI value are calculated, and the result applied to the determination method of the ground liquefaction detection system according to the second embodiment will be described.
[0076]
Table 1 shows the results of liquefaction judgments obtained by performing simulations using the method of the third embodiment and seismic experiments on actual seismic waveforms recorded at the time of earthquakes that occurred in the past. The liquefaction judgment results obtained by detecting and judging seismic waves are shown in comparison with the results of actual surveys at each observation point. As for the shaking test, it was not possible to test all the seismic waves due to the limited capacity of the shaking table. In the table, ◯ indicates that it was determined that liquefaction occurred, X indicates that liquefaction did not occur, and-indicates that the shaking test could not be performed.
[0077]
[Table 1]

[0078]
All the seismic waveforms recorded at the points where liquefaction actually occurred could be determined to have occurred. In addition, there is no false determination that liquefaction has occurred based on the seismic waveform recorded at the point where liquefaction did not occur, and liquefaction phenomenon that occurred in past earthquakes can be determined with an accuracy of almost 100%. We were able to. When the liquefaction detection system is actually driven, if the gas pipe is shut off due to an erroneous determination, a considerable amount of time is required for the subsequent recovery work.
[0079]
From the simulation results and the results of the seismic experiment performed based on the past seismic waveform, the periodic maximum acceleration A_pmaxFor earthquakes in which both the period P exceeds the threshold value, and the SI value and the ground displacement D both exceed the threshold value, it was found that almost 100% liquefaction occurs.
[0080]
Although the present invention has been described with reference to the embodiments, the present invention is not limited to these embodiments, and various modifications are possible. For example, in the second embodiment, acceleration data is obtained for each of the eight axes, but the number is not limited.
[0081]
In the above embodiment, all of the four parameters of the SI value obtained based on the acceleration waveform, the displacement of the ground, the period of the acceleration waveform, and the maximum acceleration per period (period maximum acceleration) have a predetermined threshold value. If it exceeds the limit, it is determined that the liquefaction phenomenon has occurred. However, the number of parameters is not limited to this, and if at least two of these parameters are used, it is determined more than the prior art. Accuracy is improved. Furthermore, in the above embodiment, the influence of the surface wave is eliminated by setting the maximum acceleration as the maximum acceleration for each period (that is, the maximum acceleration for the period). However, the maximum acceleration in the entire acceleration waveform is not determined for each period. A configuration may be adopted in which at least two parameters among the five parameters are used by adding acceleration (maximum acceleration).
[0082]
【The invention's effect】
As described above, according to the ground liquefaction detection method or the liquefaction detection device of the present invention, the acceleration waveform due to earthquake motion can be obtained.About multiple directions in the same horizontal planeAnd detecting at least two of the SI value, the maximum acceleration, the displacement of the ground, the period of the acceleration waveform, and the maximum acceleration for each period (period maximum acceleration) based on the acceleration waveform, Since the occurrence of the liquefaction phenomenon is detected by determining whether or not it is equal to or greater than the threshold value, it is possible to detect the liquefaction of the ground easily and quickly with high accuracy.
[0083]
In particular, in the determination, the period of the acceleration waveform is detected, and the maximum acceleration for each period (period maximum acceleration) is detected. Whether each period and the maximum acceleration are equal to or greater than a predetermined threshold value for each period. By determining whether or not, it is possible to eliminate the influence of surface waves that are not so great that the acceleration causes liquefaction even in a long period, and a more reliable determination result can be obtained.
[0084]
Further, the acceleration waveform of the seismic motion is obtained in each of the biaxial directions orthogonal to each other in the same horizontal plane and other directions different from these biaxial directions, and whether each parameter is equal to or greater than the threshold based on the plurality of acceleration waveforms. By determining, liquefaction determination with higher reliability can be performed without being affected by the installation direction of the sensor.
[0085]
Further, according to the liquefaction detection system of the present invention, since the liquefaction detection device of the present invention is provided at a plurality of points, the presence / absence of liquefaction can be accurately determined with a simple configuration, and a plurality of detection points can be determined. It is possible to easily monitor the occurrence of liquefaction and to take quick action. Furthermore, this liquefaction detection system can be configured in a system consisting only of a general seismometer capable of measuring the acceleration of the presence or absence of liquefaction, so a special device for directly observing the liquefaction phenomenon is required. Without cost.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a functional configuration of a liquefaction detection device according to a first embodiment of the present invention.
FIG. 2 is a schematic diagram illustrating a configuration of a liquefaction detection system including the liquefaction detection device illustrated in FIG.
FIG. 3 is a waveform diagram for explaining a period (zero cross period) of an acceleration waveform due to earthquake motion.
FIG. 4 is a waveform diagram for explaining the influence of surface waves on liquefaction determination.
FIG. 5 is a flowchart for explaining the operation of the liquefaction detection system shown in FIG. 2;
FIG. 6 is a vector diagram for explaining a projection calculation method.
FIG. 7 is a flowchart for explaining the operation of the liquefaction detection system according to the second embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... Seismic wave detection means, 11 ... Period detection means, 12 ... Period maximum acceleration detection means, 13 ... SI value detection means, 14 ... Ground change detection means, 15 ... Liquefaction determination means, 101 ... Liquefaction detection apparatus, 102 ... Seismic wave detection sensor, 103 ... Liquefaction determination unit, 203 ... Alarm notification unit (alarm unit), 300 ... Monitoring room (monitoring unit)

Claims (9)

An acceleration waveform due to earthquake motion is detected, and based on the acceleration waveform, at least two of the SI value, maximum acceleration, ground displacement, acceleration waveform period and maximum acceleration for each period are detected, and each is predetermined. A ground liquefaction detection method for detecting the occurrence of a liquefaction phenomenon by determining whether or not a threshold value of
The acceleration waveform is obtained in two axial directions orthogonal to each other in the same horizontal plane and directions different from the two axes, and the occurrence of a liquefaction phenomenon is detected based on the acceleration waveforms in the plurality of directions. The ground liquefaction detection method. The acceleration waveform due to earthquake motion is detected, the period of this acceleration waveform is detected, the maximum acceleration for each period is detected, and whether each period and the maximum acceleration are equal to or greater than a predetermined threshold value for each period. A ground liquefaction detection method for detecting the occurrence of a liquefaction phenomenon by judging ,
The acceleration waveform is obtained in two axial directions orthogonal to each other in the same horizontal plane and directions different from the two axes, and the occurrence of a liquefaction phenomenon is detected based on the acceleration waveforms in the plurality of directions. The ground liquefaction detection method. The acceleration waveform is detected in each of two biaxial directions orthogonal to each other in the same horizontal plane, and an acceleration waveform in a direction different from the biaxial direction is obtained by calculation based on the detected acceleration waveform. The ground liquefaction detection method according to claim 1 or 2, characterized by the above. A seismic wave detecting means for detecting an acceleration waveform due to seismic motion;
SI value detecting means for detecting SI value based on the acceleration waveform, maximum acceleration detecting means for detecting maximum acceleration, ground displacement detecting means for detecting ground displacement, period detecting means for detecting the period of the acceleration waveform, and At least two detection means of period maximum acceleration detection means for detecting the maximum acceleration for each period;
Liquefaction determining means for determining whether or not a liquefaction phenomenon has occurred by determining whether or not the detection value output of each of these detection means is equal to or greater than a predetermined threshold ,
The seismic wave detecting means obtains the acceleration waveform in each of a biaxial direction perpendicular to each other in the same horizontal plane and directions different from the two axes, and based on the acceleration waveforms in a plurality of directions detected by the seismic wave detecting means. In addition, a ground liquefaction detection device characterized by detecting the occurrence of a liquefaction phenomenon . A seismic wave detecting means for detecting an acceleration waveform due to seismic motion;
Period detecting means for detecting a period of the acceleration waveform based on the acceleration waveform;
A period maximum acceleration detecting means for detecting a maximum acceleration for each period;
Liquefaction determining means for determining whether or not a liquefaction phenomenon has occurred by determining whether or not the detection value output of each of the period detection means and the period maximum acceleration detection means is equal to or greater than a predetermined threshold ,
The seismic wave detecting means obtains the acceleration waveform in each of a biaxial direction perpendicular to each other in the same horizontal plane and directions different from the two axes, and based on the acceleration waveforms in a plurality of directions detected by the seismic wave detecting means. In addition, a ground liquefaction detection device characterized by detecting the occurrence of a liquefaction phenomenon . The seismic wave detecting means detects the acceleration waveform in each of two mutually perpendicular directions in the same horizontal plane, and calculates an acceleration waveform in a direction different from the two axial directions based on the detected acceleration waveform in the two axial directions. liquefaction detection device of the ground according to claim 4 or claim 5, wherein the those obtained by. The liquefaction detection device according to any one of claims 4, 5, and 6 installed at a plurality of points,
Communication means for transmitting detection results from the plurality of liquefaction detection devices;
A ground liquefaction detection system comprising: monitoring means for monitoring liquefaction occurrence status at the plurality of points based on the detection result. The ground liquefaction detection system according to claim 7, wherein the plurality of liquefaction detection devices are provided in a gas supply area in which a large number of gas pipes are embedded. Further, when the occurrence of a liquefaction phenomenon is confirmed in the monitoring means, a shutoff instruction is given to the gas pipe in consideration of the state of burying the gas pipe at the liquefaction detection point. 8 liquefaction detection system of the ground according.

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