Calibration Development Based on Dielectric Mixture Model for Measurement of Subgrade Water Content with TDR Method

Year 2024, Issue: 19, 67 - 82, 31.01.2024

Mehmet Özgür

https://doi.org/10.47072/demiryolu.1366737

Abstract

One of the most frequently used parameters to evaluate track performance in railways is the track modulus. Although it is affected by substructure and superstructure elements, the track modulus mainly depends on the subgrade resilient modulus. The resilient modulus is a function of water content in all soils. Consequently, the track modulus may show seasonal variations depending on the rainfall regime. Periodic measurements of the subgrade water content should be made in order to consider this situation when evaluating track performance. Oven drying is one of the primary methods used to measure soil water content. In addition to the difficulty of taking samples from the subgrade, oven drying can be considered as time consuming since the procedure is based on 24-hour drying. Alternatively, Time Domain Reflectometry (TDR) is an electromagnetic measurement method that enables real-time and remote measurement. Water content is determined with the help of the calibration equation established with the dielectric permittivity of the soil measured by TDR. In this study, TDR measurements made on 7 different soil classes, which can be considered as subgrade material and whose fine content is maximum 15%, were obtained from the literature. For the calibration equation, unlike other studies, dielectric mixture model was used instead of regression methods. The proposed calibration was evaluated with the performance metrics selected in the test set. Again, unlike other studies, the calibration, which is specific to a soil group rather than a soil class, provided 93% prediction success within a wide water content range, provided that it remains within the 2.0% error band. Additionally, it was observed that the prediction error distribution was adequately narrow and centered close to zero. In this context, TDR, whose use is extremely limited in Türkiye is recommended as a rapid method for periodic measurements of subgrade water content on railway tracks that provides high accuracy.

Keywords

Track modulus, Subgrade, Water content, TDR, Calibration

Demiryolu Taban Zemini Su İçeriğinin TDR Yöntemi ile Ölçümü için Dielektrik Karışım Modeli Yardımıyla Kalibrasyon Geliştirilmesi

Abstract

Demiryollarında hat performansını değerlendirmek için en sık başvurulan parametrelerden birisi hat modülüdür. Altyapı ve üstyapı elemanlarından etkilenmekle birlikte hat modülü büyük oranda taban zemini esneklik modülüne bağlıdır. Esneklik modülü ise tüm zeminlerde su içeriğinin bir fonksiyonudur. Bu sebeple hat modülü yağış rejimine bağlı olarak mevsimsel farklılıklar gösterebilir. Bu durumun hat performansı değerlendirilirken göz önünde bulundurulabilmesi için taban zemini su içeriğinin periyodik ölçümleri yapılmalıdır. Zemin su içeriğinin ölçülmesi için kullanılan yöntemlerin başında etüvde kurutma gelmektedir. Taban zemininden örnek almanın zor olmasının yanı sıra deney prosedürünün 24 saat kurutmaya dayalı olması nedeniyle etüvde kurutma yavaş bir yöntem olarak değerlendirilebilir. Alternatif olarak Zaman Tanım Alanında Yansıma Yöntemi (Time Domain Reflectometry, TDR), gerçek zamanlı ve uzaktan ölçüm alabilmeyi sağlayan bir elektromanyetik ölçüm yöntemidir. TDR ile ölçülen zeminin dielektrik iletkenliği ile kurulan kalibrasyon denklemi yardımıyla su içeriği belirlenir. Bu çalışmada taban zemini malzemesi olabilecek ve ince dane içeriği en fazla %15 olan 7 farklı zemin sınıfında yapılmış olan TDR ölçümleri literatürden elde edilmiştir. Kalibrasyon denklemi için diğer çalışmalardan farklı olarak regresyon yöntemleri yerine dielektrik karışım modeli kullanılmıştır. Önerilen kalibrasyon, sınama setinde seçilen performans göstergeleri ile değerlendirilmiştir. Yine diğer çalışmalardan farklı olarak bir zemin sınıfı yerine zemin grubuna özgü olan kalibrasyon, geniş bir su içeriği aralığında %2,0 hata bandında kalmak koşuluyla %93 tahmin başarısı sağlamıştır. Ayrıca tahmin hatası dağılımının yeterince dar ve sıfıra yakın merkezlenmiş olduğu görülmüştür. Bu bağlamda demiryolu hatlarında taban zemini su içeriğinin periyodik ölçümleri için Türkiye’de kullanımı son derece sınırlı olan TDR, yüksek doğruluk sağlayan hızlı bir ölçüm yöntemi olarak önerilmektedir.

Keywords

Hat modülü, Taban zemini, Su içeriği, TDR, Kalibrasyon

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