LIQUEFACTION ANALYSIS DURING THE EXCAVATION OF BUILDING PIT IN AQUEDUCT PROJECT AT BURGUM, FRIESLAND, THE NETHERLANDS

ROSE TIRTALISTYANI, Teuku Faisal Fathani, ST., MT., Ph.D.

2014 | Skripsi | TEKNIK SIPIL

Baru-baru ini, pemerintah Belanda telah berinisiasi untuk membangun jembatan air atau sering disebut aqueduct yang bertempatkan di Burgum, Belanda. Jembatan air ini diharapkan dapat selesai pada tahun 2016 mendatang. Proyek ini sekarang berada dalam tahap desain dimana penggalian tanah sebagai dasar proyek adalah isu yang sangat penting untuk kesuksesan seluruh tahap konstruksi. Dari semua data yang telah dikumpulkan, kemungkinan likuifaksi sangatlah tinggi karena keberadaan tanah berpasir dalam kondisi jenuh air. Hal tersebut pun dapat berpotensi menyebabkan ketidakstabilan lereng selama ekskavasi di area proyek. Namun, adanya potensi likuifaksi masih merupakan estimasi, sehingga penelitian lebih lanjut harus dilakukan untuk mengetahui secara pasti kemungkin likuifaksi di area proyek. Oleh karena itu, penelitian ini bertujuan untuk mengetahui kemungkinan likuifaksi dan ketidakstabilan lereng dalam proyek jembatan air di Burgum. Analisis likuifaksi dilakukan dengan dua jenis yaitu likuifaksi berdasarkan perilaku tanah di lapisan kritis dan analisis likuifaksi yang diakibatkan oleh gempa minor. Estimasi likuifaksi berdasarkan lapisan kritis didasarkan pada perilaku tanah yang dianalisis dengan pendekatan parameter (state parameter) dari berbagai metode, seperti Robertson Screening Method, Shuttle dan Jefferies Universal Framework, Ghafghazi Comprehensive Interpretation. Sedangkan gempa minor yang akan menyebabkan likuifaksi dianalisis dengan Simplified Procedure dari Idriss dan Boulanger (2008). Data yang digunakan dalam penelitian ini adalah satu Cone Penetration Test (CPT) dan satu data tambahan dengan Seismic Cone Penetration Test (SCPT) dari daerah yang dianggap paling kritis terhadap likuifaksi di lokasi proyek. Setelah analisis likuifaksi dilakukan berdasarkan berbagai metode, stabilitas lereng dilakukan dengan perangkat lunak GeoSlope. Hasil dari analisis likuifaksi menyimpulkan bahwa: (1)pendekatan parameter menunjukkan bahwa kemungkinan likuifaksi bervariasi untuk setiap metode. Robertson Screening Method menunjukkan bahwa seluruh lapisan tanah tidak akan terlikuifaksi dengan faktor aman stabilitas lereng 2.253. Metode pendekatan parameter lainnya, seperti Shuttle dan Jefferies Universal Framework dan Ghafghazi Comprehensive Interpretation menunjukkan bahwa lapisan pasir bagian bawah akan terlikuifaksi dan mengakibatkan lereng tidak stabil, ditunjukan dengan faktor keamanan 0.219. (2) Sedangkan gempa minor yang telah dianalisis dengan Idriss dan Boulanger (2008), menunjukan bahwa lereng tidak stabil karena lapisan tanah bagian bawah terlikuifaksi jika magnitudo dari gempa melebihi 5. Hal ini menyebabkan lereng menjadi tidak stabil dengan faktor aman 0.219. Namun, analisis likuifaksi tidak perlu dilakukan bila magnitudo gempa minor kurang dari 4.5, karena akselari tanah tidak melebihi 0.1g.

Recently, the Dutch Government has initiated to build a new aqueduct at Burgum, the Netherlands that is expected to finish in 2016. Nowadays, this project resides in the design phase where the excavation of open building pit as the underground working platform is a very important issue for the succeedence of whole construction phase. From the preliminary data, the estimation of liquefaction came up because the existence of non-cohesive very fine sandy soil in saturated condition. The liquefaction occurrence will also lead to slope instability at the building pit. However, the liquefaction issue was mainly a rough estimation that needed to be analyzed to get more insight. Therefore, this research was aimed to give the overview of liquefaction potential and slope stability in aqueduct project at Burgum. The liquefaction analyses were done with two types of analyses based on soil behavior in critical layer and minor earthquake. Robertson Screening Method, Shuttle and Jefferies Universal Framework, and Ghafghazi Comprehensive Interpretation were used to get the soil behavior assements in the critical soil layer, whether it was dilative or contractive to judge the liquefaction potential. Liquefaction cause by minor earthquake was analyzed with Simplified Procedure from Idriss and Boulanger (2008) using various magnitude input to know the liquefaction safety factor. To get the desired results from selected methods, this research used the data from one available cone penetration test (CPT) and one additional data by means of seismic cone penetration test (SCPT) from the most critical area in project location. After the liquefaction analyses were done with various methods, lastly slope stability was conducted with the GeoSlope software to know the slope safety factor. The outcome from the liquefaction analyses concluded that: (1)state parameter approach showed that the liquefaction possibility varies for each method. Robertson Screening Method showed that the entire soil layer will not liquefied. As a result, the building pit remained stable with safety factor of 2.253. The others state parameter methods, like Shuttle and Jefferies Universal Framework and Ghafghazi Comprehensive Interpretation gave the impression that the lower sandy layer will be liquefied. Accordingly, the building pit safety factor lessened into 0.219. (2)Whereas liquefaction caused by minor earthquake that had been analyzed with Simplified Procedure from Idriss and Boulanger (2008) was using various magnitude model as an input. The result indicated that the building pit was not resistant to liquefaction since the lower sandy layer was liquefied if the minor earthquake magnitude was higher than 5. Thus, it was resulted in the failure of the building pit with safety factor 0.219. However, the liquefaction analysis was not necessary to be conducted if the magnitude lower than 4.5 as the peak ground acceleration lower than 0.1g.

Kata Kunci : Keyword: liquefaction, state parameter, simplified procedure, minor earthquake, slope stability