Analysis of the Safety Factor for the Vertical Stability of a Building Pit

BENITO AGUS PUTRANTO, Teuku Faisal Fathani, S.T., M.T., Ph.D.

2014 | Skripsi | TEKNIK SIPIL

Belanda terbentuk pada area delta dimana 40 persen dari daratannya terletak di bawah permukaan laut. Kondisi geografis dan pengaruh muka air laut mengakibatkan tingginya muka air tanah di beberapa area di Belanda. Saat ini, Fugro terlibat dalam sebuah proyek di kota Urk, Belanda. Salah satu lingkup pekerjaannya adalah untuk menganalisa stabilitas galian terhadap tekanan air artesis. Hal ini menjadi isu dikarenakan lokasi pembangunan terletak di atas tanah lunak. Keruntuhan galian dapat terjadi jika akibat pengaruh tekanan air pada lapisan pasir yang terletak di bawah lapisan tanah lunak tersebut. Ir. De Jongh (2004) mengusulkan suatu formula yang menyertakan tahanan geser tanah sebagai faktor yang meningkatkan stabilitas vertikal suatu galian. Penelitian ini mencoba untuk menginvestigasi pengaruh tahanan geser tersebut. Penelitian ini dibagi menjadi beberapa bagian. Pertama, mengumpulkan data tanah di lokasi proyek (uji lapangan dan proyek kincir angin). Selanjutnya, melakukan analisis stabilitas sesuai dengan kondisi proyek uji lapangan menggunakan metode-metode acuan. Metode tersebut yaitu metode Terzaghi, metode NEN9997, dan metode muka air kritis. Setelah itu, penulis akan menggunakan software PLAXIS 2D untuk memodelkan kondisi galian tersebut. Pada pemodelan, penulis melakukan variasi terhadap parameter thanan geser dan juga lebar galian untuk menginvestigasi pengaruh dua parameter tersebut. Yang terakhir, penulis melakukan analisis terhadap stabilitas galian untuk proyek kincir angin dan memberikan masukan berdasarkan hasil analisis tersebut. Berdasarkan hasil analisis dan pemodelan, Terdapat beberapa kesimpulan dari penelitian ini. (1)Berdasarkan rumus-rumus acuan, faktor aman untuk proyek uji lapangan adalah lebih kecil dari 1. Dari hasil perhitungan, muka air kritis pada lapisan pasir adalah 3.08 m atau 0 mNAP. Akan tetapi, berdasarkan pengamatan di lapangan saat proyek tersebut berlangsung, tidak ada tanda-tanda keruntuhan ataupun deformasi besar yang terjadi pada muka galian tersebut. (2)Berdasrkan pemodelan dengan PLAXIS 2D, muka air kritis pada lapisan tanah adalah 3.7 m atau +0.62 mNAP. (3) Berdasarkan pemodelan, kuat geser tanah tidak memberikan pengaruh positif terhadap stabilitas galian. (4)Penelitian ini menunjukkan bahwa faktor yang mempengaruhi stabilitas galian adalah tekanan ekses pori akibat proses penggalian. Tekanan ekses pori jenis ini meningkatkan faktor aman sebesar 0.17. Untuk jangka waktu pendek (t = 0 hari) tekanan ekses pori ini memberikan pengaruh positif terhadap stabilitas galian. (5)Akibat pengaruh uplift pressure, permukaan galian akan berbentuk melengkung. (6) Galian yang lebih lebar akan menimbulkan deformasi pada muka galian yang lebih besar dibandingkan dengan galian yang lebih sempit.

The Netherlands is a typical delta area with 40 percent of its area located below the sea level. This kind of situation resulted in a high groundwater level in some parts of the Netherlands, in the accordance with the sea water level. Recently, Fugro had been involved in a wind turbines project in Urk, the Netherlands. One of the work scopes was to check the stability of the building pit against the uplift pressure. This became an issue since the construction is located on soft soil. A high uplift pressure in the sand layer below the soft layer could cause a surface rupture. However, based on the Fugro's experienced in the previous project in Culemborg, there was a possibility that the reference method which used to calculate the vertical stability against the uplift pressure underestimates the soil strength. De Jongh (2004) proposed a formula which put the undrained shear strength into account. This research is trying to investigate the contribution of this undrained shear strength to the vertical stability. In order to obtain the objective, this research was divided into several steps. The first step was collecting the soil data from the projects. The next step was doing some vertical stability calculations with three reference methods which are commonly used to calculate the vertical stability and compare it with the field test results. Those methods were the Terzaghi method, NEN 9997 method, and the critical hydraulic head method. After that, PLAXIS 2D software was used to model the building pit. For the modeling, some variations of the undrained shear strength and the building pit's width were also performed to investigate the effect of these two parameters. Lastly, some calculations are performed for the wind turbines project also with recommendation based on the results. Based on the analysis and the modeling, this research gave some conclusions. (1) Based on the reference methods, the safety factor for the field test was always less than 1. Even after adding the slope contribution, safety factor of 1 still could not be reached. From the analysis, the maximum hydraulic head of the deeper sand layer was 3.08 m or 0 mNAP. However, from the monitoring during the field test, no surface rupture or large deformation occurred in the building pit. (2)From the modeling with PLAXIS 2D, the maximum hydraulic head of the deeper sand layer was 3.7 m or +0.62 mNAP.(3) Based on the modeling from the soil data, the undrained shear strength gave no positive contribution for the stability. Thus, this research could not find the contribution of the undrained shear strength. (4)From this research, a hidden safety came from the negative excess pore pressure due to an excavation. The negative excess pore pressure gave an extra safety of 0.17. In a short-term duration (t = 0 day) this negative excess pore pressure was contributing positively to the building pit's safety. (5) Due to an uplift pressure, the soft soil column would have an arc shape.(6 ) A wider building pit generated a higher deformation compared to a narrow one.

Kata Kunci : building pit, uplift pressure, undrained shear strength, negative excess pore pressure.