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Pore water pressure is a factor which contributes to foundation stability. The change in pore water pressure in soil mass may affect the bearing capacity of the foundation. Especially for sandy soil, the increased pore pressure may cause the foundation collapse. This study investigates the stability of soil-foundation system due to the increase of pore pressure using a scaled model in the laboratory. The sands were taken from Siteba Padang and sieved pass through sieve no. 40. The calculated relative density of sample in the box model is 51.76%, which indicated the medium sand density. The model was made up to obtain saturated condition. The increased pore water pressure was performed using upwards seepage method in several steps, which are monitored through piezometer pipes. The tests results show that at the last step, the sand boil happened on the soil surface that result the sink of foundation model. It also indicates the loss of bearing capacity due to the loss of effective stress in the soil mass. The Mohr-graphs is also performed to figures the stress in the soil mass. The graph also can show the collapse points in the soil mass during the test.

Likuifaksi (liquefaction) merupakan salah satu bahaya yang ditimbulkan dari gempa bumi. Pada saat gempa terjadi, tanah mengalami perubahan sifat dari solid ke liquid akibat beban siklik yang diterima. Untuk itu, diperlukan suatu metode yang mudah dan sederhana untuk mengevaluasi potensi perilaku likuifaksi pada tanah, yang dapat diterapkan di kota Padang dan Sumatera Barat secara umumnya. Salah satunya adalah metode analisa potensi likuifaksi menggunakan data pengujian sondir di lapangan. Metode yang dititik beratkan kepada korelasi data perlawanan konus terhadap cyclic stress ratio dalam suatu grafik sederhana (Seed dan Idriss) yang menunjukkan batasan tanah mengalami likuifaksi atau tidak. Grafik dapat menunjukkan variasi nilai kritis uji sondir terhadap potensi likuifaksi telah dikembangkan untuk beberapa variasi terahadap magnitude gempa dan percepatan maksimum tanah serta pengaruh level muka air tanah. Dari hasil penelitian yang dilakukan, menunjukkan daerah penelitian yaitu GOR Haji Agus Salim dan Lapai, Padang, memiliki nilai faktor keamanan yang sangat kecil (< 0,5), dibandingkan dengan faktor keamanan yang telah ditetapkan, yaitu 1,50. Hal ini berati bahwa daerah penelitian memiliki potensi besar terhadap likuifaksi. Keywords : gempa bumi, likuifaksi, normalisasi perlawanan kerucut, in-situ test

The earthquake and tsunami predictions in the city of Padang have caused very rapid land-use changes, especially in the Kuranji watershed, where people tend to seek locations that are safe from tsunamis and liquefaction. Changes in environmental characteristics such as slope geometry conditions, vegetation density, and changes in land use will affect runoff coefficient and rainwater filtration, triggering a potential for landslides. This study aims to analyze the potential for landslides due to changes in land use in the Kuranji Watershed. The identification of land-use change is carried out using a remote sensing approach, namely the Normalized Difference Built-Up Index (NDBI). Landslide potential is determined based on the relationship between land use and runoff coefficient from 2007 to 2019. The results showed there had been an increase in the built-up area in the Kuranji watershed from 1602.212 ha (2007) to 2897.513 ha (2019). In contrast, the vegetation area has decreased. An increase in the runoff coefficient of 3.9% from 2007 to 2019. The final results of this study are thematic geospatial information obtained in the form of the relationship between changes in land use and the potential for landslides that occurred in the Kuranji watershed during the period 2007 to 2019.