Bendungan termasuk kedalam heavy construction sehingga bendungan menyimpan potensi bahaya yang besar dan memiliki resiko kerusakan fisik serta kegagalan fungsi. Salah satu kerusakan fisik serta kegagalan fungsi pada bendungan adalah erosi akibat mengalirnya air melalui lubang-lubang pada pondasi (piping). Penelitian ini bertujuan untuk mengetahui debit air bendungan yang merembes melewati as bendungan pada keadaan muka air normal dan banjir, serta mengetahui nilai faktor keamanan bendungan dari bahaya piping. Metode yang digunakan dalam penelitian ini adalah metode observasi lapangan meliputi pemetaan geologi, pengukuran geolistrik dan pemboran inti, serta perhitungan Faktor Keamanan (FK). Sampel inti batuan diuji di laboratorium untuk mengetahui sifat fisik dan mekanika tanah terutama nilai specific gravity dan void ratio. Perhitungan FK menggunakan simulasi dengan input muka air normal dan muka air banjir. Berdasarkan hasil dari pemetaan geologi, permukaan lokasi penelitian disusun oleh batupasir tuffan, tuff, dan breksi. Pengukuran geolistrik dan pemboran inti menunjukkan bahwa bagian bawah permukaan dari lokasi penelitian tersusun oleh tuff yang mendominasi bagian atas dan batupasir tuffan mendominasi bagian bawah. Debit rembesan sebelum grouting pada keadaan muka air normal adalah sebesar 14,33 m3/hari dan berubah menjadi 9,49 m3/hari, debit rembesan sebelum grouting pada kondisi muka air banjir sebesar 15,32 m3/hari dan berubah menjadi 10,17 m3/hari. Nilai faktor keamanan piping Bendungan Logung pada saat sebelum di grouting adalah 1,38 dan naik menjadi 4,77 setelah di grouting.
Movement of soil and rock masses down the slope or known as landslide is a natural activity that can pose a danger to humans. Generally, landslide can be influenced by the slope, slope materials (rock/soil), and rainfall. Morphologically, the study area, which is located in the borders of Tembalang and Banyumanik Districts, Semarang City, is a hill that is quite steep. It has a potential to experience landslides. Landslide occurences at the study area do not always occur on steep slopes. Therefore, the aim of this study is to determine the relationship between the slope materials and the occurrences of landslide. Data was obtained through mapping to determine geological conditions and enlisting landslide occurrences at the study area. Furthermore, the correlation between the number of occurrences and the type of slope materials at the location of the ground motion was carried out using information value method. Based on geological mapping, 5 types of lithology were obtained, namely: alluvium, breccia, sandstone, siltstone, and claystone. Out of 10 landslides that were found, 6 of them occurred in claystone lithology. We interpret that landslide occurrences in study area are relating to the lithology claystone with information value 0.416. The claystone, a part of the Kerek Formation, physically were easily degraded. In addition, this lithology mostly found in open slopes, where the process of erosion by water is more intense.
Java Island has volcanic arcs at the south and at the middle which are spread in a east-west pattern called Southern Mountains Zone and Quaternary Mountains Zone. The east-west pattern resembles the structural pattern produced by the Java tectonic subduction. Based on this, research was carried out to determine tectonic relationships and structural development in the Southern Mountains Zone and the Quaternary Mountains Zone. The study was conducted by structural mapping of each zone, namely the Grendulu Fault in Pacitan Regency which belongs to Southern Mountain Zone and the Cemorosewu Fault in Karanganyar Regency which belongs to Quaternary Mountains Zone. The mapping shows that the Grendulu Fault is a horizontal fault with north-south main stress, while the Cemorosewu Fault is a normal fault with nearly vertical main stress. Based on these, it can be concluded that there is no direct, but indirect tectonic relationship that works between the two: both structures developed due to Java Subduction. The structural development of the Grendulu Fault is strongly influenced by Java Subduction, which the subduction gives north-south main stress that forms this fault. While Southern Mountains formed, Kendeng Basin was formed due to loading from the mass of Southern Mountains. The formation of Kendeng Basin was continued with the formation of the Mount Lawu Complex (part of Quarternary Mountains) where the Cemorosewu Fault developed. This fault is formed as a result of mass loading of the Mount Lawu itself and triggered by the slope from Kendeng Basin to the north.
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