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Fasies sedimen Kuarter berpotensi likuifaksi Pesisir Kota Padang, Provinsi Sumatra Barat berdasarkan data inti bor dan CPTu Prahara Iqbal; Adrin Tohari; Imam A. Sadisun; Dwiharso Nugroho
Jurnal Lingkungan dan Bencana Geologi Vol 5, No 1 (2014)
Publisher : Badan Geologi

ABSTRAKKajian fasies dan sifat fisik sedimen Kuarter kaitannya dengan potensi likuifaksi di daerah Pesisir Kota Padang dan sekitarnya dilakukan dengan tujuan mendapatkan hubungan antara fasies dan karakteristik fisik sedimen Kuarter terhadap potensi likuifaksinya. Metode yang digunakan adalah observasi inti bor dan pola kurva CPTu untuk menentukan fasies yang berkembang, analisis laboratorium, dan analisis potensi likuifaksi untuk mengetahui karakteristik fisik setiap fasies. Berdasarkan asosiasi litologi, pengamatan pola kurva CPTu, analisis laboratorium, dan analisis potensi likuifaksi, terdapat tiga fasies yang berkembang di daerah penelitian dengan potensi likuifaksi yang berbeda. Fasies pasir fluvial memiliki potensi likuifaksi yang lebih tinggi daripada fasies pasir pematang pantai dan fasies lempung rawa, sedangkan fasies lempung rawa memiliki potensi likuifaksi yang paling rendah di antara fasies-fasies tersebut.Kata Kunci: Fasies sedimen Kuarter, CPTu, Likuifaksi, Kota PadangABSTRACTThe study of Facies and physical property of Quaternary sediments and its relationship with liquefaction potential was conducted at the coastal area of Padang city, West Sumatra Province in order to get the relationship between facies and physical characteristics of the Quaternary sediments and its liquefaction potential. The method used are core and CPTu patterns observations to determine the developed facies, laboratory analysis, and liquefaction potential analysis to determine the physical characteristics of each facies. Based on lithological associations, CPTu pattern observations, laboratory analysis, and liquefaction potential analysis, there are three facies that develop at the research areas with different liquefaction potential. Fluvial sand facies has greater liquefaction potential than the beach ridge sand facies and marsh clay facies, while marsh clay facies has the smallest one among two facies.Keyword: Facies and Quarternary sediments Kuarter, CPTu, liquefaction, city of Padang

Desain cut slope chart untuk evaluasi kestabilan lereng di atas badan jalan. Studi Kasus: Cinona, Cisalak dan Cijengkol, Kabupaten Bandung Barat , Jawa Barat Arifan Jaya Syahbana; Adrin Tohari; Eko Soebowo; Dwi Sarah; Khori Sugianti
Jurnal Lingkungan dan Bencana Geologi Vol 4, No 1 (2013)
Publisher : Badan Geologi

ABSTRAKTopografi daerah Jawa Barat didominasi oleh perbukitan yang menyebabkan sering terjadinya peristiwa tanah longsor pada lereng potongan di atas badan jalan, terutama pada musim hujan. Banyaknya frekuensi bencana longsor di atas badan jalan telah menimbulkan kerugian fisik akibat kerusakan jalan dan kerugian ekonomi akibat terputusnya jalur transportasi. Untuk mengurangi kejadian bencana longsor di atas badan jalan, diperlukan pengetahuan tentang desain potongan lereng yang stabil berdasarkan kondisi sifat fisik, hidrologis dan keteknikan tanah setempat yang dapat dicapai dengan penyusunan desain chart lereng potongan (cut slope chart) yang tepat. Tulisan ini bertujuan untuk menghasilkan chart kestabilan lereng kupasan jalan dengan studi kasus di daerah Cinona, Cisalak, dan Cijengkol, Kabupaten Bandung Barat, Jawa Barat. Metode penelitian yang dilakukan adalah investigasi geoteknik terdiri dari uji CPT dan bor tangan pada lereng-lereng tanah residual vulkanik, pengujian laboratorium sifat fisik, hidrologis, kuat geser tanah residual pemodelan numerik infiltrasi air hujan dan kestabilan lereng. Hasil penelitian menunjukkan chart kestabilan lereng kupasan pada badan jalan di lokasi penelitian mempunyai kecenderungan yang sama dimana angka aman menurun seiring dengan bertambahnya kejenuhan tanah lereng dan kemiringan lereng. Kemiringan lereng yang baik adalah tidak melebihi 45°. Hasil studi ini diharapkan dapat berkontribusi terhadap desain lereng kupasan badan jalan yang tepat untuk dapat mengurangi berbagai kerugian ekonomis dan korban jiwa akibat longsor.Kata kunci: badan jalan, cut slope chart, kestabilan lereng, tanah longsorABSTRACTThe topography of West Java is dominated by hills where many slope failures frequently take place on the road cut slopes particularly during the rainy season. The frequent road cut slope failures had caused physical and economical losses due to the disruption of transportation. In order to mitigate the events of slope failures above the roadway, it is important to understand the stable slope cut design based on the in situ physical, hydrological and engineering properties which can be achieved by composing the design cut slope chart. This paper aims to construct a cut slope chart for case study of Cinona, Cisalak, dan Cijengkol, West Bandung Regency, West Java. The methods employed in this study were geotechnical investigation consisted of Cone Penetration Test (CPT) and hand boring carried out in the volcanic residual soil slopes, geotechnical laboratory analysis to obtain the physical, hydrological and shear strength properties of the residual soil and numerical modelling of coupling rain water infiltration and slope stability analyses. The results show that the cut slope charts for the road cut slopes in the study area show similar tendency of decreasing factor of safety along with the increasing slope saturation and inclination. The favorable slope inclination is not exceeding 45°. This study shall contribute to the appropriate design of road cut slope to minimalize economic loss and casualties due to landslides/slope failures.Keywords: road, cut slope chart, slope stability, landslide/slope failure

Studi Air Tanah di Pantai Bosnik, Distrik Biak Timur, Pulau Biak, Provinsi Papua Hendra Bakti; Dadan Dani Wardana; Wilda Naily; Adrin Tohari; Arief Rachmat
Jurnal Lingkungan dan Bencana Geologi Vol 7, No 3 (2016)
Publisher : Badan Geologi

ABSTRAKPenelitian telah dilaksanakan di Pantai Bosnik, Distrik Biak Timur, Kabupaten Biak Numfort, Pulau Biak, Provinsi Papua. Penelitian dilakukan untuk mengetahui karakter air tanah wilayah pesisir sebagai informasi dasar bagi pengelolaan sumber daya air. Metode penelitian yang dipakai terdiri atas survei hidrogeologi permukaan, hidrokimia, dan survei geolistrik. Batuan di daerah penelitian didominasi batu gamping koral (Formasi Mokmer) dan sedikit endapan aluvium pantai. Keduanya bertindak sebagai akuifer yang dapat menyimpan dan meloloskan air dalam jumlah yang berarti. Air tanah yang terdapat pada batu gamping koral terkonsentrasi dalam porositas sekunder yang saling berhubungan. Tipe air tanah didominasi oleh tipe klorida (Na-Cl dan Mg-Cl). Sementara air tanah pada aluvium pantai dicirikan dengan tipe bikarbonat (Ca-HCO3). Mata airnya memiliki tipe karbonat (Ca-HCO3, Mg-HCO3) dan tipe Na-Cl. Batu gamping koral mempunyai tahanan jenis yang bervariasi dari 3 ohm-m – 5000 ohm-m. Tahanan jenis rendah berasosiasi dengan rongga hasil pelarutan yang terisi air payau atau pun air asin, sedangkan tahanan jenis kontras tinggi merupakan batu gamping kompak dan kering.Kata kunci: akuifer, batu gamping, porositas sekunder, air tanah, Pulau BiakABSTRACTA research has been conducted in Bosnik Coast, East Biak District, Biak Numfort, Biak Island, Papua Province. The study was conducted to determine the character of the groundwater in coastal areas as basic information for the management of water resources. The research method consists of the surface hydrogeologic survey, hydrochemical, and the geoelectric survey. The rocks in the studied area are dominated by coral limestone (Mokmer Formation) and a little alluvium coastal sediment. Both act as an aquifer that can store and release significant quantities of water. Groundwater is contained in coral limestone, concentrated in interconnected secondary porosity. The type of water dominated by the chloride type (Na-Cl and Mg-Cl). While groundwater in coastal alluvium is characterized by the bicarbonate type (Ca-HCO3). The springs have carbonate type (Ca-HCO3, Mg-HCO3) and Na-Cl type. The result of geolelectrical measurement indicated that the coral limestone had resistivity which varied from 3 ohm-m - 5000 ohm-m. The low resistivity associated with the voids or cavities filled with brackish water or salt water, while the high resistivity contrast is a compact and dry limestone.Keywords : aquifer, limestone, secondary porosity, groundwater, Biak Island

VARIATIONS OF PORE-WATER PRESSURE RESPONSES IN A VOLCANIC SOIL SLOPE TO RAINFALL INFILTRATION Adrin Tohari
JURNAL RISET GEOLOGI DAN PERTAMBANGAN Vol 23, No 2 (2013)
Publisher : Indonesian Institute of Sciences

ABSTRACT A field monitoring of hydrological response and slope movement in an active landslide area, comprising of volcanic soil, was conducted to have a better understanding of the mechanism of landslide reactivation during a rainfall period. Monitoring instruments consisted of jet-fill tensiometers, inclinometers, open stand-pipe piezometers, and a tipping-bucket rain-gauge. The records of pore-water pressure show that the hydrological responses exhibited some spatial variability, and were mainly influenced by antecedent soil moisture conditions and rainfall patterns. The ingress of wetting front was confined up to the depths of 3 m, and a transient positive pore pressure could develop at a depth of 1 m during a heavy rainfall. More sustained increase in pore-water pressure could develop at deeper soil. Meanwhile, the piezometer records show that the response of groundwater table occur more significantly in the middle and lower portions of the slope, associated with the subsurface topographical features. The analysis of antecedent moisture conditions based on the pore-water pressure measurement data indicates that simplified model of slope hydrology cannot fully explain the spatial and temporal development of the pore-water pressures observed in the soil slope

LIQUEFACTION POTENTIAL AT PADANG CITY: A COMPARISON OF PREDICTED AND OBSERVED LIQUEFACTIONS DURING THE 2009 PADANG EARTHQUAKE Adrin Tohari; Khori Sugianti; Eko Soebowo
JURNAL RISET GEOLOGI DAN PERTAMBANGAN Vol 21, No 1 (2011)
Publisher : Indonesian Institute of Sciences

ABSTRACT The September 30, 2009 Padang earthquake has resulted in damages to infrastructures and buildings and a death toll of 383 in Padang City. Numerous liquefaction and ground deformations caused by the earthquake were particularly evidence in the areas few kilometers from the coast. The paper presents results of the previous field geotechnical investigations of liquefaction potential and the recent liquefaction observations in Padang City. A microzonation map was created using the data from liquefaction potential analyses and liquefaction potential indexes. The predicted liquefaction susceptibility zones showed a good agreement with site observations. The assessment suggests that the liquefaction susceptibility decreases to the northeast away from the coastal line.

IN SITU MEASUREMENT OF THE SOIL MOISTURE CONTENT PROFILE AT A DECOMPOSED GRANITE CUT-SLOPE Adrin Tohari; Makoto Nishigaki
JURNAL RISET GEOLOGI DAN PERTAMBANGAN Vol 20, No 1 (2010)
Publisher : Indonesian Institute of Sciences

ABSTRACT Two selected sites at a decomposed granite cut-slope were instrumented with a rain gauge and a series of ThetaProbes installed at depths up to 90 cm from the slope surface. The objective of this study was to evaluate the significance of antecedent soil moisture conditions in controlling the hydrologic responses of the soils at each site. The antecedent moisture content of the soil in Site #1, in general, increases with depth. The response of the soil to saturation process is more or less confined to the shallow depth. On the other hand, the measurement data of the soil in Site #2 showed that the near surface soil exhibits higher antecedent moisture condition than the deeper soil. The ingress of the wetting front could reach more than 90 cm depth during long period of rainfall. Thus, these results suggest that the hydrological response of the soils is spatially variable and is influenced by the antecedent soil moisture conditions within the soil profile. The presence of soil layers, the distribution of macro cracks within the soil profile, and additional lateral source of moisture may also have significant influences on soil response to saturation process.

PENGARUH ZONA JENUH AIR TERHADAP KESTABILAN LERENG DI WENINGGALIH, KABUPATEN BANDUNG BARAT Moch Hilmi Zaenal Putra; Rendi Dwi Kartiko; Prihadi Soemantidiredja; Imam Achmad Sadisun; Adrin Tohari
JURNAL RISET GEOLOGI DAN PERTAMBANGAN Vol 30, No 1 (2020)
Publisher : Indonesian Institute of Sciences

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.14203/risetgeotam2020.v30.1086

Kejadian longsor dangkal di Desa Weninggalih mengakibatkan kerugian setiap tahunnya. Hal tersebut diakibatkan oleh salah satu budaya warga setempat yaitu membuat kolam ikan tanpa dilapisi lapisan kedap air sehingga diduga menjadi salah satu faktor penyebab menurunnya kestabilan lereng. Sebagai upaya untuk mengurangi risiko kerugian tersebut dibutuhkan pemahaman yang baik mengenai karakteristik longsor dan pengaruh keberadaan kolam ikan terhadap kondisi kestabilan lereng. Penelitian ini bertujuan untuk mengetahui tingkat kestabilan lereng dan pengaruh keberadaan kolam ikan untuk studi kasus di daerah Weninggalih. Metode yang digunakan dalam penelitian ini meliputi pemetaan geologi, pemetaan topografi detail, pengukuran electrical resistivity tomography (ERT), pengambilan sampel tanah, pengujian geoteknik, dan analisis kestabilan lereng. Metode kesetimbangan batas dengan perhitungan Morgenstern-Price digunakan untuk analisis kestabilan lereng. Daerah studi terdiri dari produk vulkanik berupa tanah residual dan tuf tebal yang bersifat lepas dan kuat geser dalam yang rendah. Hasil analisis ERT menunjukkan adanya perluasan zona jenuh air yang diakibatkan oleh keberadaan kolam ikan. Analisis kestabilan lereng menunjukkan bahwa kemunculan zona jenuh air yang meluas menyebabkan menurunnya nilai faktor keamanan lereng secara signifikan. ABSTRACT – The effect of water saturated zone on slope stability in Weninggalih, Bandung BaratRegency. Shallow landslides inWeninggalih Village resulted in many losses every year. One of the contributing factors is the customs of building a fishpond in the sloping ground without an impermeable liner. Consequently, the seepages from the pond cause the decrease in slope stability. To reduce the risk of loss, a better understanding of the landslide’s characteristics and the effect of the fishpond on the stability of the slope is required. The purpose of this study is to determine the level of slope stability and the influence of the fishponds to slope stability in the Weninggalih area. Methods used in this study include geological mapping, detailed topographic mapping, electrical resistivity tomography (ERT), soil sampling, geotechnical laboratory testing, and slope stability analysis. The limit equilibrium method with the Morgenstern- Price formula was used for slope stability analysis. The study area is composed of volcanic products consisting residual soil and thick tuff that are loose and having low shear strength. Results of ERT analysis showed the extension of the water saturated zone caused by the presence of fishponds. Analysis of the slope stability shows that the expanding water saturated zone causes a significant decrease in the slope safety factor.

POTENSI LIKUIFAKSI AKIBAT GEMPABUMI BERDASARKAN DATA CPT DAN N-SPT DI DAERAH PATALAN BANTUL, YOGYAKARTA Eko Soebowo; Adrin Tohari; Dwi Sarah
JURNAL RISET GEOLOGI DAN PERTAMBANGAN Vol 19, No 2 (2009)
Publisher : Indonesian Institute of Sciences

ABSTRAK Gempabumi dapat menimbulkan bahaya likuifaksi yang dapat merusakkan bangunan dan sarana infrastruktur khususnya di wilayah perkotaan di Indonesia. Investigasi geoteknik bawah permukaan telah dilakukan di daerah Patalan, Bantul, Yogyakarta, untuk mendapatkan gambaran susunan lapisan tanah dan kekuatannya, menentukan kedalaman dan ketebalan lapisan tanah yang berpotensi likuifaksi dan penurunan lapisan tanah akibat likuifaksi. Analisis potensi likuifaksi dilakukan menggunakan data CPT (cone penetration test) dan N-SPT (standard penetration test), dengan mempertimbangkan nilai percepatan getaran tanah maksimum (p.g.a) sebesar 0,25g, magnitudo gempabumi sebesar 6,2 SR dan muka airtanah setempat. Hasil penyelidikan menunjukkan bahwa lapisan tanah lepas selama gempabumi terdiri dari pasir lanauan dan lanau pasiran pada kedalaman antara 0,2 - 12,8 m. Analisis potensi likuifaksi mengindikasikan bahwa ketebalan lapisan tanah yang berpotensi terlikuifaksi bervariasi antara 0,2 m dan 5,2 m, Sedangkan penurunan total lapisan tanah terutama terkonsentrasi di wilayah bagian tengah yang terletak di jalur Patahan Opak dengan besaran antara 0,21 cm hingga 12,98 cm Zona likuifaksi dan penurunan ini berada pada lapisan sedimen bagian atas yang mengisi pada cekungan Bantul di sekitar Patahan Opak.

REKAYASA HIDRAULIKA KESTABILAN LERENG DENGAN SISTEM SIPHON: STUDI KASUS DI DAERAH KARANGSAMBUNG, JAWA TENGAH Arifan Jaya S; Adrin Tohari; Khori Sugianti; Nugroho Aji S; Sunarya Wibowo; Sueno Winduhutomo
JURNAL RISET GEOLOGI DAN PERTAMBANGAN Vol 24, No 2 (2014)
Publisher : Indonesian Institute of Sciences

Engineering measures to stabilize slopes with low water tables have been widely developed but mostly required high operating costs. This research conveys a subsurface drainage engineering method widely used in European countries but not yet applied in Indonesia: Siphon. This method aims to lower groundwater level, hence increases slope stability. Siphon installation was carried out by boring 3 holes to 4 m depth and constructing monitoring wells at each hole to check the groundwater level. Siphon system was fitted up to the toe slope leading to an outlet. Comparison of measured discharge and calculated discharge was made to verify siphon parameters. It is conclude that mathematical approach could provide back analysis of parameters changing the discharge values. The changing parameters are roughness and coefficient of discharge. The siphon roughness changed to 0.018, 0.017 and 0.018 and coefficient of discharge changed to 0.0589, 0.0193, and 0.0348 for siphon 1, 2 and 3 respectively. The changing roughness values indicated a blockage in the siphon system due to the trapped fine grained soils. To overcome this problem the filter and siphon wells must be regularly cleaned to prevent fine materials to enter the system and being attached to the rubber tube wall.ABSTRAKPerekayasaan kestabilan lereng dengan karakteristik muka airtanah dangkal telah banyak dikembangkan, walaupun cenderung memerlukan nilai operasional yang tinggi. Salah satu metode metode perekayasaan drainase bawah permukaan yang telah dikembangkan negara maju di Eropa namun belum diaplikasikan di Indonesia adalah metode siphon. Metode ini merupakan sistem rekayasa hidraulika yang bertujuan untuk menurunkan muka airtanah sehingga kestabilan lereng meningkat. Penelitian ini dilakukan untuk mengetahui parameter/faktor apa yang berubah sehingga mempengaruhi nilai debit Siphon, dengan menggunakan pendekatan matematis. Pemasangan sistem Siphon telah dilakukan dengan cara membuat lubang bor sedalam 4 m sebanyak 3 titik dan sumur pemantauan pada tiap titik yang berfungsi sebagai pengecekan muka air tanah. Sistem siphon dipasang sampai pada kaki lereng yang berujung pada outlet. Perbandingan debit hitungan matematis dan debit terukur dilakukan sehingga didapatkan verifikasi parameter yang digunakan. Pada penelitian ini diperoleh kesimpulan dengan pendekatan matematis bisa dilakukan analisis balik mengenai parameter apa yang mengubah nilai debit. Parameter yang berubah pada penelitian ini ialah nilai kekasaran dan koefisien debit. Perubahan nilai kekasaran Siphon 1, Siphon 2, dan Siphon 3 menjadi masing-masing 0,018, 0,017, dan 0,018 dan koefisien debitnya masing-masing 0,0589, 0,0193, dan 0,0348. Perubahan nilai kekasaran yang lebih besar mengindikasikan adanya material sumbatan yaitu tanah butiran halus yang berada pada sumur Siphon dan masuk ke sistem Siphon. Untuk mengatasinya, filter yang ada dan sumur Siphon perlu dibersihkan sehingga material sumbatan tidak masuk ke dalam sistem dan menempel pada dinding selang.

Microzonation of Soil Amplification Based on Microtremor, Spt and Cptu Data in Bandung Basin Anggun Mayang Sari; Eko Soebowo; Afnindar Fakhrurrozi; Arifan Jaya Syahbana; Adrin Tohari
JURNAL RISET GEOLOGI DAN PERTAMBANGAN Vol 29, No 1 (2019)
Publisher : Indonesian Institute of Sciences

Bandung Basin consists of Sunda-Tangkuban Perahu volcanic deposit that is made of lake sediment and an alluvial fan with fine to coarse-grained materials such as clay, silt and sand. The area is surrounded by several earthquake sources such as the Lembang, Cimandiri, and Baribis Faults. Therefore, it is important to understand soil dynamic problems with respect to seismic sources and soil properties. This research aims to investigate velocity amplification of the Bandung Basin using microtremor measurements and site classification based on the Standard Penetration Test (SPT) and Cone Penetrometer Test (CPTu). Velocity amplification was analyzed using the horizonal to vertical H/V spectral ratio, and site classification was determined using and values. Microzonation maps were developed using Geographical Information System (GIS) to determine the correlation between soil velocity amplification and site class. The results revealed that velocity amplification levels in the Bandung Basin vary with a range of 1.3 to 26.5. Site classification ranges from very dense soil and hard rock (class C), stiff soil (class D) to soft clay soil (class E). Site class E dominates the southeast part of the Bandung Basin with a high value of soil amplification. This scientific information is critical for further spatial planning focusing on infrastructure and residential building. Cekungan Bandung berasal dari endapan vulkanik Sunda-Tangkuban Perahu yang terdiri dari sedimen danau dan kipas aluvial dengan material berbutir halus hingga kasar seperti lempung, lanau, dan pasir. Daerah ini juga dikelilingi oleh beberapa sumber gempa yakni sesar Lembang, Cimandiri, dan Baribis. Oleh karena itu masalah dinamika tanah yang dipengaruhi oleh sumber seismik dan sifat tanah perlu menjadi perhatian. Tujuan penelitian ini adalah untuk mengetahui amplifikasi kecepatan tanah di wilayah cekungan Bandung dengan menggunakan alat mikrotremor dan metode site classification berdasarkan uji SPT dan CPTu. Amplifikasi kecepatan dianalisis menggunakan perbandingan rasio spektral H/V dan site classification yang diperoleh dengan menggunakan nilai dan . Peta mikrozonasi disusun menggunakan teknik Sistem Informasi Geografis (SIG) untuk menentukan korelasi amplifikasi tanah dan site class tanah. Hasil penelitian menunjukkan bahwa tingkat amplifikasi kecepatan di Cekungan Bandung bervariasi, berkisar 1,3 hingga 26,5. Site classification berkisar dari tanah yang sangat padat dan batuan keras (kelas C), tanah kaku (kelas D) hingga tanah lempung lunak (kelas E). Hasil penelitian menunjukkan site class E mendominasi bagian selatan Cekungan Bandung dengan nilai amplifikasi tanah yang tinggi. Informasi ilmiah ini diperlukan untuk perencanaan tata ruang kedepannya, dengan fokus pada infrastruktur dan bangunan tempat tinggal.

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