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PENGANTAR MEKANIKA TANAH SI - 2222

Dosen: Erza Rismantojo

Jadwal Perkuliahan

Kamis atau Jumat ? Ujian Tengah Semester 9 – 13 Maret 2015 Akhir Perkuliahan Jumat 30 April 2015 Ujian Akhir Semester 4 – 20 Mei 2015

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Asisten: Christian Tirtawardhana Auberta Philanta

REFERENSI

Das, B.M., 1995, “Principles of Geotechnical Engineering”, PWS Engineering, Boston, USA

Holtz, R. D. and Kovacs, W. D., 1981, “An

Introduction to Geotechnical Enginering” Prentice-Hall

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MATERI KULIAH (sebelum UTS)

Minggu ke 1 – 7: Proses pembentukan tanah Pengenalan Geologi Teknik Weight-volume relationships, plasticity, soil structure Klasifikasi tanah menurut AASHTO dan USCS

(ASTM) Pemadatan tanah dan CBR (California Bearing Ratio) Permeability dan Seepage

Minggu ke 8 (10 – 14 Maret 2014): UTS

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MATERI KULIAH (setelah UTS)

Minggu ke 9 – 15: Permeability dan Seepage Tegangan dalam tanah Compressibility dan Consolidation

Minggu ke 16 (5 – 20 Mei 2014): UAS

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SISTEM PENILAIAN

Tugas: 20% Ujian Tengah Semester: 40% Ujian Akhir Semester: 40% Tugas dikumpulkan 1 minggu setelah diberikan.

Tugas yang terlambat tidak akan dinilai. Tidak ada Tugas/Ujian tambahan untuk

perbaikan nilai.

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PENDAHULUAN

Mekanika Tanah adalah cabang dari ilmu mekanika yang berurusan dengan gaya-gaya yang bekerja pada masa tanah.

Rekayasa Geoteknik adalah salah satu spesialisasi rekayasa yang menggunakan prinsip-prinsip Mekanika Tanah.

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Low viscosity, low shear strength

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Kerak Bumi

Kerak Bumi

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Pergerakan Kerak Bumi berdasarkan data Satelit GPS (NASA) tanggal 17 April 2008

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Jenis-jenis Pergerakan di Perbatasan antar Kerak Bumi

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Mineral

A mineral is a naturally occurring solid chemical substance that is formed through geological processes and that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and does not have a specific chemical composition.

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Mineral Class

Silicate Class : terdiri dari silicon dan oxygen. Umumnya rocks ≥ 95% silicates Contoh: feldspars, quartz, olivines, pyroxenes, amphiboles, garnets,

and micas Carbonate Class: mengandung anion (CO3)2− . Contoh: calcite (CaCO3), dolomite CaMg(CO3)2

Sulfate Class (sulfate anion SO42- , contoh: gypsum)

Halide Class (halide anions: F-, Cl-, Br-, I-. Contoh: NaCl, salts) Oxide Class (contoh: hematite (iron oxide)) Sulfide Class (contoh: pyrite (iron sulfide, FeS2)) Phosphate Class (PO4

3-) Element Class (contoh: silver, gold, copper) Organic Class

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BATUAN (ROCKS)

DEFINISI BATUAN (ROCK) menurut Rekayasa Sipil (Teknik Sipil): Material yang keras (hard) dan tahan lama (durable) Material yang tidak dapat digali kecuali dengan menggunakan bahan

peledak (blasting)

JENIS BATUAN BERDASARKAN ASAL USUL PEMBENTUKANNYA: Batuan Beku (Igneous Rocks) Batuan Sedimen (Sedimentary Rocks) Batuan Metamorf (Metamorphic Rocks)

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SIKLUS PEMBENTUKAN BATUAN

Metamorphism

Transportation, Erosion,

weathering

Compaction, cementation, crystallization

Sedimentary Rock

Sediment

Igneous Rock

MagmaMetamorf Rock

melting

Transportation, Erosion, Weathering

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SIKLUS PEMBENTUKAN BATUAN

BATUAN BEKU (Igneous Rocks): Batuan yang berasal dari magma cair, lahar yang

membeku karena penurunan suhu. Intrusive igneous rocks: diorite, gabbro, granite and

pegmatite that solidify below Earth's surface; Extrusive igneous rocks such as andesite, basalt,

obsidian, pumice, rhyolite and scoria that solidify on or above Earth's surface.

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CONTOH BATUAN BEKU Coarse-grained: Granite, Syenite, Gabbro,

Diorite

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Granite is a coarse-grained, light colored, intrusive igneous rock that contains mainly quartz and feldspar minerals.

CONTOH BATUAN BEKU Fine-grained: Rhyolite, Trachyte, Basalt,

Andesite

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Andesite is a fine-grained, extrusive igneous rock composed mainly of plagioclase with other minerals such as hornblende, pyroxene and biotite.

CONTOH BATUAN BEKU Obsidian (high-silica glass)

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Obsidian is a dark-colored volcanic glass that forms from the very rapid cooling of molten rock material. It cools so rapidly that crystals do not form.

CONTOH BATUAN BEKU

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Pumice is a light-colored vesicular igneous rock. It forms through very rapid solidification of a melt. The vesicular texture is a result of gas trapped in the melt at the time of solidification.

CONTOH BATUAN BEKU

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Welded Tuff is a rock that is composed of materials that were ejected from a volcano, fell to Earth, and then lithified into a rock. It is usually composed mainly of volcanic ash and sometimes contains larger size particles such as cinders (pyroclastic material similar to pumice).

Masalah-masalah yang berhubungan dengan penggunaan batuan beku

Beberapa jenis batuan beku dengan tekstur butiran halus (fine-grained) dan mengandung silika dapat bereaksi dengan kandungan alkali semen hasil reaksi alkali silika menyebabkan pertambahan volume sehingga merusak beton

Contoh: Andesite, Rhyolite, Volcanic Breccia, Obsidian, Pumice

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Alkali-Silica Reaction (ASR)

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Characteristic crack pattern associated with the alkali–silica reaction affecting a concrete step barrier on an US motorway (photograph courtesy of the Federal Highway Administration, US Department of Transportation).[1][2]

Ca(OH)2 + H4SiO4 → Ca2+ + H2SiO42− + 2 H2O → CaH2SiO4 · 2 H2O

Permasalahan Penggunaan Batuan Beku

Batuan beku berbutir kasar (coarse-grained) biasanya tidak baik untuk bahan konstruksi jalan, batu pelindung tanggul, karena tidak tahan terhadap abrasi

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Masalah-masalah yang berhubungan dengan penggunaan batuan beku

BATUAN SEDIMEN (SEDIMENTARY ROCKS)

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Sedimentary rocks are formed by the accumulation of sediments. Types of sedimentary rocks: 1) detrital sedimentary rocks such as breccia,

conglomerate, sandstone and shale, that are formed from solid particles derived from physical and chemical weathering of rock, transported, deposited, compacted, and cemented into rocks

2) chemical sedimentary rocks such as rock salt and some limestones, that form when dissolved materials precipitate from solution; and,

3) organic sedimentary rocks such as coal and some limestones which form from the accumulation of plant or animal debris.

BATUAN SEDIMEN (SEDIMENTARY ROCKS)

95% bagian terluar kerak bumi (16 km) adalah batuan beku dan batuan metamorf (5% adalah batuan sedimen)

75% batuan yang tersingkap di permukaan bumi adalah batuan sedimen

99% dari batuan sedimen adalah: shale (46%), sandstone (32%), dan limestone (22%)

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Detrital Sedimentary Rocks

Sedimen Ukuran Batuan Sedimen Gravel >2mm Conglomerate (partikel bulat)

Breccia (partikel angular) Sand 2 – 1/16 mm Sandstone

Silt 1/16 – 1/256 mm Siltstone (shale)

Clay < 1/256 mm Claystone (shale)

Silt and clay Shale

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Contoh Batuan Sedimen

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Breccia is a clastic sedimentary rock that is composed of large (over two millimeter diameter) angular fragments. The spaces between the large fragments can be filled with a matrix of smaller particles or a mineral cement which binds the rock together.

Contoh Batuan Sedimen

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Conglomerate is a clastic sedimentary rock that contains large (greater then two millimeters in diameter) rounded particles. The space between the pebbles is generally filled with smaller particles and/or a chemical cement that binds the rock together.

Contoh Batuan Sedimen

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Sandstone is a clastic sedimentary rock made up mainly of sand-size (1/16 to 2 millimeter diameter) weathering debris. Environments where large amounts of sand can accumulate include beaches, deserts, flood plains and deltas.

Contoh Batuan Sedimen

Contoh Batuan Sedimen

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Shale is a clastic sedimentary rock that is made up of clay-size (less then 1/256 millimeter in diameter) weathering debris. It typically breaks into thin flat pieces.

Contoh Batuan Sedimen

Contoh Batuan Sedimen

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Coal is an organic sedimentary rock that forms mainly from plant debris. The plant debris usually accumulates in a swamp environment. Coal is combustible and is often mined for use as a fuel.

Contoh Batuan Sedimen

CHEMICAL SEDIMENTARY ROCKS

Carbonates: Limestone (Mineral calcite: CaCO3) Dolomite [Mineral dolomite: CaMg(CO3)2]

Evaporites: Gypsum (CaSO4.2H2O) Rock Salt (NaCl)

Fossil Rocks: Fossiliferous Limestone (Calcite fosil berukuran

pasir atau lebih besar)

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Limestone is a rock that is composed primarily of calcium carbonate. It can form organically from the accumulation of shell, coral, algal and fecal debris. It can also form chemically from the precipitation of calcium carbonate from lake or ocean water. Limestone is used in many ways. Some of the most common are: production of cement, crushed stone and acid neutralization.

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Rock Salt is a chemical sedimentary rock that forms from the evaporation of ocean or saline lake waters. It is also known by the mineral name "halite". It is rarely found at Earth's surface, except in areas of very arid climate. It is often mined for use in the chemical industry or for use as a winter highway treatment. Some halite is processed for use as a seasoning for food.

Detroit Salt Mine

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PENGGUNAAN BATUAN SEDIMEN

Reaksi alkali-carbonate pada beton Portland cement (High alkali cement > 0.6% Na2O) menyebabkan kerusakan pada beton

CaMg(CO3)2 + 2 NaOH —> CaCO3 + Na2CO3 + Mg(OH)2

Limestone dan Dolomite adalah material

yang baik untuk konstruksi Siltstone, shale, quartz sandstone biasanya

tidak baik sebagai material konstruksi

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Batuan Metamorf

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Metamorphic rocks have been modified by heat, pressure and chemical process usually while buried deep below Earth's surface. Exposure to these extreme conditions has altered the mineralogy, texture and chemical composition of the rocks. There are two basic types of metamorphic rocks: 1) foliated metamorphic rocks such as gneiss, phyllite, schist and slate which have a layered or banded appearance that is produced by exposure to heat and directed pressure; and, 2) non-foliated metamorphic rocks such as marble and quartzite which do not have a layered or banded appearance. (http://geology.com/rocks/metamorphic-rocks.shtml)

Batuan Metamorf

Terbentuk akibat berubahnya komposisi kimiawi dan tekstur dari batuan, perubahan ini disebabkan oleh panas dan tekanan

Selama proses batuan asal tidak mencair Terjadi kristalisasi mineral and perubahan

struktur komposisi mineral yang membentuk mineral baru

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Gneiss is foliated metamorphic rock that has a banded appearance and is made up of granular mineral grains. It typically contains abundant quartz or feldspar minerals.

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Quartzite is a non-foliated metamorphic rock that is produced by the metamorphism of sandstone. It is composed primarily of quartz.

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Slate is a foliated metamorphic rock that is formed through the metamorphism of shale. It is a low grade metamorphic rock that splits into thin pieces.

JENIS BATUAN METAMORF

Shale Slate (fine grained, slaty cleavage) Phyllite: memiliki komposisi yang hampir

sama dengan Slate dengan lebih banyak kandungan mineral muscovite (phyllosilicate mineral of aluminium and potassium)

Schist (metamorf lanjutan dari Slate) Gneiss (berasal dari coarse-grained igneous

rock granite & gabbro) Marble (marmer)

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MASALAH-MASALAH DENGAN BATUAN METAMORF

Foliated metamorphic rocks menghasilkan butiran yang pipih dan lonjong sewaktu dipecah mudah pecah pada waktu konstruksi pemadatan hot mixed asphalt

Coarse-grained gneiss mudah terabrasi Marble cavities (lubang akibat terlarutkan

oleh aliran air) masalah kebocoran bendungan, pondasi bangunan

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PROSES PELAPUKAN

Proses Pelapukan secara Mekanis (mechanical weathering) Kembang susut batuan akibat perubahan suhu panas dingin Tekanan yang diakibatkan membekunya air di dalam

rongga/retakan di batuan

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PROSES PELAPUKAN

Kehilangan tekanan di permukaan

Batuan yang semula berukuran besar pecah menjadi butiran kecil. Tidak terjadi perubahan komposisi kimia

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PROSES PELAPUKAN

Proses Pelapukan secara Kimiawi (chemical weathering) Reaksi kimia mengubah mineral yang terkandung dalam

batuan asal menjadi mineral baru H2O + CO2 H2CO3 (Carbonic Acid) 2K(AlSi3O8) + 2H+ + H2O 2K++ 4SiO2 + Al2Si2O5(OH)4

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Orthoclase (salah satu mineral di dalam batuan beku seperti granite)

Silica Kaolinite (clay mineral)

Hasil pelapukan di tempat batuan asal disebut Tanah Residual

Glacial soils (glaciers) Alluvial soils (transportasi oleh air) Lacustrine soils (pengendapan di danau) Marine soils (pengendapan di laut) Aeolian soils (transportasi oleh angin) Colluvial soils (transportasi oleh gravitasi)

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Transportasi Hasil Pelapukan Batuan

Transportasi Hasil Pelapukan Batuan

Residual Soil

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a) top soil and colluvium

c) young residual soil

b) mature residual soil

d) weathered rock.

Alluvial soils (transportasi oleh air)

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ALLUVIAL soil is formed when a soil carrying stream gradually loses its carrying capacity with decreasing velocity. In slowing down, a river does not have sufficient power to keep the large particles of soil suspended; these particles settle to the riverbed. Further decrease in velocity causes smaller particles to settle. As the river becomes slow and sluggish (as in the lowlands where its gradient becomes small), it holds only the extremely fine particles in suspension. These particles are deposited, finally, at the mouth of the river, where they form DELTAS of fine-grained soil

Transportasi Hasil Pelapukan Batuan

Lacustrine soils (pengendapan di danau)

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Freshwater lake deposits are called LACUSTRINE soils. They are fine grained soils resulting from material brought into freshwater lakes by streams or rivers.

Transportasi Hasil Pelapukan Batuan

Aeolian soils (transportasi oleh angin)

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Transportasi Hasil Pelapukan Batuan

Colluvial soils (transportasi oleh gravitasi)

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COLLUVIAL soil consists of mixed deposits of rock fragments and soil materials accumulated at the bases of steep slopes through the influence of gravity.

Transportasi Hasil Pelapukan Batuan

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Clay Mineral

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Clay Mineral Aluminum silicates Terdiri dari 2 unit dasar:

Silica Tetrahedron Alumina Octahedron

Silica Tetrahedron Silica sheet

Clay Mineral

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Alumina octahedron Octahedral sheet (Gibbsite sheet) Disebut Brucite sheet jika atom Aluminum diganti Magnesium

Clay Mineral

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Silica-gibbsite sheet

Clay Mineral

Kaolinite Illite Montmorillonite Chlorite Halloysite Vermiculite Attapulgite

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Clay Mineral

Kaolinite Gabungan lembaran “silica-gibbsite sheet” Tebal lembaran = 7.2 Angstrom (1 Angstrom = 10-10 m) Berbentuk “platelets” berdimensi lebar atau

panjang 1000 – 20000 Angstrom, tebal 100 – 1000 Angstrom

Specific surface = 15 m2/g

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Kaolinite

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Kaolinite

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Kaolinite

CLAY MINERAL

ILLITE Gibbsite sheet + 2 Silica sheet (di atas dan bawah) Lembaran illite diikat oleh ion potassium (K) Ion K menetralkan muatan negatif yang

disebabkan oleh penggantian atom silicon dengan aluminum di “tetrahedral sheet”

Dimensi lateral 1000 – 5000 Angstrom, tebal 50 – 500 Angstrom

Specific surface = 80 m2/g

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ILLITE

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CLAY MINERAL

Montmorillonite Struktur hampir sama dengan Illite Atom aluminum di dalam octahedral sheet diganti

oleh magnesium dan iron (besi) Tidak ada ion potassium diantara lembaran Dapat menarik banyak molekul air ke dalam

ruangan yang di antara lembaran Dimensi lateral = 1000 – 5000 Angstrom Tebal = 10 – 50 Angstrom

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Montmorillonite

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Montmorillonite

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Clay Mineral

Permukaannya memiliki muatan negatif karena: Isomorphous substitution

Al Si (tetrahedral sheet) Mg Al (octahedral sheet)

Ketidaksempurnaan pada ujung-ujung/sisi-sisi lembaran clay yang putus

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Clay Mineral

Muatan negatif pada permukaan clay dinetralkan oleh kation (ion positif) yang ada disekitarnya Ca2+, Mg2+, Na+, K+

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Diffuse Double Layer

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Diffuse Double Layer

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