Dosen: Erza Rismantojo PENGANTAR MEKANIKA TANAH SI

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PENGANTAR MEKANIKA TANAH

SI - 2222

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

Asisten:

 Christian Tirtawardhana

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

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

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

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.

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

Jenis-jenis Pergerakan di

Perbatasan antar Kerak Bumi

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.

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 (PO₄3-)

 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)

SIKLUS PEMBENTUKAN BATUAN

Metamorphism Transportation, Erosion, weathering Compaction, cementation, crystallization Sedimentary Rock Sediment Igneous Rock Magma Metamorf Rock melting Transportation, Erosion, Weathering

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 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.

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

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

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,

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+ _{+ H}

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

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

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

Contoh Batuan Sedimen

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

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

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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

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.

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)

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:

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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.

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

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.

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.

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)

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

PROSES PELAPUKAN

 Proses Pelapukan secara Mekanis (mechanical

weathering)

 Kembang susut batuan akibat perubahan suhu panas dingin

 Tekanan yang diakibatkan membekunya air di dalam

PROSES PELAPUKAN

 Kehilangan tekanan di permukaan

 Batuan yang semula berukuran besar pecah menjadi

butiran kecil. Tidak terjadi perubahan komposisi kimia

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

Orthoclase

(salah satu mineral di dalam batuan beku seperti granite)

 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

 Residual Soil

a) top soil and colluvium

c) young residual soil b) mature residual soil

 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

 Lacustrine soils (pengendapan di danau)

Freshwater lake deposits are called LACUSTRINE soils. They are fine grained soils resulting from material

brought into freshwater lakes by streams or rivers.

 Aeolian soils (transportasi oleh angin)

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 Colluvial soils (transportasi oleh gravitasi)

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

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Clay Mineral

Clay Mineral  Aluminum silicates Terdiri dari 2 unit dasar:

 Silica Tetrahedron

 Alumina Octahedron

Clay Mineral

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Alumina octahedron Octahedral sheet (Gibbsite sheet)

Disebut Brucite sheet jika atom Aluminum diganti Magnesium

Clay Mineral

Clay Mineral

 Kaolinite  Illite  Montmorillonite  Chlorite  Halloysite  Vermiculite  Attapulgite 63

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

Kaolinite

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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

ILLITE

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

Montmorillonite

Clay Mineral

 Permukaannya memiliki muatan negatif

karena:

 Isomorphous substitution

 Al  Si (tetrahedral sheet)

 Mg  Al (octahedral sheet)

 Ketidaksempurnaan pada ujung-ujung/sisi-sisi

Clay Mineral

 Muatan negatif pada permukaan clay

dinetralkan oleh kation (ion positif) yang ada disekitarnya

 Ca2+, Mg2+, Na+, K+

Diffuse Double Layer