KEKUATAN GESER TANAH

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KEKUATAN GESER TANAHDefinisi:perlawananinternaltanahtiapsatuanluasterhadapkeruntuhanatau pergeseran sepanjang bidang runtuh dalam satu elemen tanah Tujuan Studi kekuatan geser tanah: Untuk analisis masalah kestabilan tanah, seperti : daya dukung stabilitas talud (lereng) tekanan tanah ke samping pada turap/tembok penahan dll KEKUATAN GESER TANAHDasarTeori (Hukum Gesekan Newton): W T R | T > Wgeser T < Wdiam T = Wlabil |ot |= = == =tan:tanAWATtegangan dalamWTKEKUATAN GESER TANAH Basic mechanics applied to soils Equilibrium and compatibility The principle of equilibrium states that if a body is in state of rest, the net force acting on the body in any direction is zero. Thus, in the figure above, the sums of forces in the horizontal and vertical directions will be zero: AH = H1 + H2 + H3 + H4 AV = V1 + V2 + V3 + V4 The principle of compatibility states that any movements or changes in shape or volume must be compatible with no material being lost or gained. When loads are applied to bodies of soil it is assumed that the solid content remains constant (though displaced or re-arranged), but that there may be changes in the volume of water and or air. KEKUATAN GESER TANAH Mechanical Characteristic of soils Soils are not solid materials, as are steel and concrete, but are made up of separate particles, surrounded by voids, which may contain water or air or both. The particles in sands are rotund, while in clays they are flaky. Changes in shape and volume, and the strength of soil, are controlled by effective stress, i.e. the difference between the (external) total stress and the (internal) pore pressure. Under different circumstances of occurrence soils may be: - very dense, ranging through dense, intermediate and loose, to very loose - very dry and hard, ranging through stiff and soft, to very wet and soft. KEKUATAN GESER TANAH Compressibility Characteristic of soils Soils are compressible if the voids contain air. Soils saturated with water are compressible ONLY if drainage can take place. Compression results in a change in the volume of voids (changes in the volume of the grains are negligible) Loose sands are more compressible than dense sands. Wet or soft clays are more compressible than dry or hard clays. Normally consolidated clays are more compressible than overconsolidated clays. KEKUATAN GESER TANAH Strength Characteristic of soils Soil strength is basically frictional, and is controlled by effective stress. The strength of a soil is defined as the maximum sustainable shear stress that can be developed under certain specified conditions: Undrained strength:(e.g. in saturated clay) is constant with respect to effective normal stress, but decreases with increasing water content. Drained strength:increases with effective normal stress. KEKUATAN GESER TANAH Stiffness and Elasticity Stiffness is the relationship between strain and the stress that will induce it. Stiffness behavior relates to the changes in strain that accompany changes in stress (due to loading and unloading). A stress-strain curve is used to characterize stiffness behavior. The stiffness modulus is simply the slope of the stress-strain curve. KEKUATAN GESER TANAH Stiffness Moduli When the loading on a body changes the strain induced will be one or a combination of three types: oDirect or linear strain:changes in length, breadth, diameter, etc. oVolumetric strain:changes in volume oShear strain:changes in shape KEKUATAN GESER TANAH Stiffness Moduli In bodies of elastic material the three stiffness moduli (E, K and G) are related to each other and to Poissons ratio (u). It assumed that the material is elastic and isotropic (i.e. linear stiffness is equal in all directions). The following relationships can be demonstrated (for proofs refer to a text on the strength of materials) G = E/ 2(1-u) K = E/ 3(1-2u) u=Poissons ratio for soils, is in the range 0.2 - 0.5 KEKUATAN GESER TANAH Stress-Strain Behaviour of Soils The stress-strain behaviour of soils is similar to that of other engineering materials, i.e. elastic and plastic deformation occur as in steel, concrete, etc. It is the variability in behaviour that distinguishes soils from other materials. For example, the same soil at different water contents or states of compaction will exhibit a wide range of stiffness and strength characteristics. Relatively recent soils (< 1 Ma old)tend to deform plastically at relatively low stresses, but with deformation related linearly to the logarithm of stress. Geologically old soils, soils that have been overlain by other soils, rocks or ice, are harder and tend to behave elastically (or nearly so) at the levels of stress associated with civil engineering projects. KEKUATAN GESER TANAH Behaviour of Soft Soils Soft soils (including recent natural soils and reconstituted soils) with no history of previous greater loading behave inelastically; the stress-strain diagram being curved even at low stresses. The slope of the curve (E = doa/dca) decreases as the stress increases, eventually reaching an ultimate stress which remains constant as straining continues. Irreversible plastic strain occurs, so that upon unloading permanent deformation ( cp) remains. KEKUATAN GESER TANAH Behaviour of Soft Soils Stiffness values vary with different stress regimes, e.g. uniaxial, triaxial, one-dimensional. Typical values should not be used in design, except in preliminary stages, feasibility studies, etc. Compression tests are required to ascertain reliable and representative values (refer to the Soil Mechanics Reference: Compression and swelling) Typical E range(MPa) Normally consolidated clays0.2 - 4 Organic alluvial clays and peats0.1 - 0.6 KEKUATAN GESER TANAH Behaviour of Hard Soils Stiff and hard soils have often become overconsolidated, i.e. they have a stress history in which loading has increased stresses and then unloading has decreased them. Deformation is a consequence of changes in effective stress. A low stresses the behaviour is elastic (or nearly so), with the stiffness remain almost constant. After the yield point has been reached, plastic deformation occurs Work softening occurs in heavily overconsolidated soils, so that the ultimate stress will be lower than the peak stress. KEKUATAN GESER TANAH Behaviour of Hard Soils Stiffness values vary with different stress regimes, e.g. uniaxial, triaxial, one-dimensional. Typical values should not be used in design, except in preliminary stages, feasibility studies, etc. Compression tests are required to ascertain reliable and representative values (refer to the Soil mechanics Reference: Compression and swelling) Typical E range(MPa) Unweathered overconsolidated clays20 - 50 Boulder clay10 - 20 Keuper Marl (unweathered)>150 Keuper Marl (moderately weathered)30 - 150 Weathered overconsolidated clays3 -10 KEKUATAN GESER TANAH State of Stress and Strain The three orthogonal axial directions are defined in soil mechanics similarly to other engineering disciplines, except that some special considerations apply. The direction of the z-axis is usually vertical; with positive = downward-with-depth in site problems. Stresses relating to soil masses are almost always compressive. Distinction must be made between total stresses and effective stresses, e.g. o =total normal stress o=effective normal stress (note the prime) oz oy ox KEKUATAN GESER TANAH State of Stress and Strain oz oa =total axial compressive stress oa=effective axial stress=oa - u u = internal pore pressure ca =axial strain (due to oa) Uniaxial stress and strain KEKUATAN GESER TANAH State of Stress and Strain Triaxial stresses and strains KEKUATAN GESER TANAH State of Stress and Strain Biaxially symmetrical stresses and strains In a triaxial arrangement, normal stresses (ox, o y, o z) and normal strains (cx, cy, cz) are aligned on three orthogonal axes. In the triaxial test, axial directions are referred to as axial or radial. Also the two radial values will be equaly, i.e. are biaxially symmetrical. oz = oa= total axial stress ox = oy = or= total radial stress oz = oa= effective axial stress = oa - u ox = oy = or = effective radial stress = or - u u = pore pressure ca = axial strain cr = radial strain KEKUATAN GESER TANAH State of Stress and Strain Biaxially symmetrical stresses and strains The normal stresses are principal stresses (o1, o2, o3) The deviator and mean normal stresses are defined as: q = deviator stress = oa - or = oa - or(= o1 - o3) p= mean total normal stress= 1/3 (o1 + o2 + o3) = 1/2 (oa + 2or) p = mean effective normal stress= 1/3 (o1 + o2 + o3) = 1/2 (oa + 2 or) = p - u KEKUATAN GESER TANAH State of Stress and Strain Plane strain Plane strain conditions occur, for example, under the centre of long strip footings and retaining walls. Strains only occur in a vertical plane, perpendicular to this plane the strain is zero. ozor ov= vertical total stress ozor ov= vertical effective stress czor cv= vertical strain oxor oh= horizontal total stress oxor oh= horizontal effective stress oy = horizontal stress in the direction of zero strain =K0 ov (where K0 = coefficient of earth pressure at rest) KEKUATAN GESER TANAH State of Stress and Strain Plane strain Mohr circle and the invariants s and t KEKUATAN GESER TANAH Shear Stress and Strain Engineers shear strain is defined as the angle of distortion of a rectilinear element. t=shear stress (acting tangentially) =shear strain (angular distortion due to the shear stress) On either side of a slip plane the shear stress (t) has reached a limiting value (tf), which may be termed shear strength,e.g. as in the shear box (or direct shear) test The stress-displacement (t/dx) curve will have characteristics similar to those of a shear-stress/shear-strain curve, but cannot be used to measure shear stiffness. KEKUATAN GESER TANAH State of Stress and Strain Stiffness Parameters: Changes in volume in soil masses are due to changes in effective stress. Changes in shape can be related to both total and effective stresses. E= Stiffness (Youngs) modulus for direct (axial) straining, i.e. when oor = 0 = ooa/ ocai.e.. in terms of total stresses Eu= Stiffness modulus measured in undrained conditions, i.e. when ocv = 0 E= Stiffnessmodulus for direct (axial) straining, i.e. when oor = 0 =ooa/ oca i.e. in terms of total stresses Eo= Youngs modulus for one-dimensional compression, i.e. when ocr = 0 K= Bulk modulus (isotropic stress)= oo/ ocv G= Shear modulus=ot/o KEKUATAN GESER TANAH State of Stress and Strain Stiffness Parameters: Tangent and Secant values of E Since the stress-strain behaviour of soil produces a curve, E is not constant, but decreases with increasing stress.Practical measures of the slope (e.g. to obtain an E value for design) can be either a secant value or a tangent value. For problems where the stress is simply raised from zero by Ds, the secant value is appropriate. For problems involving small strains around a given level of stress (point A), the tangent value is more suitable. STRENGTHANDFAILURE KEKUATAN GESER TANAH Strength and Failure The strength of a material is often expressed in terms of the applied stress, e.g. - in a tie rod: tensile strength - a concrete cube: compressive strength - in bolts: shear strength In all cases, however, strength is related to a characteristic maximum shear stress. The magnitude of maximum shear stress is given by the radius of the Mohr circle. KEKUATAN GESER TANAHKriteria Keruntuhan Menurut MOHR-COLOUMB Keruntuhanterjadipadasuatumaterialakibatkombinasikritis antarategangannormaldangeser,danbukanhanyaakibat tegangan normal maksimum atau tegangan geser maksimum saja. tf = f(o)| : sudut geser-internal c : kohesi tf = c + o tan|KEKUATAN GESER TANAHMohr circle for tensile strength In soil mechanics, since stresses are invariably compressive, the sign convention for the Mohr circle axes is:compressive stresses are positive and plotted on thex-axis to the right. Note that the tensile strength will be given by otf= diameter of the Mohr circle. The shear strength (tf) is given by the radius of the Mohr circle KEKUATAN GESER TANAHMohr circle for compressive strength Note that the angle of the potential plane of failure is 45 Maximum shear strength = 1/2 oc KEKUATAN GESER TANAHMohr circle for shear strength The example in the figure is that of a saturated clay slope, but the same concept of shear strength applies in all soil constructions. The stresses at failure (slipping in this case) are: tf= shear stress = radius of the Mohr circle on = normal effective stress = the x-coordinate of the circle centre KEKUATAN GESER TANAHMohr circle for water The stresses at a point within a liquid are equal;they plot at a single point. Thus, liquids have no shear strength. KEKUATAN GESER TANAHStrength Criteria Strength criteria relate to the maximum sustainable shear stress, i.e. the shear stress at failure. For soils, there are really two criteria by which strength (and therefore failure) may be defined, but one of these is modified to provide a third. oTresca - applies to failure in metals and undrained soils oMohr-Coulomb (when c = 0) - applies the critical state failures in soils oMohr-Coulomb (when c > 0) - applies to peak state failures in soils. KEKUATAN GESER TANAHThe Tresca Criterion At any level of normal stress the Mohr circle diameter (oa - or) remains constant. The failure envelope is therefore parallel to the on axis and the strength independent of normal stress. Failure occurs when the Mohr circle increases in diameter to touch the failure envelope: Undrained shear strength, tf=cu(or su)=oa - or=constant Strength criteria Related to undrained condition In terms of total stresses KEKUATAN GESER TANAHThe Mohr-Coulomb (c = 0) Criterion The Mohr-Coulomb (c = 0) criterion relates to the drained critical state strength of soils. Shear strength increases linearly with normal stress; the strength being zero at zero normal stress. As the normal stress increases the Mohr circle diameters increase; circles representing failure stress ultimately touch a straight-line failure envelope. o Critical state shear strength, tf =on tan | o |(or |c)= the critical angle of friction Strength criteria Related to drained critical state strength of soils KEKUATAN GESER TANAHThe Mohr-Coulomb (c > 0) Criterion Strength increases linearly with normal stress, i.e. circle Mohr diameters increase; circles representing failure stress touch a failure envelope. At low stresses the failure envelope is curved, but for practical purpose a straight line is fitted with a practical normal stress range which gives the cohesion intercept on the shear stress axis. Peak state shear strength, tf =c + on tan |p o c=the cohesion intercept o|p= the peak angle of friction Strength criteria Related to drained peak state strength of condition KEKUATAN GESER TANAHThe Mohr-Coulomb (c > 0) Criterion Strength increases linearly with normal stress, i.e. circle Mohr diameters increase; circles representing failure stress touch a failure envelope. At low stresses the failure envelope is curved, but for practical purpose a straight line is fitted with a practical normal stress range which gives the cohesion intercept on the shear stress axis. Peak state shear strength, tf =c + on tan |p o c=the cohesion intercept o|p= the peak angle of friction Strength criteria Related to drained peak state strength of condition BASIC GEOTECHNICAL STRUCTURAL TYPES BASIC GEOTECHNICAL STRUCTURAL TYPES Foundations Retaining walls Slopes BASIC GEOTECHNICAL STRUCTURAL TYPES Foundation: The main variables are load (F), size (B) and founding depth (D). The main design criteria are settlement and stability. Sub-types:shallow, deep, piles; pads, strips, rafts; cellular, caissons BASIC GEOTECHNICAL STRUCTURAL TYPES Retaining Wall: These are basically vertical structures subject to horizontal loading. May be gravity walls, deriving stability from their own weight or embedded walls, which are considered to be weightless. The main variables are depth of support provided (H), depth of embedment (D), base size (B), type of soil. Design criteria include overturning, sliding, cracking, bending, ground-bearing stability and settlement. BASIC GEOTECHNICAL STRUCTURAL TYPES Slope Stability: Basically two types: o Cut slopes: excavations, cuttings - construction decreases loading. o Built slopes: embankments, dams - construction increases loading. Effects of seepage are important - soil strength varies with pore pressures. Both short term and long term stability can be critical. KEKUATAN GESER TANAHKemiringan Bidang Keruntuhan Akibat Geser Tegangannormaldantegangangeser pada bidang runtuh:uo ot 2 sin23 1 =nuo o o oo 2 cos2 23 1 3 1++=no1 o3 on tf tn o1 o3 o3 < o1KEKUATAN GESER TANAHPada saat runtuh: tf = tn|((u |.|\| o o+ |.|\| o o+ = uo otan 2 cos2 2c 2 sin23 1 3 1 3 1atau | u u+ | o+ o = otan cos 2 sinc tan22133 1.. (a) Kriteria keruntuhan Mohr-Coloumb: tf = c + o tan|KEKUATAN GESER TANAHPersamaan di atas memberikan hubungan baru: 2450|+ = uUntukharga-hargao3danctertentu,kondisiruntuhakanditentukanolehharga minimumdariteganganutamabesaro1.Bilahargao1minimum,makaharga (1/2.sin2u-cos2 u.tan|) pada Persamaan (a) haruslah maksimum. Sehingga: ( ) 0 tan cos 2 sindd221= | u uu0 tan . cos . sin 2 sin cos2 2= | u u + u u| u u+ | o+ o = otan cos 2 sinc tan22133 1o1 o3 on tf tn o1 o3 o3 < o12450|+ = uKEKUATAN GESER TANAHGambar disamping menunjukkan gam-baranseparuhlingkaranMohryang mewakilikondisiteganganpadasaat keruntuhanpadasuatumassatanah. Garis keruntuhan yang dinyatakan oleh persamaantf=c+otan|me-nyinggung lingkaran Mohr pada titik X. Jadi,keruntuhangeseryangterjadi padabidangtertentudapatkitanyata-kandenganlingkaranberjari-jariOX, dan bidang tersebut harus membentuk kemiringansudutu=450+|/2ter-hadap bidang utama besar. Lingkaran Mohr dan Garis Keruntuhan KEKUATAN GESER TANAHBilahargau=450+|/2dimasukkankedalamPersamaan(a)dankemudian disederhanakan, akan menghasilkan: ( ) ( )2 223 145 tan . c 2 45 tan .| |+ + + o = oAkantetapi,Persamaan(b)jugadapatdenganmudahditurunkandengan menggunakan lingkaran Mohr dan ilmu ukur sederhana. .. (b) KEKUATAN GESER TANAHBeberapa Cara Penentuan (Pengujian) Kekuatan Geser Tanah: 1. Uji Geser Langsung (direct shear test) 2. Uji Triaxial (triaxial test) 3. Uji Kuat Tekan Bebas (unconfined compressive strength test) 4. Uji Vane Shear 5. Dll. KEKUATAN GESER TANAHUji Geser Langsung Ni Ti batu pori tanah batu pori ring perata beban meja Ni:beban vertikal (normal) Ti:gaya horisontal yang diperlukan untuk menggeser ring (tanah) A:luas penampang tanah si:lintasan yang diperlukan sampai tanah tergeser KEKUATAN GESER TANAHUji Geser Langsung KEKUATAN GESER TANAHPercobaan dengan Menggunakan Pasir AN11 = o Uji 1:Uji 2:Uji 3:AT11 = t;; s1 AN22 = o; ; ; s2 ; s3 AN33 = oAT22 = tAT33 = tHasil Uji: s t tf1 tf2 tf3 o3 o2 o1 o t o1o2o3 tf1 tf=o . tan| tf2 tf3 | | : sudut geser dalam KEKUATAN GESER TANAHPercobaan dengan Menggunakan Lempung | : sudut geser dalam o t o1o2o3 tf1 tf=c + o . tan| tf2 tf3 | c c : kohesi [kN/m2]KEKUATAN GESER TANAHLuas Sample : A = (5.08 * 5.08) cm2 No. Uji Arah NormalArah Geser GayaTeganganGayaTegangan kgkg/cm2 kgkg/cm2 190.3487515.440.210924 2140.5425018.300.32166 3321.24000219.100.739993 4451.74375327.261.05638 UJI GESER LANGSUNG CONTOH TANAH PASIR KEKUATAN GESER TANAHTahanan Gesery = 0.6022x00.20.40.60.811.20 0.5 1 1.5 2Teg. Normal [kg/cm2]Teg. Geser [kg/cm2]| = atan(0.6022) = 310 c = 0 KEKUATAN GESER TANAHUJI GESER LANGSUNG CONTOH TANAH LEMPUNG Diameter Sample : D = 5.0 cm No. UjiArah NormalArah Geser GayaTeganganGayaTegangan kgkg/cm2 kgkg/cm2 1271.37454514.060.715782 2402.03636318.060.919418 3472.39272720.411.039054 4542.74909122.431.141891 KEKUATAN GESER TANAH| = atan(0.312) = 17.320 c = 0.2868 kg/cm Tahanan Gesery = 0.312x + 0.286800.20.40.60.811.21.40 0.5 1 1.5 2 2.5 3Teg. Normal [kg/cm2]Teg. Geser [kg/cm2]KEKUATAN GESER TANAHPengamatan Hasil Uji Geser Langsung: Diagram tegangan geser vs. perubahan tinggi benda uji karena pergerakan menggeser untuk tanah pasir padat dan lepas (uji geser langsung) KEKUATAN GESER TANAHHal umum yang dapat ditarik dari gambar di atas berkaitan dengan variasi tegangan geser penghambat dan perpindahan geser, yaitu: 1. Pada pasir lepas (renggang), tegangan geser penahan akan membesar sesuai denganmembesarnyaperpindahangesersampaitegangantadimencapai tegangangeserruntuhSetelahitu,besartegangangeserakankira-kira konstan sejalan dengan bertambahnya perpindahan geser.2. Padapasirpadat,tegangangeserpenghambatakannaiksejalandengan membesarnya perpindahan geser hingga tegangan geser runtuh (maksimum) tf tercapai.Hargatfinidisebutsebagaikekuatangeserpuncak(peakshear strength).Bilateganganruntuhtelahdicapai,makategangangeser penghambatyangadaakanberkurangsecaralambatlaundengan bertambahnyaperpindahangesersampaipadasuatusaatmencapaiharga konstan yang disebut kekuatan geser akhir maksimum (ultimate shear strength). KEKUATAN GESER TANAHUji Triaxial: PRINCIPLES OF THE TRIAXIAL COMPRESSION TEST The triaxial compression test is used to measure the shear strength of a soil under controlled drainage conditions. In the conventional triaxial test, a cylindrical specimen of soil encased in a rubber membrane is placed in a triaxial compression chamber, subjected to a confining fluid pressure, and then loaded axially to failure. Connections at the ends of the specimen permit controlled drainage of pore water from the specimen. The test is called "triaxial" because the three principal stresses are assumed to be known and are controlled. Prior to shear, the three principal stresses are equal to the chamber fluid pressure. During shear, the major principal stress, o1 is equal to the applied axial stress (P/A) plus the chamber pressure, o3. The applied axial stress, o1 - o3 is termed the "principal stress difference" or sometimes the "deviator stress". The intermediate principal stress, o2 and the minor principal stress, o3 are identical in the test, and are equal to the confining or chamber pressure hereafter referred to as o3. KEKUATAN GESER TANAH1. Consolidated-drainedtestatau drained test (CD test) 2. Consolidated-undrainedtest (CU test) 3. Unconsolidated-undrainedtest atau undrained test (UU test) Tiga tipe standar dari uji triaxial yang biasanya dilakukan: Uji Triaxial: KEKUATAN GESER TANAHUji Triaxial: PENGUJIAN KUAT GESER DENGAN TRIAXIAL Uji Triaxial: cell body load ring cell piston strain dial gauge load dial gauge porous discs 1. Thecellpressureconnection to the chamber 2. Thebackpressure connectiontothetopofthe sample 3. The pore pressure connection Three essential connections to triaxial cell: KEKUATAN GESER TANAHUji Triaxial: TABUNG TRIAXIAL SELANG PENYALUR DAN PENGUKUR TEKANAN (BEBAN) Kran u Aod Aod o3o3 o3 o3 Kran u o3o3 o3 o3 Tahap 1: Confining PressureTahap 2: Shear Pressureo3 : konstan Aod : bertahap sampai runtuh (Aod)fPrinsip Uji Triaxial Pemberian Beban: KEKUATAN GESER TANAHKEKUATAN GESER TANAHConfining PressureShear Pressure Jenis UjiKranTeg. Air Pori (u)KranTeg. Air Pori (u) CDBukau = uc = 0Bukau = uc+Aud = 0 CUBukau = uc = 0Tutupu = uc+Aud = Aud UUTutupu = uc Tutupu = uc+Aud Perbedaan Tipe Standard Pengujian Triaxial KEKUATAN GESER TANAHHasil Uji Triaxial CD Garis keruntuhan untuk tegangan efektif dari uji CD pada pasir dan lempung NC Total = Effective c ~ 0 KEKUATAN GESER TANAHHasil Uji Triaxial CD Garis keruntuhan untuk tegangan efektif dari uji CD pada lempung OC KEKUATAN GESER TANAHContoh 9-2: Hasil uji triaxial cara air teralirkan-terkonsolidasi (CD) pada tanah lempung NC adalah sebagai berikut:o3= 276 kN/m2 (Aod)f= 276 kN/m2 Tentukan: a) Sudut geser, | b) Sudut u (sudut antara bidang keruntuhan dengan bidang utama besar/major principal plane) c) Tegangan normal o dan tegangan geser tf pada bidang keruntuhan KEKUATAN GESER TANAHPenyelesaian: Untuk tanah NC, persamaan garis keruntuhannya adalah: tf = o tan | Pada uji triaxial baik tegangan utama besar maupun kecil pada saat terjadi keruntuhan adalah: o1 = o1 = o3 + ((Aod)f = 276 + 276 = 552 kN/m2 dan, o3 = o3 = 276 kN/m2 KEKUATAN GESER TANAHAtau sin | =333 . 0276 552276 552' '' '3 13 1=+=+o oo o| = 19.45o o o o73 . 54245 . 1945245 = + = + =|u b) a) Lingkaran Mohr dan garis keruntuhan dapat dilihat pada gambar depan, dimana: sin | =(( +(( =2' '2' '3 13 1o oo oOAABKEKUATAN GESER TANAHDengan memasukkan harga o1 = 552 kN/m2, o3=276 kN/m2, dan u = 54,73o di atas akan didapatkan2kN/m 368.03 54.73) cos(22276 5522276 552' = ++=dan, 2fkN/m 130.12 54.73) sin(22276 552 = =c) dengan menggunakan persamaan (6-8) dan (6.9):o (pada bidang keruntuhan) = uo o o o2 cos2' '2' '3 1 3 1++dan,uo ot 2 sin2' '3 1 =fKEKUATAN GESER TANAHContoh 9-4: Dua buah benda uji dari tanah lempung yang sama mula-mula dikonsolidasi dengan tegangan penyekap sebesar 600 kN/m2. Kemudian kedua benda benda uji tersebut diujitriaxialCDdengantekananpenyekapyangberbedadanjauhlebihkecildari tegangan penyekap mula-mula di atas. Hasil kedua uji tadi adalah sebagai berikut:Benda uji 1 :o3=100 kN/m2 (Aod)f =410.6 kN/m2 Benda uji 2 :o3 =50 kN/m2 (Aod)f =384.37 kN/m2 Tentukan parameter-parameter dari kekuatan geser sampel tanah.KEKUATAN GESER TANAHPenyelesaian: Untuk benda uji 2, tegangan-tegangan utamanya adalah: o3 = o3 = 50 kN/m2 o1 = o1 = o3 + ((Aod)f = 50 + 384.37= 434.37 kN/m2 Untuk benda uji 1, tegangan-tegangan utama pada saat runtuh adalah: o3 = o3 = 100 kN/m2 o1 = o1 = o3 + ((Aod)f = 100 + 410.6 = 510.6 kN/m2 KEKUATAN GESER TANAH..(a) |.|\|+ + |.|\|+ =245 tan 2245 tan 100 6 . 5101 1 2| |o ocBenda uji 2: |.|\|+ +|.|\|+ =245 tan 2245 tan 50 37 . 4341 1 2| |o oc..(b) Kedua benda uji ini adalah terkonsolidasi lebih (OC). Jadi, dengan menggunakan hubungan pada Persamaan (9-7): |.|\|+ + |.|\|+ =245 tan 2245 tan ' '1 1 23 1| |o oo ocBenda uji 1: KEKUATAN GESER TANAHDengan memasukkan | =12 ke Persamaan (a), didapatkan : |.|\|+ + |.|\|+ =21245 tan 221245 tan 100 6 . 5102 o oc510.6 = 152.5 + 2.47c c = 145 kN/m2 Bila Persamaan (a) dikurangi Persamaan (b) didapat: |.|\|+ =245 tan 50 23 . 761 2|oo o51223 . 76tan2451 1=((= +|atau| = 12o 50 KEKUATAN GESER TANAHHasil Uji Triaxial CU Garis keruntuhan untuk tegangan total & efektif dari uji CU pada pasir dan lempung NC KEKUATAN GESER TANAHHasil Uji Triaxial CU Garis keruntuhan untuk tegangan total dari uji CU pada lempung OC KEKUATAN GESER TANAHContoh 9-5: Sebuahbendaujidaritanahpasirjenuhairdiberitekananpenyekap(confining pressure)sebesar60lb/in2.Kemudianteganganaksialdinaikkantanpa memperbolehkanterjadinyadrainase(daridankedalambendauji).Bendauji tersebutmencapaikeruntuhanpadasaatteganganaksialmencapai50lb/in2. Tegangan air pori pada saat runtuh adalah 41.35 lb/in2.Tentukan:a) Sudut geser kondisi CU b) Sudut geser kondisi CD KEKUATAN GESER TANAHPenyelesaian: bagian a) Pada saat runtuh, o3 = 60 lb/in2 o1 = o1 = o3 + (Aod)f = 60 + 50 = 110 lb/in2(Aod) f = (Aod)failure = (Aod)pada saat runtuh Dari gambar: sin |(cu) =| || | ' '' '3 13 1o oo o+=OAAB| || |294 . 01705060 11060 110= =+= atau |(cu) =17.1oKEKUATAN GESER TANAHPenyelesaian: bagian b) o3 = o3 - (Aud)f = 60 41.35 = 18.65 lb/in2 o1 = o1 - (Aud)f = 110 41.35 = 68.65 lb/in2 sin |(CD) =| || | ' '' '3 13 1o oo o+=OAAB| || |5727 . 03 . 875065 . 18 65 . 6865 . 18 65 . 68= =+=|(CD) = 34.94oKEKUATAN GESER TANAHHasil Uji Triaxial UU Lingkaran Mohr untuk tegangan total dan garis keruntuhan dari uji UU KEKUATAN GESER TANAHHasil Uji Triaxial UU Contoh Kasus Penggunaan Paramater CD: KEKUATAN GESER TANAHKEKUATAN GESER TANAHContoh Kasus Penggunaan Paramater CD: KEKUATAN GESER TANAHContoh Kasus Penggunaan Paramater CU: KEKUATAN GESER TANAHContoh Kasus Penggunaan Paramater CU: KEKUATAN GESER TANAHContoh Kasus Penggunaan Paramater CU: KEKUATAN GESER TANAHContoh Kasus Penggunaan Paramater UU: KEKUATAN GESER TANAHContoh Kasus Penggunaan Paramater UU: KEKUATAN GESER TANAHContoh Kasus Penggunaan Paramater UU: COMPARISON OF THE TRIAXIAL AIND THE DIRECT SHEAR TEST The advantages of the triaxial test over the direct shear test are: Progressive effects are less in the triaxial. The measurement of specimen volume changes are more accurate in the triaxial. The complete state of stress is assumed to be known at all stages during the triaxial test, Whereas only the stresses at failure are known in the direct shear test. The triaxial machine is more adaptable to special requirements. The advantages of the direct shear test are: Direct shear machine is simpler and faster to operate. A thinner soil sample is used in the direct shear test, thus facilitating drainage of the pore-water from a saturated specimen. COMPARISON OF THE TRIAXIAL AIND THE DIRECT SHEAR TEST KEKUATAN GESER TANAHUji Kuat Tekan Bebas uufcq= = =2 21otKEKUATAN GESER TANAHUji Vane Shear T h d T T = Me + Ms + MeT:momen torsiMe:momen tahanan pada muka atasdan bawah silinder runtuhMs: momen tahanan pada dinding silinder runtuh ) ( c dh) ( M2du st =8du e3c M | t =Dimana:d:diameter baling-baling h:tinggi baling-baling KEKUATAN GESER TANAHa) | = tahanan geser termobilisasi dianggap berbentuk segi tiga b) | = 2/3 tahanan geser termobilisasi dianggap seragam c) | = 3/5 tahanan geser termobilisasi dianggap berbentuk parabola cu d/2d/2d/2d/2 d/2d/2 T = Me + Ms + Me||.|\|| + t =4d2h dc .3 2u||.|\|| + t=4d2h dTc3 2u Uji Vane Shear KEKUATAN GESER TANAHUji Vane Shear VANE SHEAR LABORATORIUM VANE SHEAR LAPANGAN KEKUATAN GESER TANAHFAKTOR-FAKTOR YANG MEMPENGARUHI BESARNYA KEKUATAN GESER TANAH: 1. Keadaan tanah: ukuran butiran, angka pori, bentuk 2. Jenis tanah: kerikil, pasir, lanau, lempung, berpasir, berlempung 3. Kadar air: terutama pada lempung 4. Jenis dan tingkat beban: pembebanan yang terlalu cepat menghasilkan tekanan air pori yang berlebih 5. Anisotropis: kuat geser arah tegak lurus berbeda dengan arah sejajar bidang geser KEKUATAN GESER TANAHFAKTOR-FAKTOR YANG MEMPENGARUHIHASIL UJI KUAT GESER DI LABORATORIUM: 1. Metoda pengujian 2. Derajat ketergangguan contoh tanah 3. Kadar air contoh tanah saat diuji 4. Tingkat reganganKEKUATAN GESER TANAHMETODA EMPIRIS PENENTUAN KUAT GESER: (korelasi cukup memadai untuk rentang harga-harga LL = 20 45 dan PI = 15 30) Index Plastisitas, PI [%] Sudut Geser Dalam.| [..0] 0 30 20 20 10 40 406080100 0 Lempung KEKUATAN GESER TANAH020406080100 30 20 10 40 0 Sudut Geser Dalam.| [..0] Persentase Lempung [% < 0.002 mm] Batas Nilai METODA EMPIRIS PENENTUAN KUAT GESER: Pasir