SOIL MECHANIC 2 - kuliahonline.unikom.ac.id
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SOIL MECHANIC 2
Kuat Geser Tanah (2)
Sherly Meiwa , ST., MT
Department of Civil Engineering
Universitas Komputer Indonesia
Bandung, 2020
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Kekuatan Geser Tanah (Shear Strength)
Tujuan Pembelajaran
• Dapat menjelaskan kriteria keruntuhan serta konsep kuat geser tanah
• Dapat menjelaskan prosedur pengujian kuat geser tanah dan interpretasi
hasilnya
Pokok Bahasan
• Kriteria keruntuhan tanah
• Kohesi dan sudut geser dalam tanah
• Uji kuat Geser Tanah di Laboratorium dan interpretasinya
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Kekuatan Geser Tanah
Definisi
Perlawanan internal tanah tiap satuan luas terhadap keruntuhan atau
pergeseran sepanjang bidang runtuh dalam satu elemen tanah.
Tujuan Studi Kuat Geser Tanah
Untuk analisis masalah kestabilan tanah seperti: daya dukung pondasi;
stabilitas lereng (landslide stability); tekanan tanah aktif/pasif pada konstruksi
dinding penahan tanah (turap, retaining wall)
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s
Kekuatan geser tanah terbentuk oleh 3 komponen:
1. Komponen gesekan/friksi
2. Kohesi dan Adhesi
3. Interlocking antar partikel tanah
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Dasar Teori
Hukum Gesekan Newton
Berat Sendiri
Tegangan Normal
Tegangan Geser
Gaya
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Kriteria Keruntuhan (Mohr-Coulomb)
Mohr (1980)
Berat Sendiri
Tegangan Normal
Tegangan Geser
Gaya
Keruntuhan terjadi pada suatu material
akibat kombinasi kritis antara
tegangan normal dan geser
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Kriteria Keruntuhan (Mohr-Coulomb)
𝑊 = 𝑁
W
∅′
T
tan∅′ =𝑇
𝑊
Dalam tegangan
tan∅′ =𝑇/𝐴
𝑊/𝐴
tan∅′ =τ
σ
τ = σ tan ∅′
𝝉 = 𝒄 + 𝝈 𝐭𝐚𝐧 ∅′
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Kriteria Keruntuhan (Mohr-Coulomb)
Kriteria keruntuhan Mohr-Coulomb
Keruntuhan pada Material diakibatkan oleh kombinasi kritis antara teg. normal
dan geser serta sifat-sifat mekanis tanah yang dinyatakan dalam bentuk:
(Gaya/luas kN/m2)
(Gaya/luas kN/m2)
(Gaya/luas kN/m2)
(degrees atau o)
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Kriteria Keruntuhan (Mohr-Coulomb)
∅
𝒄tf
σ
Friction
angle
Cohesion
t
σ
tf adalah tegangan geser maksimum tanah pada
tegangan normal s sebelum terjadi keruntuhan
(Kondisi tegangan total)
𝝉 = 𝒄 + 𝝈 𝐭𝐚𝐧 ∅′
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Kriteria Keruntuhan (Mohr-Coulomb)
∅
𝒄tf
σ
Sudut geser
Kohesif
t
σ
(Kondisi tegangan total)
𝝉 = 𝒄 + 𝝈 𝐭𝐚𝐧 ∅′
tf adalah tegangan geser maksimum tanah pada
tegangan normal s sebelum terjadi keruntuhan
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Kriteria Keruntuhan (Mohr-Coulomb)
∅’
𝒄′tf '
σ ′
Sudut geser efektif
Kohesif
efektif
t
σ
tf ’adalah tegangan geser maksium
tanah pada tegangan normal efektif s’
sebelum terjadi keruntuhan
(Kondisi tegangan efektif)
𝝉′ = 𝒄′ + 𝝈′ 𝐭𝐚𝐧 ∅′
σ ′ = σ − uu = tegangan air pori
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Kriteria Keruntuhan (Mohr-Coulomb)
∅’
𝒄′Kohesif
efektif
t
σ
Kuat geser terdiri atas dua komponen:
kohesif dan friksi
𝝉′ = 𝒄′ + 𝝈′ 𝐭𝐚𝐧 ∅′
σ ′ = σ − uu = tegangan air pori𝝈′𝐟 𝐭𝐚𝐧 ∅′
𝒄′
𝝈′𝐟
Komponen kohesif
Komponen friksi
c dan f adalah parameter dalam kuat geser tanah. semakin
tinggi nilainya, semakin besar kuat geser tanah tersebut.
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Keruntuhan Geser pada Tanah
Tanah umumnya runtuh akibat geser
Dinding Penahan
Tanah
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Keruntuhan Geser pada Tanah
Tanah umumnya runtuh akibat geser
Dinding Penahan
Tanah Tahanan geser
yang termobilisir
Bidang runtuh
Saat terjadi keruntuhan, tegangan geser di permukaan keruntuhan telah
mencapai kekuatan gesernya (tahanan geser yang termobilisir)
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Keruntuhan Geser pada Tanah
Tanah umumnya runtuh akibat geser
Dinding Penahan
TanahTahanan geser
yang termobilisir
Saat terjadi keruntuhan, tegangan geser di bidang runtuh telah mencapai
kekuatan gesernya (tahanan geser yang termobilisir)
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Mekanisme Keruntuhan Geser
Butiran tanah bergeser satu
sama lain sepanjang bidang
keruntuhan tanpa adanya
butiran tanah yang hancurFailure Surface
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Mekanisme Keruntuhan Geser
Saat terjadi keruntuhan,
tegangan geser sepanjang
bidang runtuh (t) telah
mencapai kuat gesernya (tf)
s t
t
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Tegangan pada Lingkaran Mohr
s’Elemen Tanah
t’
q
s ’1
s ’1
s ’3 s ’3
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Lingkaran Mohr dan Selubung Keruntuhan
Elemen Tanah pada beberapa lokasi
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X
Y
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Lingkaran Mohr dan Selubung Keruntuhan
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Lingkaran Mohr dan Selubung Keruntuhan
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Lingkaran Mohr dan Selubung Keruntuhan
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Arah Bidang Runtuh
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s’
t’
q
s ’1
s ’1
s ’3 s ’3(90 – q)
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Arah Bidang Runtuh
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(90 – q)q
(180 – q)
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Arah Bidang Runtuh
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(90 – q)q
(180 – q)
f
90 – q
(180 – q)
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Arah Bidang Runtuh
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f
90 – q
(180 – q)
0o = f + 90o – 2q
180o = f + (90o – q) + (180o – q)
2q = f + 90o
q = f/2 + 45o
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Arah Bidang Runtuh
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f
90 – q
(180 – q)
0o = f + 90o – 2q
180o = f + (90o – q) + (180o – q)
2q = f + 90o
q = f/2 + 45o
q
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Pengujian parameter kuat geser tanah
1. Uji Geser Langsung (Direct Shear Test)
2. Uji Triaxial (Triaxial Test)
3. Uji Kuat Tekan Bebas (Unconfined
Compressive Strength Test)
Pengujian
Laboratorium
Pengujian
Lapangan
1. Uji Vane shear
2. Pressure Meter
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Laboratory Test
shc shc
svc
svcRepresentative
soil sample
shc shc
svc z
Before Construction After and during Construction
+ Ds
svc + Ds
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Laboratory Test
Simulating Field Condition
in the Laboratory
0
0
0
0
Representative
soil sample
taken from the site
shc shc
svc
svc
1. Set the specimen
In the apparatus
and apply the initial
Stress condition
shc shc
svc + Ds
svc + Ds
t
svc
svc
t
0 0
svc + Ds
svc + Ds
2. Apply the
Corresponding field
stress condition
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Aplikasi
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Unconfined Compression Test
0 0
svc + Ds
svc + Ds
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Uji Geser Langsung (Direct Shear Test)
Ni : Beban Vertikal (normal)
Ti. : Gaya horisontal yang diperlukan untuk
menggeser ring (tanah)
A : luas penampang tanah
Si : lintasan yang diperlukan sampai tanah
tergeser
t
svc
svc
t
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Direct Shear Test pada Tanah Pasir
Hasil Uji
= sudut geser dalam
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Direct Shear Test pada Tanah Lempung
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Uji Geser Langsung (Direct Shear Test)
Contoh Tanah Pasir
Luas Sample: A = (5.08*5.08) cm2
No. Uji
Arah Normal Arah Geser
Gaya Tegangan Gaya Tegangan
kg kg/cm2 kg kg/cm2
1 9 0.348751 5.44 0.210924
2 14 0.542501 8.30 0.32166
3 32 1.240002 19.10 0.739993
4 45 1.743753 27.26 1.056638
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Uji Geser Langsung (Direct Shear Test)
Φ = a tan(0,6022) = 31o
c = 0
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Uji Geser Langsung (Direct Shear Test)
Contoh Tanah Pasir
Diameter Sample: D = 5.08 cm
No. Uji
Arah Normal Arah Geser
Gaya Tegangan Gaya Tegangan
kg kg/cm2 kg kg/cm2
1 27 1.374545 14.06 0.715782
2 40 2.036363 18.06 0.919418
3 47 2.392727 20.41 1.039054
4 54 2.749091 22.43 1.141891
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Uji Triaxial (Triaxial Test)
Tiga tipe standard dari uji triaxial
yang biasanya dilakukan
1. Consolidated-drained test atau drained
test (CD test)
2. Consolidated-undrained test (CU test)
3. Unconsolidated-undrained test atau
undrained test (UU test)
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Uji Triaxial (Triaxial Test)
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Uji Triaxial (Triaxial Test)
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Uji Triaxial (Triaxial Test)
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Uji Triaxial (Triaxial Test)
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Prinsip Uji Triaxial
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Perbedaan Tipe Standard Pengujian Triaxial
Jenis Uji
Confining Pressure Shear Pressure
Kran Teg. Air Pori (u) Kran Teg. Air Pori (u)
CD Buka u = uc = 0 Buka u = uc + Δud = 0
CU Buka u = uc = 0 Tutup u = uc + Δud = 0
UU Tutup u = uc Tutup u = uc + Δud