Kuat Geser Tanah Direct Shear Test (Uji Geser Langsung) Reza P. Munirwan, ST, M.Sc

Determination of shear strength parameters of soils (c, f or c’, f’) Percobaan Laboratorium menggunakan sampel tanah tidak terganggu (undisturbed sample) Percobaan Lapangan Vane shear test Torvane Pocket penetrometer Fall cone Pressuremeter Static cone penetrometer Standard penetration test Percobaan lab yang umum, Direct shear test Triaxial shear test Percobaan laboratorium lainnya, Direct simple shear test, torsional ring shear test, plane strain triaxial test, laboratory vane shear test, laboratory fall cone test

Percobaan Laboratorium z svc + Ds shc Selama dan sesudah konstruksi Kondisi Lapangan z svc shc Sebelum konstruksi Model sampel tanah

Percobaan Laboratorium svc + Ds shc Traxial test Percobaan Laboratorium Simulasi kondisi lapangan di laboratorium Model sampel tanah dari lapangan Step 1 Atur alat tes dan sampel sedemikian rupa untuk kondisi tegangan awal svc shc svc t Direct shear test Step 2 Berikan tegangan seperti yang akan diberikan dilapangan

Skema alat percobaan direct shear Direct shear test Skema alat percobaan direct shear

Direct shear test Persiapan sampel pasir Porous plates Direct shear test adalah percobaan yang paling sesuai untuk kondisi percobaan consolidated drained khususnya pada tanah granular (contoh: pasir/sand) atau lempung keras/stiff clays Persiapan sampel pasir Komponen dari shear box Persiapan sampel pasir Porous plates

Direct shear test Persiapan sampel pasir Pressure plate Specimen preparation completed Pressure plate Leveling the top surface of specimen

Direct shear test Steel ball P Prosedur Percobaan Pressure plate Step 1: Apply a vertical load to the specimen and wait for consolidation P Pressure plate Porous plates Proving ring to measure shear force S

Direct shear test Steel ball P Prosedur Percobaan Pressure plate Step 1: Apply a vertical load to the specimen and wait for consolidation P Prosedur Percobaan Pressure plate Steel ball Proving ring to measure shear force S Porous plates Step 2: Lower box is subjected to a horizontal displacement at a constant rate

Direct shear test Shear box Loading frame to apply vertical load Dial gauge to measure vertical displacement Shear box Proving ring to measure shear force Dial gauge to measure horizontal displacement Loading frame to apply vertical load

Direct shear test Analisis hasil percobaan Note: Cross-sectional area of the sample changes with the horizontal displacement

Direct shear tests pada pasir Stress-strain relationship Shear stress, t Shear displacement Dense sand/ OC clay tf Loose sand/ NC clay tf Change in height of the sample Expansion Compression Shear displacement Dense sand/OC Clay Loose sand/NC Clay

Direct shear tests pada pasir Cara menentukan parameter kuat geser c dan f Shear stress, t Shear displacement tf3 Normal stress = s3 tf2 Normal stress = s2 tf1 Normal stress = s1 Shear stress at failure, tf Normal stress, s f Garis keruntuhan Mohr – Coulomb

Direct shear tests pada pasir Hal penting menggenai parameter kuat geser c and f pada pasir Percobaan Direct shear adalah drained dan tekanan air pori terdissipasi, maka u = 0 Pasir = tanah non-kohesi, maka c = 0 Kesimpulan, f’ = f dan c’ = c = 0

Direct shear tests pada lempung In case of clay, horizontal displacement should be applied at a very slow rate to allow dissipation of pore water pressure (therefore, one test would take several days to finish) Failure envelopes for clay from drained direct shear tests Shear stress at failure, tf Normal force, s f’ Normally consolidated clay (c’ = 0) Overconsolidated clay (c’ ≠ 0)

Interface tests on direct shear apparatus In many foundation design problems and retaining wall problems, it is required to determine the angle of internal friction between soil and the structural material (concrete, steel or wood) Where, ca = adhesion, d = angle of internal friction

Kelebihan percobaan direct shear Drainase yang cepat dapat diperoleh, karena ketebalan sampel percobaan yang kecil. Dapat digunakan untuk menggetahui parameter kuat geser pada pertemuan tanah. Kekurangan percobaan direct shear Arah bidang keruntuhan sudah diketahui. Luas permukaan sliding berubah seiring dengan berjalannya proses percobaan. Ketidak seragaman distribusi kuat geser sepanjang bidang runtuh.

Lets do some examples

Beban Geser pada Keruntuhan (kN) Contoh Soal Hasil-hasil pengujian yang diperoleh dari pengujian geser langsung (direct shear) dengan ukuran sampel 60mm x 60mm pada tanah lempung berpasir sebagai berikut: Percobaan Beban Normal (kN) Beban Geser pada Keruntuhan (kN) 1 0.20 0.205 2 0.40 0.26 3 0.80 0.355 Gambar garis selubung kegagalannya. Tentukan nilai dari parameter kuat gesernya.

Tegangan Normal (kN/m2) Penyelesaian Percobaan Tegangan Normal (kN/m2) Tegangan Geser(kN/m2) 1 56 57 2 111 72 3 223 99

Penyelesaian Shear stress at failure, tf (kN/m2) Normal stress, s (kN/m2)

In-situ shear tests Vane shear test Torvane Pocket Penetrometer Pressuremeter Static Cone Penetrometer test (Push Cone Penetrometer Test, PCPT) Standard Penetration Test, SPT

In-situ shear tests Vane shear test (suitable for soft to stiff clays) Torvane Pocket Penetrometer Pressuremeter Static Cone Penetrometer test (Push Cone Penetrometer Test, PCPT) Standard Penetration Test, SPT

Vane shear test This is one of the most versatile and widely used devices used for investigating undrained shear strength (Cu) and sensitivity of soft clays Applied Torque, T PLAN VIEW Vane D H Disturbed soil Rupture surface Bore hole (diameter = DB) h > 3DB) Vane T Rate of rotation : 60 – 120 per minute Test can be conducted at 0.5 m vertical intervals

In-situ shear tests Vane shear test Torvane (suitable for very soft to stiff clays) Pocket Penetrometer Pressuremeter Static Cone Penetrometer test (Push Cone Penetrometer Test, PCPT) Standard Penetration Test, SPT

Torvane Torvane is a modification to the vane

In-situ shear tests Vane shear test Torvane Pocket Penetrometer (suitable for very soft to stiff clays) Pressuremeter Static Cone Penetrometer test (Push Cone Penetrometer Test, PCPT) Standard Penetration Test, SPT

Pocket Penetrometer Pushed directly into the soil. The unconfined compression strength (qu) is measured by a calibrated spring.

Swedish Fall Cone (suitable for very soft to soft clays) Cu ∞ Mass of the cone ∞ 1/(penetration)2 Soil sample The test must be calibrated

In-situ shear tests Vane shear test Torvane Pocket Penetrometer Pressuremeter (suitable for all soil types) Static Cone Penetrometer test (Push Cone Penetrometer Test, PCPT) Standard Penetration Test, SPT

Pressuremeter Air Coaxial tube Water Pre – bored or self – bored hole Guard cell Measuring cell

In-situ shear tests Vane shear test Torvane Pocket Penetrometer Pressuremeter Static Cone Penetrometer test (Push Cone Penetrometer Test, PCPT) (suitable for all soil types except very course granular materials) Standard Penetration Test, SPT

Static Cone Penetrometer test 40 mm Static Cone Penetrometer test 40 mm 40 mm 40 mm Cone penetrometers with pore water pressure measurement capability are known as piezocones

In-situ shear tests Vane shear test Torvane Pocket Penetrometer Pressuremeter Static Cone Penetrometer test (Push Cone Penetrometer Test, PCPT) Standard Penetration Test, SPT (suitable for granular materials)

Standard Penetration Test, SPT SPT is the most widely used test procedure to determine the properties of in-situ soils Number of blows for the first 150 mm penetration is disregarded due to the disturbance likely to exist at the bottom of the drill hole 63.5 kg 0.76 m Various correlations have been developed to determine soil strength parameters (c, f, ect) from N The test can be conducted at every 1m vertical intervals 0.15 m Number of blows = N1 Number of blows = N2 Number of blows = N3 Drill rod Standard penetration resistance (SPT N) = N2 + N3

Standard Penetration Test, SPT SPT (Manual operation)

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