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

Transkripsi

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

2 Determination of shear strength parameters of soils (c, f or c, f ) Percobaan Laboratorium menggunakan sampel tanah tidak terganggu (undisturbed sample) Percobaan Lapangan Percobaan lab yang umum, 1.Direct shear test 2.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 1. Vane shear test 2. Torvane 3. Pocket penetrometer 4. Fall cone 5. Pressuremeter 6. Static cone penetrometer 7. Standard penetration test

3 Percobaan Laboratorium Kondisi Lapangan Model sampel tanah s vc z s vc + Ds z s hc s hc s hc s hc s vc s vc + Ds Sebelum konstruksi Selama dan sesudah konstruksi

4 Percobaan Laboratorium Simulasi kondisi lapangan di laboratorium 0 s vc s hc s vc + Ds s hc s vc + Ds 0 0 s hc s hc s vc t 0 Model sampel tanah dari lapangan Step 1 s vc Atur alat tes dan sampel sedemikian rupa untuk kondisi tegangan awal t Step 2 s vc Berikan tegangan seperti yang akan diberikan dilapangan

5 Direct shear test Skema alat percobaan direct shear

6 Direct shear test 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 Porous plates Komponen dari shear box Persiapan sampel pasir

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

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

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

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

11 Direct shear test Analisis hasil percobaan s Normalstress Areaof Normalforce(P) crosssection of thesample t Shear stress Shear resistancedevelopedat thesliding surface(s) Areaof crosssection of thesample Note: Cross-sectional area of the sample changes with the horizontal displacement

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

13 Shear stress, t Direct shear tests pada pasir Cara menentukan parameter kuat geser c dan f Normal stress = s 3 Normal stress = s 2 t f3 t f2 t f1 Normal stress = s 1 Shear displacement Shear stress at failure, t f f Garis keruntuhan Mohr Coulomb Normal stress, s

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

15 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, t f f Normally consolidated clay (c = 0) Normal force, s Overconsolidated clay (c 0)

16 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) P Soil S Foundation material t f c a s' tan Where, c a = adhesion, = angle of internal friction

17 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.

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19 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) Gambar garis selubung kegagalannya. Tentukan nilai dari parameter kuat gesernya.

20 Penyelesaian s Normalstress Areaof Normalforce(P) crosssection of thesample t Shear stress Shear resistancedevelopedat thesliding surface(s) Areaof crosssection of thesample Percobaan Tegangan Normal (kn/m2) Tegangan Geser(kN/m2)

21 Shear stress at failure, t f (kn/m2) Penyelesaian Normal stress, s (kn/m2)

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

23 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

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

25 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

26 Torvane Torvane is a modification to the vane

27 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

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

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

30 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

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

32 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

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

34 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)

35 Standard Penetration Test, SPT SPT is the most widely used test procedure to determine the properties of in-situ soils 63.5 kg 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 Various correlations have been developed to determine soil strength parameters 0.76 (c, m f, ect) from N The test can be conducted at every 1m vertical intervals Drill rod 0.15 m 0.15 m 0.15 m Number of blows = N 1 Number of blows = N 2 Number of blows = N 3 Standard penetration resistance (SPT N) = N 2 + N 3

36 Standard Penetration Test, SPT SPT (Manual operation)

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