.. Pengujian Triaksial

Pengujian ini adalah salah satu pengujian yang terpenting dalam mekanika batuan untuk menentukan kekuatan batuan di bawah tekanan triaksial. Percontoh yang digunakan berbentuk silinder dengan syarat-syarat sama pada pengujian kuat tekan. Dari hasil pengujian triaksial dapat ditentukan :

- Strength envelope (kurva instrinsic) atau selubung kekuatan

- Kuat geser atau shear strength

- Sudut geser dalam,φ

- Kohesi, C

• Triaxial test– Young modulus– Poisson’s ratio

– Shear strength (cohesion, angle of friction)

Triaxial Shear Strength

Apparatus and ProceduresGeneral description is given in Section 2.4.4 and, as applicable to rock testing, in Section2.5.3 and in Table 2.28.

Strength Values

Studies have been made relating analysis of petrographic thin sections of sandstone to estimates of the triaxial compressive strength (Fahy and Guccione, 1979). Relationships have been developed for approximating peak strengths for rock masses (Hoek and Brown, 1980).

Triaxial Shear Test

Purpose

Total or effective stress parameters, either in compression or extension, are measured in the triaxial shear apparatus. The test method is generally unsuited for measuring ultimate strength because displacement is limited and testing parallel to critical surfaces is not convenient. Apparatus includes a compression chamber to contain the specimen (Figure 2.49) and a system to apply load under controlled stress or strain rates, and to measure load, deflection, and pore pressures (Figure 2.50). Specimens are usually 2.78 in. in diameter as extruded from a Shelly tube, or 1.4 in. in diameter as trimmed from an undisturbed sample. Specimen height should be between 2 and 2.6 times the diameter.

General Procedures

Rate of loading or strain is set to approximate field-loading conditions and the test is runto failure (Bishop and Henkel, 1962). Confining pressures: Tests are usually made on three different specimens with the same index properties to permit defining a Mohr diagram. Test method variations are numerous. Specimens can be preconsolidated or tested at their stress conditions as extruded. Tests can be performed as drained or undrained, or in compression or extension. The various methods, parameters measured, and procedures are summarized in Table 2.21. CU tests are covered in ASTM D4767-02.

(c) Uji Kekuatan Triaksial

Uji kekuatan triaksial bertujuan untuk mengetahui karakteristik kekuatan tanah yang mencakup informasi rinci pengaruh tekanan lateral, tekanan air pori, drainase dan konsolidasi. Uji dapat dilakukan dengan mengacu pada standar uji SNI 03-4813 Metode Pengujian Triaksial Untuk Tanah Kohesif Dalam Keadaan Tanpa Konsolidasi dan Drainase, SNI 03-2455 Metode Pengujian Triaxial A, SNI 03-2815-1992 Metode Pengujian Triaxial B atau ASTM D 2850 Test Method for Unconsolidated, Undrained Compressive Strength of Cohesive Soils in Triaxial Compression, D 4767 Test Method for Consolidated-Undrained Triaxial Compression Test on Cohesive Soils.

8) Triaxial Test

(a) Triaxial A (Tanah)

(1) Alat ukur beban aksial mempunyai ketelitian ± 1,0 %

(2) Kecepatan gerak vertikal mempunyai ketelitian ± 0,1 %

(3) Piston harus dijaga agar tetap sentri dengan penyimpangan tidak melebihi 1,3 mm

(4) Alat ukur perubahan volume mempunyai ketelitian ukur ± 0,05 %

(5) Ketelitian alat ukur deformasi ± 0,02 %

(6) Ketelitian alat ukur panjang dan diameter tanah ± 0,1 %

(7) Ketelitian timbangan ± 0,05% dari massa contoh

(b) Triaxial B (Batu)

(1) Kecepatan pembebanan tidak boleh berdeviasi lebih dari 10% dari kecepatan yang ditentukan

(2) Kecepatan pembebanan untuk uji tekan satu arah harus dipilih antara 2 dan 15 menit

(3) Ketelitian alat ukur deformasi dan regangan minimal 0,0025 mm