Internal soil erosion in the form of piping causes excessive seepage and progressively enlarged channels inside levees and earth dams. This paper reports the experimental study on the effect of piping on soil's shear strength, which can be used to realistically evaluate the stability of existing levees and earth dams that have been subjected to prolonged erosion. Triaxial compression tests are performed on a sandy clay soil. A soil specimen (10.2cm in diameter) without piping channel is first tested as a control sample. Then a single hole is formed using a mini hand auger in the soil during the specimen preparation to simulate a piping channel. Three piping inclinations are studied: vertical, inclined, and horizontal. For each piping channel inclination, three hole diameters are tested. After saturation, consolidated-undrained (CU) compression tests are performed to simulate the slow piping formation followed by a quick hydraulic loading during a flooding. For each channel inclination and diameter, three-stage compressions on the same specimen are conducted to obtain three Mohr's circles in order to obtain the cohesion and the internal friction angle of the soil element with piping. The experiments showed deteriorating effect of piping on effective internal friction angle (φ'). φ' decreases with the increase of diameter of vertical holes. It was also observed that φ' is not affected by the channel diameters when the piping channel is horizontal or inclined, although φ' does decrease by an average of 28 of specimens with horizontal and inclined holes when compared with that of the control specimen.