Due to the growth of the economy in Thailand, the electricity transmission system must be regularly maintained so that electricity is effectively and safely distributed for uses in daily life and industries. Natural disasters often cause damage to transmission towers. In addition, some damage has occurred due to vandalisms and soil excavations near transmission towers. Foundation scouring and tilting typically occur at transmission towers. In practice, repair of damage to a transmission tower typically begins with transferring the foundation pressure of the damaged tower to other firm foundation soil using a guy fixed to a guy anchor. The transmission tower is then restored to its original condition. In this study, a series of field anchorage strength tests were performed in the Bangkok metropolitan region which is well-known for its very soft to soft clay deposits. Additionally, Kunzelstab penetration tests were performed to evaluate the undrained shear strengths of the clay with depth at the test locations. The measured ultimate anchorage strengths were then compared with values estimated theoretically on the basis of measured undrained shear strengths. The measured values were found to be approximately 70 % of the theoretical values. This was probably due to the clay disturbance caused by anchor installation. Based on this fact, the ultimate anchorage strengths were then estimated for other clay conditions.