Abstract
A 750m deep borehole was drilled in Tsukuba city for an underground water level obeservation well. Using this borehole, hydraulic fracturing test, ASR (Anelastic Strain Recovery) tests and DSCA (Differential Strain Curve Analysis) tests were performed at a depth around 670m, to measure in-situ underground stress. From a break down pressure of 49MPa, we estimated the maximum and minimum horizontal principal stresss were about 40MPa and 30MPa, respectively. In ASR tests, the maximum and intermediate principal strains were located horizontally and the minimum principal strain was located vertically. Assuming the vertical stress was overburden pressure (12MPa), the maximum and minimum horizontal principal stresses were 21MPa and 15MPa, respectively.
The underground stresses around Tsukuba city is in the condition that two principal horizontal stresses are greater than a vertical stress. This underground stress condition is different from the results of stress relieving method conducted on many underground openings in Japan.
