Hydraulic fracturing stress measurements in Hong Kong

Since 1990, a total of 149 hydrofrac stress measurements to about 200 m depth were conducted in 18 boreholes as part of several geotechnical site investigation programs in the Hong Kong area. The in situ tests were carried out by using the wireline hydrofrac technique to move the straddle packer tool within the 76 mm or 101 mm diameter boreholes. Although the tests were performed both in fractured and unfractured crystalline rocks and the boreholes are located in areas of pronounced topographic relief, the results yield a consistent orientation of the maximum horizontal stress of N 108°±28°. Above 150 m depth, the vertical stress S_v due to the weight of the overburden with given rock density is the minimum principal stress, while the few deeper data available suggest that the minimum horizontal principal stress is the least principal stress. The derived stress magnitudes can be reasonably summarized by the following normalized stress-ratios:

where z is the depth in meters and S_h and S_H are the minimum and maximum horizontal principal stresses. Due to the considerable scatter of the stress data at shallow depth above 100 m, it is suggested that further detailed in situ stress measurements be undertaken in areas where large-scale underground excavations are planned.