Various typical results are introduced of in-situ stress measurements by overcoring tests during last two decades. The cases can be divided into three types: in-situ stress in lower strength rocks, in-situ stress in higher strength rocks, in-situ stress in high stress region. The deepest borehole is 90m in depth. The measured highest and the lowest (tensile) natural stresses are 64.6MPa and 1MPa. Some general laws of natural stress in the shallow earth crust are drawn from the above results. The relationship between natural stress and Young''s moduli of rocks and the influence of natural stresses on rock engineering are discussed.
On donne les différents résultats typiques des mesures des contraintes in-situ obtenues par l''essai de surcarottage pendant les dernières deux décades. Les cas se divisent en trois catégories: les contraintes in-situ dans le massif rocheux avec base resistance, celles dans le massif rocheux avec haute resistance, et celles dans les régions de hautes contraintes. Les contraintes naturelles maximale et minimale sont 64.6MPa et 1MPa (tension). Quelques lois sont decouvertées concernant la distribution des contraintes naturelles dans la partie superficielle de la croûte terrestre. Le rapport entre les contraintes naturelles et les modules d''elasticité et l''influence des contraintes naturelles sur le génie géomechanique sont discutés dans ce papier.
Im vorliegenden Beitrag werden die typische Ergebnise während der letzten Jahrzehnten von den verschiedenen in-situ Spannungsmessungen mittels der Uberkernnungstechnik berichtet und in den 3 Arten: die in-situ Spannung in klein-Festigkeit Felsen, die in-situ Spannung in groβ-Festigkeit Felsen und die in-situ Spannung in groβ-Spannung Bereiche sich teilt. Die Teufe der tiefsten Bohrung beträgt 90m. Die höchste und niedrigste Spannung sind 64.6MPa und 1MPa. Aus den obigen resultate wird die allgemeine Gesetze für Primärspannung in der oberflächlichen Erdrinde formuliert. Anschlieβend werden die Beziehung zwichen der Primärspannung und des E-Moduls des Felsen und der Einfluβ der Primärspannung auf Felsbau disskutiert.
In-situ stress measurements have been carried out in our institute since 1964. Some kinds of methods and instruments have been employed, such as partial stress relief, stress recovery, over coring technique and strain gauge, vibrating wire, borehole deformeter, etc.. After a great mass of lab and field tests, overcoring technique with a borehole deformeter and bioaxial apparatus has been selected as the better one. The details about it can be found elsewhere (Zhu 1985). This method has successfully been adopted by many institutes and engineering departments in about twenty sites in China (Fig. l). The examples introduced in this paper are mainly abstracted from the tests which were carried out by the authors. The terminology and explanation used to describe in-situ stress are various, some are similar and some are different. The terminology quoted here comes from Bielenstein & Barron''s suggestion (Bielenstein & Barron, 1971), as follows. They classified in-situ stresses into two groups, i.e., natural one and induced one, the latter is artificial stress components due to removal or addition of materials. It is superposped on the former one which exists prior to any excavation. The natural stress field can be composed of gravitational stresses (due to the mass of overburden), tectonical stresses and, residual stresses. Tectonic stresses may be active tectonic stresses and remanent tectonic stresses (due to post tectonic events which have only been partially relieved by natural processes).
It is impossible to accumulate higher stress in rocks with lower strength. Hence the influence of natural stress in soft rock on the geotechnical engineering is always neglected. As a matter of fact, although in-situ stress is lower in soft rocks, the deformation of the rock engineering would probably large and lead to failure due to its lower modulus of deformation and creep behaviour. During the construction of the Gezhouba Hydropower Station, large universally dislocations along the weak intercalations of rock were found after several months of excavation of the deep foundation pit by extensive observations and monitoring (Tseng et aI, 1978) The dislocations have the following characteristics, their directions are neither perpendicular to the slope of the pit nor along the dipping of the weak intercalation, but intersect the axis of an angle of 58° (NE510), Their orders are big, the biggest one is 80mm; their influence is relatively wide and deep (300m away from the excavation line) and the dislocations lasted about two years. The above phenomenon could hardly be explained by the reasons of gravitational stress relief, rock swelling and the effect of blasting. However, most of engineers in-situ did not believe existing tectonic and residual stresses there at the biginning, because the valley is very wide(over 2000m),the elevation difference between the bottom and both banks of the river is only 100–150m,rock formation is weak and ''soft (siltstone and sandstone), occurrence is gentle(NE20- 40°/SE 6°), weak intercalations are numerous, but joints and cracks do not develop.
