TECHNICAL REPORT Grouting in Sedimentary and Igneous Rock

2004:12 TECHNICAL REPORT Grouting in Sedimentary and Igneous Rock STIG BERNANDER Grouting in Sedimentary and Igneous Rock Rock Igneous and Sedimentary in Grouting BERNANDER STIG with Special Reference to Pressure Induced Deformations Stig Bernander 2004:12 2004:12 Luleå University of Technology Department of Civil & Environmental Engineering, Division of Structural Engineering 2004:12 ⎪ ISSN: 1402 - 1536 ⎪ ISRN: LTU - TR -- 04/12 -- SE Technical report 2004:12 Grouting in Sedimentary and Igneous Rock with Special Reference to Pressure Induced Deformations Stig Bernander Division of Structural Engineering Department for Civil & Environmental Engineering Luleå University of Technology SE-971 87 Luleå Phone (+) 46 920 49 10 00 Fax (+) 46 920 49 19 13 http://www.ltu.se The author of this report is Adjunct Professor Emeritus at the Division of Struc- tural Engineering, Luleå University of Technology, SE-97187 LULEÅ, Sweden. He can also be reached at his home address: Tegelformgatan 10, SE-431 36 MÖLNDAL, Sweden. Data concerning the author: 1972 – 1991 Head of the Engineering Department, Skanska West, Gothenburg, Sweden. 1980 – 1998 Adjunct professor, Luleå University of Technology, Luleå, Swe- den. 1992 – Consulting engineer, CONGEO AB, Mölndal, Sweden. Preface The scope of this report is cement-based grouting for sealing of soil and rock formations in normal civil engineering projects. It does not address hydraulic fracturing at great depths of the kind practised in the Petroleum Industry, where the objectives are contrary to grouting for reduction of permeability. Grouting with the aim of tightening and reinforcing the sub-ground holds a rather special position among the specialities of civil engineering for the simple reason that the result of grouting work cannot usually be readily inspected. Hence, the ways in which the sub-ground is actually affected by the treatment largely remains unknown. Although injection of fluids under high pressure into the ground may seem straightforward, the interpretation of how the zone subject to treatment is affected by the grouting work presents many difficulties. Tightness and closure of a treated area may of course be checked by pumping tests, but the actual physical impact and change, to which a soil or rock formation is subjected, are seldom observed or documented, and therefore not very well un- derstood even by those responsible for the grouting operations. For instance, in many case records known to me, where excavation subsequent to grouting work has actually been carried out, only a minute fraction of the injected volumes of grout (as estimated often less than 1 %) have been identified within the treated area. A vital question is then: “Where is the balance of the grout con- sumed to be found?” The answer may be a complex matter but evidently the grout is somewhere outside - mostly far outside the zone intended to be treated. Grouting strategies embrace several spheres of civil engineering, such as: − Geology - for structure and crack pattern of a rock formation; − Soil mechanics - for soil structure, stress/strain properties and permeability; − Structural mechanics - for the assessment of deformations, grout propagation and stresses related to the impact of subjecting ground to the action of a fluid under high pressure, usually far in excess of in situ states of stress; − Hydraulics - not only covering Newtonian fluids but also the transient consistency properties of stiffening grouts - for the prediction of grout propagation and spread; − Detailed knowledge of the rheological properties of different grouts, a vast subject deserving to be regarded as a discipline in its own right. iii The complexity generated by these interacting factors, and the fact that the actual physical modification of the ground by the grouting work is hardly ever observed or known in its entirety, leaves the interpretation of results achieved open to guesswork and often to unfounded speculation. There are few other domains in civil engineering, where the engineers involved allow themselves such a range of different opinions and conceptual understandings, as when interpreting the effects of grouting in soil or rock formations. The issues are admittedly complicated, but the only reliable and rational way to optimize grouting strategies is in my view to resort to the basic general laws known to engineering science, particularly in the fields of structural mechanics, soil mechanics and hydraulics. In a former capacity as head of the engineering department of a major contractor, and later as consulting engineer, I have in the past been involved in grouting operations in connection with important civil engineering projects. Grouting work, as it is practiced and represented in related technical literature and research, has in my view largely been focused on grout consistency and the ability of grouts to penetrate the voids and cracks in ground formations in their primordial states - i.e. when still unaffected by grouting pressures. Insufficient attention has - in my opinion - been given to a number of other vital aspects of the grouting process. Notably, little effort has been made on the study of how pressure-induced deformations in the ground affect - and often decisively control - the outcome of grouting operations. It is hoped that the structure-mechanical applications to this effect, which are pre- sented in this report will increase the appreciation of the importance of analyzing pressure-induced deformations in connection with grouting work. As a rule, grouting work turns out to be adequately successful, although notewor- thy failures are reported from time to time. However, with a better understanding of the mechanical response of the ground to high grouting pressure, and of the way injected grout is “accommodated” in the formation, it must be possible to achieve improved results in respect of both economy and target objectives. Impor- tantly, the impact on the environment can be predicted with greater certainty. Among other things I have found that the following aspects of grouting work are often overlooked or disregarded in current practice: − The practicability of low pressure grouting according the principle of permeation (impregnation) in natural soil deposits and rock formations by cement- iv based grouts is often considerably overrated. The actual penetrability of suspension grouts in natural sediments and fissured rock is in my opinion almost consistently overestimated in grouting practice. The aim to attain target volumes of injected grout thus often results in inducing the operator to raise pressures to levels leading to the opening of the medium by hydraulic fracturing. In my experience, closer studies of case records of penetration grouting usually reveal clear evidence of a high frequency of hydraulic fracture events with associated mechanical deformation. In the writers’ opinion, few cases of intended genuine permeation grouting would in fact be successful without unin- tended hydraulic fracturing. For instance, Ewert (1996 a,b) shows convincingly how several case records of penetration grouting according to the GIN principle in dam construction, actually to a major extent describe hydraulic fracture events. (GIN= Grouting In- tensity Number = pressure × injected volume, Cf Section 5.3) Garshol (2001) advocates using extremely high grouting pressures in combina- tion with limitation of grout take. (Cf Section 5.3) − Time and again, in connection with grouting work, discussion arises, in which experienced engineers claim to be performing ‘permeation grouting’ even when injection pressures in the order of 5 to 15 times the in situ overburden stresses are being used. Under such conditions, the incidence of hydraulic fracturing is obviously a very likely event. − As mentioned above, the importance of the issue as to how pressure-induced deformations affect grouting has – in the writers’ opinion – been notoriously underestimated in grouting practice. It is, for instance, rather symptomatic of the state-of-the-art in this field of engineering that a report such as Pettersson & Molin (1989), which constitutes a comprehensive and excellent review of objectives, techniques and procedures in the field of grouting, hardly comments upon the importance of deformation to grouting success. The need for making at least rough assessments of the magnitude of pressure induced displacements and their implications with respect to grout spread is not dealt with although reference is made to 37 items of work related to grouting technique and research. In Vägverket (2000), dealing with grouting specifications for tunnelling issued by the Swedish National Road Administration, the effect of deformation on the penetrability of suspension grouts is not even mentioned. This is indeed remarkable, as deformation of the sub-ground is an inevitable and quantifiable reality, constituting a powerful mechanism for grouting success. In the authors opinion it constitutes the main reason why grouting - in most instances – can actually be depended on as a viable method for reducing permeability and reinforcing the sub-ground. v − When grouting according to the principle of permeation as defined in Section 3 of this report, the stability of grout and its viscous properties are crucial for achieving optimum results. However, more attention should be paid to the question as to how the deformations in the ground, associated with the injection pressures actually applied in practice, affect the requirements regarding the properties of grouts. Clearly, when deformations tend to increase the initial width of a crack many times over, this must be a vital factor to be considered in this context. − Grouting engineers often prescribe injection pressure limits with the good in- tent of controlling heave of the ground surface and environmental damage. However, the fact is that locally applied high temporary pressure as such, deep down in a bore hole, is not likely to have much effect in terms of lift or other damage at the surface. Instead, the decisive factor, generating heave of ground and related damage, is the quantity of grout actually forced into the ground, and the manner in which it has been injected.

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