Cement grouting

This expose´ on cement grouting is meant to update the general technical knowledge of the reader on grouting using cementicious (particulate) suspensions and to acquaint him with some of the later – and in a way ‘demystifying’ – experiences and findings related to the different technologies of grouting. It is meant for the learned, semi-experienced who have had some personal contact with grouting projects, or have designed and executed projects. Grouting generally is used to fill voids in the ground (fissures and porous

structures) with the aim to increase resistance against deformation, to supply cohesion, shear-strength and uniaxial compressive strength or finally – and even more frequently – to reduce conductivity and interconnected porosity in an aquifer. Grouting uses liquids which are injected under pressure into the pores and

fissures of the ground (sediments and rock). Liquid grout mixes consist of mortar, particulate suspensions, aqueous solutions and chemical products like polyurethane, acrylate or epoxy. Piston or screw-feed pumps deliver grout through open boreholes into fissures in rock, through lances, perforated pipes and packered or sleeved pipes into sedimentary soils. By displacing gas or groundwater, these fluids fill pores and fissures in the ground and thus – after setting and hardening – attribute new properties to the subsoil. The degree of saturation with – and the properties of – the hardened grout are responsible for the degree of improvement achieved. Grouting originates from mining and applications in hydro-engineering,

and although its history (starting with Berigny in France) now dates back approx. 200 years, these two sectors remain where today’s applications prevail. City excavations for high-rise structures and subways (Metro) have been prominently added to these examples. Figure 4.1 (Rodio South Africa) shows that one of the major achievements has been in early days the operation of long-range supply pipe lines for particulate grout in deep level mining. The speedy provision of grout mix to even remote areas of large

mines has more than once helped to save the mine from flooding, gas accident or collapse. Another example shows (Figure 4.2; Insond, Austria) how structural repair of broken concrete in double curvature arch dams has created another modern application of grouting with epoxy resins of high viscosity and strength. The particular feature of this repair was, that cracks

and fissures in concrete and rock had to be bonded under water and still, had to be conditioned so as to transfer even tensile forces of >2MPa. Lombardi provided the design of this repair work and took the occasion to apply his concept of grouting intensity number (GIN) (see also Section 4.2.5) at this major repair project. More than 130 000m of mainly smalldiameter coredrilling were carried out and 200 tonnes of epoxy resins were injected at pressures of between 60 and 120 bar. Another typical example of applications of grouting may be given with the grouting of horizontal barriers (blankets) in the sands below city excavations in Berlin. Internationally renowned agencies and institutions established headquarters in the revived city centre and around Potsdamer Platz, requiring more than 250 000m2 of deep, water-sealing blankets in pervious sands during the 1990s. Figure 4.3 shows respective foundation works for the new Offices of the German President of State. To reduce seepage during excavation of construction pits at gradients of approx. 10, it was necessary to reduce permeabilities to around 1 107m/s, which corresponded to seepage values of 1.5 l/s per 1000m2. Microfine binders barely met the requirement, and it was mainly silicates and aluminate-hardeners which were used to supply a soft gel in a single-shot treatment campaign (grouting one phase only through one single port outlet), sufficient to achieve the required impermeabilization and to withstand washout for more than 12 months. Costs are of course at all times a matter of the market, and therefore

difficult to generalize. To satisfy commercial requirements however, it is a general rule that grouting is only viable, if the process may be accomplished

within an acceptable lapse of time by pumping (in the case of permeation grouting) as speedily as feasible – namely, at pressures below ground fracturing, e.g. where the ground would be detrimentally deformed – fluids into voids. When considering frequently observed technical and operative ‘bound-

aries’ however, like:

. average grouting-rate per pump: 5-20 l/min;

. average man-hour [H] per operative pump-hour [h]: 1.1-3.5H/h;

. average man-hour per ton of cement of a neat OPC-grout: 5-10H/ton;

. an average minimum borehole-spacing equal to the thickness of the treatment, or <3m;

. average percentage of voids in sediments on which to base grout consumption: 28-38 per cent; in rocks however only 0.5-3.5 per cent;

. average metre of borehole per m3 of soil/rock grouted: 0.25-0.8m/m3;

. average depreciation plus interest, cost for maintenance and repair of equipment and machinery, together: 3.6-4.1 per cent per month.