Technologies

Rock anchors come in many shapes and sizes. Post-tensioned rock anchors actively transfer loading between the anchored structure and its underlying rock mass, thereby lowering the structure's centre of gravity. Passive rock anchors have high capacity to resist event-induced tensile loading, such as loading arising from periodic seismicity or seasonal buoyancy. Importantly in the case of post-tensioned or passive rock anchors installed to retrofit existing dams, the rock anchors also enhance the dams' resistance to sliding and overturning.

A rock anchor consists of a tendon and an anchorage. The tendon can consist of one or more solid steel bars, a single hollow bar, or a bundle of several high tensile steel strands. The tendon always features a bond length, and often features a free stressing length, over which a debonding mechanism or process ensures that the tendon is free to elongate without obstruction during tensioning. Drilling, grouting and tension testing are the three principal components of any rock anchor construction project. In the case of post-tensioned rock anchors, every installation is at least proof-tested by applying tension equal to 133% of the rock anchor's design loading.

Geo-Foundations has installed rock anchors at more than 150 projects across Canada, including over 40 dams. We are a full service rock anchor contractor; we complete, with our own forces, every component of the construction process, including drilling, consolidation grouting, water tightness testing, post-grouting, installation, and performance and proof tensioning.

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Micropiles are a deep foundation system consisting of small diameter (305 mm or smaller), drilled, replacement elements consisting of high strength steel and cement grout, that transfer axial loading to the earth via grout-to-ground sidewall friction. Micropiles are typically used in situations where above-ground or below-ground conditions render the use of conventional deep foundations impractical, too difficult, or too risky to the health of neighbouring structures or services.

Micropiles are classified by numerous sub-types, each determined by the combination of widely varying drilling and grouting installation processes. Drilling methods include double-head duplex, rotary percussion, continuous grout flush, and rotary with polymer flush just to name a few. Grouting methods include straight tremie injection, dynamic grouting, pressure grouting during casing retraction, and tremie-injection with post-grouting.

Geo-Foundations has designed and constructed more than 100 micropile projects across Canada. The vast majority of these projects included at least one static compression load test to 200% of micropile design loading. At the Pembroke Courthouse, in boulder-rich till with no embedment in rock, load testing was successfully completed to 2600 kN applied compression and 1400 kN applied tension. At the Art Gallery of Ontario, with rock sockets constructed 18 metres below surface in shale bedrock, static compressive load testing was successfully completed to 7100 kN on a single 273 mm diameter micropile, and to 8050 kN on a pair of 194 mm diameter micropiles.

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Jet Grouting is a unique and highly innovative drilling and grouting technology used to create - in situ - large diameter soil/cement columns using small diameter drill tooling. The three principal jet grouting methods are classified by the number of fluids employed: single fluid, double fluid and triple fluid, representing, respectively, cement grout, cement grout and air, and cement grout, air and water. Soil/cement columns are created during drill rod retraction. Specialized drill rods, manufactured to carry the various jet grouting fluids independent of one another to the tip of the drill string, are installed to depth using conventional rotary drilling methods. Upon commencement of jet grouting, the drill stem is rotated and retracted with the jet grout fluid(s) being injected under very high pressures and flow rates. The high energy jetting action achieves large diameter soil/cement columns through erosion, displacement and mixing of the injected grout with the in situ soils.

Column size is dependent on soil type, soil density, injection pressures and flow rates of the various fluids employed, rotation speed, lift rate and type of system used. Due to the number of parameters that contribute to column size, it is standard practice to install and subsequently sample or exhume several test columns to calibrate and adjust the jet grouting parameters and design.

Typically, jet grouted columns are constructed in an array or sequence of some kind to provide an improved soil mass. Geo-Foundations has completed more than a dozen jet grouting projects using the double fluid system (cement grout jacketed in air). These projects involved the consolidation and strengthening of water-bearing, cohesionless native soils ahead of excavation by tunnel boring machine, as well as in situ construction of deep hydraulic barrier walls. Other jet grouting applications include underpinning, slope stabilization and excavation support.

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Hot Bitumen Grouting is the most commonly used form of hot melt grouting. It is used to cut off high magnitude, subterranean water inflows.

The principle behind hot bitumen grouting is simple - it is able to perform in high flow situations due to its inherent anti-washout nature. The hot bitumen grout is injected at temperatures approaching 210° Celsius. At this temperature, the grout has a viscosity only slightly greater than water at room temperature. Unlike even the most viscous chemical resin grouts or the stiffest cement-based mortar grouts, which each have curing processes that are time-dependent, hot bitumen's curing is thermally driven. Upon contact with the ever-replenishing heat sink of the passing inflow water, the hot bitumen quickly turns from its injected fluid state to a highly viscous, tenaciously sticky, elasto-plastic state and eventually, after enough hot bitumen is injected, the aperture through which the inflow passes becomes plugged.

The two most notable Hot Bitumen Grouting projects completed by Geo-Foundations - in West Virginia and Missouri - both involved the elimination of fresh water inflows into limestone quarries via karstic void networks. Karst limestone is characterized by its high incidence of solution cavities, which can, in extreme examples like the Missouri quarry, manifest themselves in cavernous voids that occur along the formations' bedding planes. At the Missouri quarry, where the inflow entered the quarry via two sets of voids, each at least 6 metres high at depths exceeding 80 metres and 100 metres respectively, a total inflow of greater than 2,205 L/sec was successfully eliminated after less than seven hours of hot bitumen injection.

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Soil nailing is a slope stabilization technique that involves the installation of an array of several closely spaced earth anchors, which, via grout-to-earth friction, work together in tension and bending resistance to "knit" the slope's soil together, consequently creating a self-supporting earth mass. Most soil nailing schemes include slope facing treatments anchored in place by the heads of the individual soil nails. The most common application of soil nailing in Geo-Foundations' market is for stabilizing slopes that are inaccessible to large conventional pile driving or drilled shaft boring equipment. In all cases, soil nailing preserves the slope's existing soils without large cuts and fills - this aspect is particularly advantageous when the slope being treated is vegetated with mature growth that can, by conscientious design and installation considerations, be preserved intact.

Soil nails themselves are drilled and grouted soil anchors, consisting of a single steel (or, in rare cases, fibreglass) bar tendon - typically 25 mm to 45 mm diameter continuously threaded bar - encapsulated in a cement grout body with typical nominal diameter of 100 mm to 150 mm. By far the most common tendon / installation technique combination is hollow bar continuous grout flush. Soil nail heads usually consist of a steel plate fixed in place against the slope surface by a nut threaded on to the soil nail tendon. Slope facing treatment, when used, can consist of some manner of meshing - steel or synthetic - in combination with some form of bio-treatment like seed-impregnated coconut husk matting or mulch cover with hydro-seeding.

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Anchored shotcrete can be used to construct retaining walls, for temporary excavation support, and to underpin structures. Universally constructed from the top downwards, anchored shotcrete retains, rather than replaces, the existing soil behind its facing. Anchored shotcrete walls can be permanent, stand-alone structures, or, as is more common, temporary excavation support built to enable construction of permanent, framed structures.

Unlike conventional wall construction systems that rely on either compaction of fill in layers behind the wall, or settlement-inducing movements before the wall can resist load, anchored shotcrete features post-tensioned soil anchors and spraying of shotcrete directly onto native soil. The pre-loading limits movements both directly by imparted axial force, and indirectly by enhancement of shotcrete-to-earth friction at the back of the wall. The soil anchor arrangement is the key to this pre-loading mechanism; the soil anchors are closely spaced and lightly loaded, and inclined on a shallow angle typically not exceeding 15° to the horizontal. It is not uncommon for anchored shotcrete walls to be designed without any footing at the base of the wall, even in cases where the anchored shotcrete is constructed as permanent underpinning.

The top-downwards construction sequence is common to both anchored shotcrete and conventional underpinning, and usually consists of a single-level or multi-level 3-panel or 4-panel sequence. It is the mode of support of the structure being underpinned that differs between the two techniques. Conventional underpinning entirely replaces the soil that was the underpinned structure's original bearing support, with a pier that transfers the bearing to a deeper horizon. Conversely, anchored shotcrete underpinning preserves the soil providing bearing support to the structure, and compensates for the excavation-induced loss of lateral support by pre-loading the shotcrete facing and mechanically ensuring no reduction in bearing capacity within the retained soil mass.

Geo-Foundations has completed anchored shotcrete projects of all descriptions, most notably at Renaissance ROM, Toronto Western Hospital, Lowe's of York and the Union Station Revitalization Project.

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Rock grouting has a long history of use in dam construction and rehabilitation, and can be applicable to challenges in mining, tunnelling, rock mechanics and environmental remediation. Rock grouting is typically performed to reduce the hydraulic conductivity of, or more appropriately, across a rock mass by injection of grout into the rock's joints and fissures.

The end product of the in situ treatment of a rock mass by grouting is commonly referred to as a grout curtain, which can be considered as a two-dimensional 'structure' across which a significant hydraulic gradient can be resisted. An extreme example of such a grout curtain is Geo-Foundations' work at the 55-metre deep outfall shaft at the Niagara Tunnel project, where, once the shaft was excavated, the negative side of the grout curtain was air at atmospheric pressure and the positive side was ground water under 55 metres of hydrostatic pressure.

Grout curtains are constructed by successive orders (primary, secondary, etc.) of drilling and grouting of multiple grout holes, each of which is drilled, cleaned, water-tested and grouted - usually with cement-based suspension grout - in sequence. With appropriate equipment, personnel and instrumentation, a sophisticated grouting program can be performed with verifiable results. Real time monitoring and recording of pressures and flows allows for the adjustment and optimization of grouting parameters in response to the evolving hydraulic signature of the rock mass. Analysis of this recorded body of work, coupled with an appropriate number of hydraulic conductivity tests, can unmistakably show the reduction in hydraulic conductivity (measured in Lugeons) achieved over time and from one stage of grouting to the next.

Geo-Foundations has a rich history of experience in rock grouting, including construction of several grout curtains below existing dams and proposed mine tailings ponds.

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Soil Permeation Grouting is typically used to reduce soil permeability, improve soil cohesion, improve the structural characteristics of the soil, or, as often as is the case, a combination of some or all of these goals. It involves the injection of grout at low pressures into the soil matrix in an effort to permeate or encapsulate the individual soil grains without otherwise disturbing the natural state of the soil.

Sleeve pipes are the key enabling apparatus involved. They are installed prior to grouting by being placed and encapsulated in a weak mix of bentonite and cement in a series of drilled holes intersecting the target soil mass. A sleeve pipe, typically 38 mm to 75 mm diameter PVC or steel pipe, contains several sets of small holes along its length that are enveloped by protective, expandable, rubber sleeves. The sleeves act as one-way valves, keeping fluids outside the pipe from getting in, but allowing fluid on the inside - once sufficiently pressurized - to get out. A device called a packer is used to isolate one or more sleeves at a time. Grout is injected via the grout pump at surface, through the grout tube to the packer, then finally across the sleeve and into the interstitial space between the individual soil grains.

Grout is injected into the soil slowly and under relatively low pressure in order to avoid excessive hydrofracturing of the soil. Grout spread is governed by soil type, degree of soil compaction, grain size distribution, grout type (eg. solution or suspension), gel time, grout rheology and grouting pressure. Real time monitoring of pressure, flow and cumulative grout take is used to constantly evaluate and determine the direction of the grouting program. Its implementation also conveniently results in measurement of the completed work for compilation of records and computation of useful data such as theoretical grout spread and determination of the hydrofracturing threshold.

Geo-Foundations' has completed a number of soil permeation grouting projects involving cement grouts, microfine cement grouts, sodium silicate grouts, acrylate grouts and polyurethane grouts. This technology is particularly useful in sandy soils with less than 20% fines. A common application is pre-treatment of cohesionless soils close to sensitive structures to prevent undermining the structure during excavation or tunnelling.

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Compaction grouting is also commonly known by the generic names Low Mobility Displacement Grouting or simply Low Mobility Grouting. The technique involves the delivery of low mobility grout via open ended tubes (drill casings) drilled or driven to pre-determined depths. The grout, typically containing cement, sand, fly ash and water, is placed in stages, from bottom of the hole upwards, with pressure-based and volume-based refusal criteria. Between consecutive stages, the casing is retracted upwards until the delivery tube is completely removed.

Stage dimensions and volume refusal criteria are determined on a hole by hole basis, after drilling, but prior to grouting. Grout injection rate and pressure are dictated by the soils' ability to redistribute the increased stresses and dissipate pore water pressures and are often modified as the grouting program progresses.

The low mobility grout displaces the existing soil creating grout bulbs in situ. Benefits of this process include densification and consolidation of the soil and creation of vertical grout columns.

As the technique involves displacement, it is particularly effective in disturbed soils. Most of Geo-Foundations' past compaction grouting jobs have come about as a result of our clients needing to repair disturbed soil for reasons such as over-excavation during tunneling, long-term migration of fine particles through broken sewer pipes, water main rupture, or past poor compaction of placed fill soils.

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Due to extremely high overturning moments induced by the height of the tower and the forces acting on the blades, wind turbines founded on spread footings feature very broad footings for the simple fact that a spread footing has no resistance to uplift - all of the overturning moment has to be taken out in compression from one side of the foundation only at any one time. This fact results in large footprint bases, and since the bases are heavily reinforced, the consequence is high volumes of concrete and rebar. The same set of circumstances result in similarly large bases being required for turbines founded on driven piles when those piles have no uplift capacity.

It is possible, given beneficial geology at a wind turbine site where the turbines are not founded directly on bedrock, to dramatically reduce the size of the turbine base by applying innovation to the foundation design. For instance, at a site where the soils are too compressible to support a reasonably sized conventional spread footing, but where rock or another suitably hard stratum is present at relatively shallow depth, deep foundations such as micropiles, drilled piles or internally post-tensioned bored piles can be used. The added cost of the piling system is balanced or outweighed by the sizeable reduction in excavation size and concrete and rebar quantities.

For turbine bases founded directly on bedrock, rock anchors can be used to reduce the size of the turbine base. Rock anchored bases work on the exact opposite principle of broad spread footings. In the case of the spread footing, all of the overturning is resisted by compression of the bearing soils at the half of the base under compression while none of the overturning is taken by uplift resistance of the half of the base under uplift. In the rock anchor case, all or practically all of the overturning resistance is taken by the rock anchors at the half of the base under uplift.

Geo-Foundations has installed over 1750 rock anchors for wind turbine bases.

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