GSI System Designer s Guide

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1 G O N AR - S Y S T E M S I N TE R N AT I O N AL GSI System Designer s Guide Gonar - Systems International Ltd ul. Obroki Katowice Tel Fax gsi@gonar.com.pl

2 Contents: Foreword Features of GSI System Technical Data for GSI System Drill rods Drill bits... 6 Coupling sleeves Nuts and washer plates Centralisers Corrosion protection Design Anchoring Nailing Micro piles Close References Page 2

3 Foreword Based on the experience of the Gonar Ltd and Królmet Ltd companies, GONAR SYSTEMS INTERNATIONAL Ltd has introduced a system of ground anchors onto the market. It is a unified system intended for producing soil nails, ground anchors and micro piles with the use of the same components (a drill bit, drill rods, a centraliser, a coupling sleeve, a washer plate, and a nut). The system has been designed for use in ground engineering (geotechnics, geotechnical and structural engineering, foundation works, mining, and environmental engineering). The aim of this guide is to familiaise you with the benefits offered by the GSI System, its possible applications, as well as basic design rules. Page 3

4 Features of GSI System The GSI System allows the use of a single technological process in order to produce soil nails, ground anchors, and micro piles. A threaded drill rod used for these purposes, fitted with an appropriate drill bit simultaneously performs the role of the drill conduit, is used for jet grouting, and serves as the leave-in-place type component of the system reinforcement. On starting the rotary percussion drilling, the initial grouting commences through the inner opening. The centralisers located on the drill rods provide for an even distribution of the suspension. This jet grouting method allows the voids occurring in the soil medium 1 to be filled, as well as driving out the water, and the earth which became loose due to drilling holes in the walls, thus ensuring that the borehole is completely filled with the injection medium. After drilling the borehole, the final grouting takes place. The drill rod together with the drill bit stay in the borehole as the reinforcement. Additionally, there is the possibility of extending the drill rods by connecting them by means of the coupling sleeves. As a result of the jet grouting, a homogeneous grout body with an actual diameter depending on the type of soil is obtained. The GSI System is presented in detail in Figure 1. to: Using the GSI System allows large technical and economic benefits to be obtained owing lower costs of drilling equipment due to its smaller dimensions; smaller area designated for the building site; the possibility of applying the system under difficult soil conditions and for the complicated geometry of walls; unlimited length of drill rod sets (coupling by means of the system sleeves); simultaneous drilling and jet grouting that improves the structure of the surrounding soil; small impact on the environment and the surrounding buildings during the works (low noise and vibration levels); drilling without using the casing pipes (lower investment costs). 1 In this guide, the term of soil medium denotes both soil and rock. Page 4

Nakrętka sferyczna Podkładka Ośrodek gruntowy Tekst oryginalny po polsku śerdź gwintowana (ocynkowana ogniowo) Tuleja Centralizator śerdź gwintowana (pokryta warstwa epoksydową) Buława iniekcyjna

5 Nakrętka sferyczna Podkładka Ośrodek gruntowy Tekst oryginalny po polsku śerdź gwintowana (ocynkowana ogniowo) Tuleja Centralizator śerdź gwintowana (pokryta warstwa epoksydową) Buława iniekcyjna Koronka wiertnicza Tłumaczenie opisu rysunku Nr 1 Spherical nut Washer plate Soil Tłumaczenie na język angielski Threaded drill rod (hot galvanised) Coupling sleeve Centraliser Threaded drill rod (epoxy-coated) Grout body Drill bit Fig. 1 Schematic section of the soil nail installed in the soil Page 5

6 Technical data for GSI System Drill rods The drill rods are made from the 28Mn6 steel grade. They possess drunken (the R25, R32, R38, and R51 Type drill rods) or trapezoidal (the T76 Type drill rod) threads, which undoubtedly improves the bonding of the injection medium to the drill rod compared to smooth pipes or those made from reinforced steel. External threads are made by the cold rolling of the smooth pipes. Technical data are shown in Table 1. Table 1 Technical data Unit. R25 R32L R32N R32S R38 R51L R51N T76N T76S Outside diameter mm Inside diameter mm Cross-section area mm Thread type 2 - f f f f f f f t t Ultimate load kn Yield load kn Typical design load capacity Weight kg/m Length of sections m 1, 2, 3, 4, 6 or according to customer s order Table 1. Technical data for drill rods kn Drill bits The GSI System offers various types of the leave-in-place drill bits intended for various types of soil medium. The available types of drill bits are presented in Figure 2. Under hard conditions, the drill bits can be used with sintered carbide tips that enhance the life of the drill bits. For the GSI Systems, the drill bits with the following outside diameters are available: 38, 42, 51, 64, 76, 90, 100, 115, 130, 150, 200, 300 mm. Moreover, it is possible to use the adapter for the systems with bigger diameters of the drill rod. For example the R32 system, the R32 R38 adapter, the R38 φ115 button bit. 2 f drunken thread; t trapezoidal thread Page 6

krzyŝowa słupkowa asymetryczna Ŝwirowa Tekst oryginalny po polsku asymetryczno-słupkowa krzyŝowo-słupkowa Tłumaczenie opisu rysunku Nr 2 cross bit button bit asymmetrical bit gravel bit Tłumaczenie

7 krzyŝowa słupkowa asymetryczna Ŝwirowa Tekst oryginalny po polsku asymetryczno-słupkowa krzyŝowo-słupkowa Tłumaczenie opisu rysunku Nr 2 cross bit button bit asymmetrical bit gravel bit Tłumaczenie na język angielski asymmetrical button bit cross button bit Fig. 2. Types of drill bits for the GSI Systems 1st row hardened bits, 2nd row bits with sintered carbide tips The availability of the appropriate drill bits for various types of soil medium is shown in Table 2. Rod type R25 R32 R38 R51 T76 Table 2 Bit Cohesive diameter soil mm Sand Gravel Soft rock Hard rock Table 2. The availability of appropriate drill bits for various types of soil medium Page 7

8 Coupling sleeves The coupling sleeves are intended for connecting the drill rods and bear the symbols R25, R32, R38, R51 and T76. They are manufactured from the 28Mn6 fine-grained high-grade steel with a machined internal thread. Fig. 3. The coupling sleeve for the GSI Systems Nuts and washer plates The system nuts and washer plates are manufactured for each type of drill rods. The offer includes spherical nuts, too. The nuts are made from the 42CrMo4 steel and heat-treated to a hardness of HRC, while the washer plates are made from the St3S steel. Centralisers Fig 4. Spherical nut and convex washer plate The system centralisers are manufactured from the 42CrMo4 steel for each type of drill rods and with various outside diameters matched with the inside diameters of drill bits. The diameters available are given in Table 3, and the appearance of the centraliser is depicted in Figure 5. GSI System R25 R32 R38 R51 Table 3 Outside diameter T Table 3. The available outside diameters of centralisers Page 8

9 The centraliser in a GSI System ensures the alignment of the system during drilling, and consequently a uniform grout covering. The covering uniformness provides for better protection against corrosion. When selecting a centraliser, it should be remembered that the outside diameter has always to be smaller than the outside diameter of the drill bit. Fig 5. The centraliser for the GSI Systems Corrosion protection The corrosion protection method is exactly matched with the manner of use. The GSI Systems can be used as temporary elements with the usability below two years as well as elements permanently built in. The following protection types are used, depending upon the needs: For temporary use the corrosion protection, which the grout covering forms. The appropriate thickness and uniformity of the thickness is obtained by the use of a centraliser of a suitable size. The recommended thickness of the covering should amount to 10 mm in rocky ground while it should be doubled in soily ground. For permanent use or temporary use in an aggressive environment, the drill rods can be made from hot galvanised steel. Moreover, they can additionally be epoxy-coated or made from stainless steel. Page 9

10 drill rod without anticorrosive coating Fig. 6. Corrosion protection methods for drill rods hot galvanised drill rod drill rod epoxy-coated Page 10

11 Part common for anchors, nails and micro piles Design Proceeding scheme Geoengineering problem Geological engineering conditions Method Soil nails Ground anchors Micro piles Assumed load Assumed load Assumed load Selection of drill rods fulfilling the load capacity condition Selection of drill rods fulfilling the load capacity condition Selection of drill rods fulfilling the load capacity condition Determination of the length of grout body Determination of the length of drill rod Determination of the length of drill rod Determination of the length of drill rod Before commencing the design, the detailed results of the geotechnical survey should be obtained from the investor or a survey should be ordered from a geotechnical specialist company. Besides the lithology and geology, the following data should be obtained: description and classification of the soil (graining, humidity, bulk density, consolidation degree, consistence boundaries) shear strength, compressibility permeability and water conditions aggressiveness of the soil medium and ground water Page 11

12 Selection of drill rod A drill rod in the GSI System constitutes the reinforcement and is selected from the condition for axially tensed elements according to the PN-B-03264:2002 standard: where: N Sd N Rd (1) N Sd design load to which the GSI System structure is exposed, kn N Rd design load capacity of the drill rod, kn The design load capacity of the drill rod is selected according to Table 1 (connecting drill rods with the use of coupling sleeves does not reduce the load capacity of the rods) External load capacity The load capacity is the maximum axial draw-off force which an anchor is able to transfer without causing dislocations. In other words, it is the ultimate resistance of the side surface at the contact grout body-soil medium. The external load capacity depends on the parameters of the soil medium and the dimensions of the grout body. The external load capacity is calculated by the equation below: π DB τ N zw = (2) η where: N zw external load capacity, kn D B design diameter of grout body, m τ maximum lateral friction on the side surface of grout body, kpa η factor of a safety The design diameter of the grout body is calculated by the equation below: D = α (3) B D O where: D B design diameter of grout body, m D O nominal borehole diameter (outside diameter of drill bit), m α a factor depending on the type of soil medium Page 12

13 the coefficient α is selected according to Table 4 or based on experience well proven under field conditions. Type of soil Designation PN-B PN-EN ISO Table 4 Factor α Gravel ś Gr 1,3 1,4 Sandy gravel śp sagr 1,2 1,4 Sand with gravel Po grsa 1,2 1,3 Coarse sand Pr C 1,1 1,2 Medium sand Ps M, Sa 1,1 1,2 Fine sand Pd F 1,1 1,2 Dusty sand Pπ sisa 1,1 1,2 Dust π Si 1,1 1,2 Clay, silt G, I Cl 1,2 Rocks RQD < 50 % 1,1 Rocks RQD > 50 % 1,0 Table 4. Dependence of the factor α on the type of soil medium Anchoring Before commencing the design of a retaining structure with the use of ground anchors it is best to carry out tests on the anchors on the building site, because the results of such tests are the most appropriate basis to design the construction. Overall length of anchor The overall length of the anchor is the free anchor length L free and the length of the grout body L fixed placed inside the load-bearing zone beyond the soil wedge or outside the zone of the non-load-bearing ground. To ensure the optimum embedding of the drill rod, the grout body should be placed at least two (2) metres behind the fracture surface (Fig.7). To design the free length, it is necessary: 1. to determine the soil wedge ϕ γ = 45 + (4) 2 where: γ angle of inclination of the failure surface, φ angle of internal soil friction, Page 13

14 2. to determine deep sliding plane 2 γ K = ϕ (5) 3 where: γ K angle of inclination of the deep sliding plane, φ angle of internal soil friction, 3. to draw, in an appropriate scale, the calculated soil wedge and the deep sliding plane and mark out the determined anchoring level (point A) 4. to draw a straight line from the anchoring point (point A) at an appropriate angle α = (10 35 ) to the intersection with the line determining the deep sliding plane. The point of intersection of these two straight lines (point B) determines the centre of the grout body designed. 5. to determine the length L (the segment between points A and B) 6. to calculate the overall length of the anchor using the equation below: L fixed L C = L' + (6) 2 where: L c overall length of anchor L distance between the anchoring point and the centre of the grout body L fixed length of the grout body α A γ powierzchnia odłamu γ K głęboka powierzchnia poślizgu L fixed B Fig. 7. Method of determining the length of ground anchors Page 14

Nailing The soil nails (passive ground anchors) are used for reinforcing the soil, i.e. for the enhancement of the strength properties of the soil medium.

15 Nailing The soil nails (passive ground anchors) are used for reinforcing the soil, i.e. for the enhancement of the strength properties of the soil medium. The use of the GSI System for carrying out this type of work makes evident all the advantages of the system. Drilling a borehole with direct injections does not cause the soft earth to slide downwards from the walls of the borehole and consequently fill it. At the same time, this nail production method stabilises the soil medium improving its characteristics because all interspaces and voids in the zone surrounding the borehole become filled with the injection medium. This is especially important when carrying out work in poor load-bearing soil medium (e.g. Carpathian flysch). Fig. 8. Application of the TOP DOWN method for undercutting the slope Soil nailing is widely used for: carrying out the protection of walls of deep excavations Here, a well-proven TOP DOWN method (Fig. 8) is used. It consists of erecting the walls in a stagewise manner, in the following order. construction of an excavation of a depth that enables two rows of nails production of soil nails and protection of the wall face (e.g. shotcreting, panels) removal of a subsequent soil layer These operations are repeated until the desired depth is achieved. The soil nails ensure the stability of the entire system producing a light-weight retaining structure. stability protection of the slopes being undercut For slopes with a smaller inclination, the GSI nails are used while the slope face is protected using the flexible technique: steel wire meshes, biotextiles or geosynthetics, which allow the regreening of the slope protection surfaces. protection of landslides Page 15

16 This type of work fully demonstrates the capacity and advantages of the GSI System, and namely the possibility of working in difficult terrain and the quick and effective installation of the nails. protection of debris slopes In areas where the effect of falling rock fragments and crumbs takes place, especially in mountain environments, nailing is used in conjunction with increased strength steel wire nets. Owing to the dynamics of the effect, the net and fixing nails have to assure an adequate strength and longevity. When designing and producing ground nails, it is necessary to remember: to design the number of nails and horizontal and vertical intervals between them on the basis of previously determined characteristics of soil medium; that in order to perform its role, the ground nail has to pierce the sliding plane; to begin nailing immediately after an excavation has been completed so that ground stress relief will not occur. The nail density (quantity) is calculated by the equation below: N zw P = (7) a b where: P nail density N zw external load capacity of the nail, kn a b horizontal and vertical intervals for the nails, m m Instead of the length of the grout body, the length of the section of the nail that extends beyond the sliding plane is taken. Cohesion of the nailed soil medium resulting from the insertion of nails changes: c uz cu + c 1 = (8) where: c uz modified cohesion of soil medium after the insertion of nails, kpa c u initial cohesion of soil medium, kpa c 1 increase in cohesion of soil medium resulting from the insertion of nails, kpa Page 16

17 The value of c 1 is calculated by the equation below: 1+ sinϕ c1 = P (9) 2 cosϕ where: φ angle of internal friction P nail density Further calculations are performed on the basis of the modified value c uz while replacing the initial cohesion of soil with the modified cohesion of the soil medium. Micro piles In a similar way as to when designing ground anchors, the correct design of a micro pile consists in the proper selection of the reinforcement depending on its design load capacity. The same equations should be used as for the ground anchors. When calculating the required length of grout body and determining the overall length of the micro pile, care should be taken that its length ensures the foundation in load-bearing soil as the entire load capacity is determined just by this layer. In areas where rocks occur at a shallow depth, the borehole should be deepened so much as to embed the pile at a depth of 0.5 m in bedrock. When carrying out works in cohesive media, particularly in soft plastic and plastic ones, attention should be given to the possibility of the buckling of the micro pile. During the design of protections with the use of the GSI System, the best way is to avail oneself of computer-aided analytical methods (Slope/W, Talren, Stability, and the like) as well as such relatively young disciplines as numerical methods based on: Finite Element Method (Plaxis, Crisp, Z_Soil, and the like) Edge Element Method (Beasy, and the like) Finite Difference Method (Flac, Flac 3D, and the like) Page 17

18 Close The objective of this guide is to familiarise you with the benefits offered by the GSI System, its functionality and simple use. The subsoil strengthening methods based on the use of anchor systems continue to gain more and more attention, year by year, and supersede traditional methods. These efficient and economically advantageous methods are effective in many situations. A welldeveloped system consisting of many components that can be interconnected in any manner offers almost unlimited possibilities of its applications. At the same time, simple calculation methods allow a quite fast determination of technical parameters and the initial estimation of costs. We have presented the basic information on the preparatory design. It is always to be remembered, however, that in every case the design has to be prepared by an authorised designer. References PN-EN 1537:2002 Execution of special geotechnical work. Ground anchors. PN-B-02481:1998 Geotechnics. Basic terms, letter symbols and units of measure. PN-B-03264:2002 Concrete, reinforced concrete and prestressed constructions. Structural analysis and design Jarominiak A.: Light-weight retaining structures (in Polish: Lekkie konstrukcje oporowe). WKiŁ, Warszawa 2000 Page 18

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