Dowel bars and rockbolts, technologies and machines for their installation

Dowel bars and rockbolts, technologies and machines for their installation

Introduction

It is known that the most advanced method used in the building practice for stabilization of open pit walls, slopes, etc. is the reinforcement by rods (dowel bars) and rockbolts. The reinforcement with rods is a passive method that prevents a wedge of failure on a potential slip surface by the dowel bars. The rods are driven or indented into soil, or are installed in a pre-drilled hole with a cement-sand mortar, and a massive reinforced soil retention wall is formed as a result (Fig. 1).

The rods are rockbolts with a die-rolled or worm profile, from 12 mm to 28 mm in diameter. By experiments and in actual construction practice, it has been found that the rod reinforcement parameters as are indicated in Figure 1, should be within the following ranges:

0.4H £ l £ 0.8H, 0.5 m £ a £ 1.5 m, 0° £ a £ 30°, 12 mm £ d_a£ 28 mm, 0° £ b £ 20°.

The shotcrete thickness should vary within 50 mm£ d £ 150 mm.

The dynamic method of strengthening a soil mass makes it possible to pull the unstable wedge of failure to the stable part of a soil body. The reinforcement efficiency depends on the number of rockbolts and their prestressing force. In this case, the proper consolidation of a concrete wall or a slope involves less rockbolts than in the case with rods. Nevertheless, the rock bolting is able to meet competition with the rod reinforcement only in the event of:

— a simple, well-priced and reliable design,

— ability to come into play immediately after installation into a soil body,

— simple technologies that need no much input of manual work and time.

** Institute of Mining, Siberian Branch, Russian Academy of Sciences, Russia.

** Siberian State University of Communications, Novosibirsk, Russia.

Researchers of the Institute of Mining SB RAS developed a dilation rockbolt that met the above listed requirements (Stazhevsky 1983). Figure 2 shows the schematics of the dilation rockbolt and stresses that develop under a pulling force P. The rock bolting structure includes

Fig. 1. Rod reinforcement of a slope

H – depth of an open pit; l – length of a rod; da– diameter of a rod; a – spacing of rods; b – spacing of rods girtwise the slope;a – incline of rods; b – incline of the slope; d – shotcrete thickness

Rys. 1. Wzmocnienie prêtowe zbocza:

H – mi¹¿szoœæ odkrywki; l – d³ugoœæ prêta; da– œrednica prêta; a – rozstaw prêtów; b – rozstaw prêtów oczepowych w zboczu;a – nachylenie prêtów; b – nachylenie zbocza; d – gruboœæ torkretu

Fig. 2. Rock bolting of a slope

1 – pull bar; 2 – foot; 3 – pickup; 4 – nut; 5 – wall of a hole; 6 – granular material; 7 – wedge of failure;

8 – slip surface; 9 – stable body Rys. 2. Mocowanie œrubowe zbocza w skale

1 – prêt rozporowy; 2 – stopa; 3 – podbieracz; 4 – nakrêtka; 5 – œcian¹ otworu; 6 – materia³ ziarnisty;

7 – klin zabezpieczaj¹cy; 8 – powierzchnia poœlizgu; 9 – masa stabilna

simplest and utilizing purposefully the nature of granular materials, i.e. dilation and friction, allows creation of reinforcement such that satisfies all the above-formulated requirements.

Developers of the Institute of Mining SB RAS have designed a unique range of universal portable pneumatic percussion machines (PPM) with an end-to-end axial channel (Esin 1986). Table 1 presents technical characteristics of PPM.

The basic functionality of these machines is to drive into soil and pull out, at any inclines, rods, bearing tubes and production pipes with external diameters from 12 mm to 65 mm and 2–10 m in length with the purpose of installation of rock bolting.

The machine applies impact load to the mentioned elements, in particular, dowel bars, through the specially built-in self-advancing gripping mechanism. The latter delivers dy- namic force in the prescribed cross-section of a rod or a pipe, and, in so doing, eliminates their loss of longitudinal stability in the course of drivage. This design feature tells the new pneumatic percussion machines form the other devices for the same task, and relatively small weight of PPM allows operation in rather constrained conditions and on falsework (Fig. 3).

TABLE 1 Technical characteristics of PPM

TABELA 1 Charakterystyka techniczna PPM

Parameter

Model

PPM-3 PPM-35 PPM-65

Length [m] 0.7 0.8 0.8

Weight [kg] 18 30 60

Rockbolt diameter [mm] 12-25 20-30 35-65

Impact energy [J] no less than 40 100 180

Impact frequency [Hz] no less than 10 10 9

Air rate [m³/min] 1.5 2.5 4.5

Compressed air pressure [MPa] 0.6 0.6 0.6

The rock bolt reinforcement with PPM is implemented in layers, starting from the soil body surface, and in floors, usually up to 2 m high (refer to Fig. 1). Within each floor, a surface cover is installed to prevent from rockslides and weathering, then rock bolts are driven into the soil body and are connected to the protection cover. The cover may be made of shotcrete, geotextile or other materials. Every next underlying floor is treated similarly. The so-strengthened soil mass functions as a monolithic retaining wall.

The technology of rock bolting with using PPM has been tested in large-scale production activities (Kritsky 2003). It was successfully applied at the Novosibirsk subway construc- tion, in particular, in the course of building the “Marx Avenue” Station, in the space of more than 600 m of the surface of the pit wall with an incline of ~80° and a height of nearly 21 m.

The experience of the pneumatic percussion machines enables recommending them as the highly productive mechanization means for the rod reinforcement. As experts say, the rock bolting of soil slopes by the method described above ensures cost cutting by 45 % against the conventional technologies, through lower labor content, reduced cost of materials and operation of the machines.

We have emphasized above that the new pneumatic percussion machines are capable of drivage of the dilation rockbolts. Figure 4 illustrates schematically the installation of the

Fig. 3. Rock bolting with using PPM Rys. 3. Mocowanie œrubowe za pomoc¹ PPM

material retainer 6 is additionally compacted while being filled. The compaction is also performed by PPM. With using net 4 (Fig. 2), manually or by a net setter, the rockbolt is tensioned to the design value.

The tests on the proving ground showed that the dilation rockbolts with d = 30 mm, L = 1000–1200 mm and a retainer made of sand to 2 mm in size, depending on the physico-mechanical properties of a soil mass, can develop the pullout resistance up to

~10 kN. Displacement of the installation is governed by the soil deformation properties and pre-compaction of the retainer.

Wall of bar 1. The filling is carried out either in the course of taking out the insertion bar (Fig. 4c) or later (Fig. 4d). With the purpose of reducing displacements of the rockbolt when loaded, granular material retainer 6 is additionally compacted while being filled. The

Fig. 4. Installation of the dilation rockbolt

1 – installation bar; 2 – rockbolt pull-bar; 3 – pneumatic percussion machine; 4 – granular material Rys. 4. Instalacja œruby dylatacyjnej

1 – prêt instalacyjny; 2 – prêt rozporowy; 3 – pneumatyczna maszyna uderzeniowa; 4 – materia³ ziarnisty

compaction is also performed by PPM. With using net 4 (Fig. 2), manually or by a net setter, the rockbolt is tensioned to the design value.

The tests on the proving ground showed that the dilation rockbolts with d = 30 mm, L = 1000–1200 mm and a retainer made of sand to 2 mm in size, depending on the physico-mechanical properties of a soil mass, can develop the pullout resistance up to

~10 kN. Displacement of the installation is governed by the soil deformation properties and pre-compaction of the retainer.

The up-to-date ring machines are capable of driving rods and pipes with a diameter up to 65 mm, which limits construction of rock bolting with the increased bearing capacity. With the aim of meeting this demand, a purpose-oriented technology was elaborated at the Institute of Mining SB RAS. The technology offers holing for rockbolts with retainers by mobile pneumatic percussion machines, pneumatic punchers, that were designed at the Institute of Mining as well (Fig. 5) (Gurkov 1990).

Fig. 5. View of pneumatic puncher IP-4605 Rys. 5. Widok pneumatycznej wiertarki udarowej IP-4605

TABLE 2 The technical characteristics of the pnaumatic punchers

TABELA 2 Charakterystyka techniczna pneumatycznych wiertarek udarowych

Parameter

Pneumatic puncher model

SO-134 IP-4603 IP-4605

Diameter of the puncher and driven hole [mm] 155 130 95

Length of the machine [mm] 1700 1400 1500

Weight of the machine [kg] 150 90 55

Single impact energy [J] 550 220 80

Impact frequency [Hz] 5.8 6.2 5.4

Compressed air pressure [MPa] 0.6 0.6 0.6

Air rate [m³/min] 8.0 5.0 4.0

Driven hole length [m] 50 50 50

Penetration velocity [m/h] 1-40 1-40 1-50

walls. During the rock bolting, holes approximately 12 m deep were driven at an angle of nearly 20° towards horizon by pneumatic puncher IP-4603 having the casing diameter of 130 mm. To hold the pre-set direction, the machine was launched from the special launch platform.

The rockbolt pull-bars were made of reinforcing steel with the die-rolled section and had the diameter of 30 mm. The foot was represented by an orbiculate steel plate 126 mm in diameter and 30 mm in thickness. The fill material was river sand 0.5–2.0 mm in size, its moisture was 12–15%. The retainer was made by pneumatic mud jack model RN-1.

The tests showed that in clay loam and san loam having density of ~15–18 kN/m³, internal friction angle of ~16–20°, cohesion of ~20–30 kPa and natural humidity of ~17%, the rockbolts with retainers generate the pullout resistance of the order of 100–120 kN. The time spent for installation of one rockbolt is 1–1.5 hours.

The study results described in this paper and approbation of the rockbolting procedures on proving grounds and at construction sites make it possible to recommend the discussed rockbolts and machines for the mining practice, in particular, for prevention of hazardous deformations of pit walls and slopes composed of clayey or sandy-clayey rocks. The studies devoted to the improvement of the designs and technologies continue.

REFERENCES

[1] E s i n N . N . , K o s t y l e v A . D . , G u r k o v K . S . , S m o l y a n i t s k y B.N., 1986 – Pneumatic Percussion Machines for Drivage of Boreholes and Blastholes. Novosibirsk: Nauka.

[2] G u r k o v K.S., K l i m a s h k o V.V., K o s t y l e v A.D., P l a v s k i k h V.D., R u s i n E.P., S m o l y a n i - tsky B.N., T u p i t s y n K.N., C h e p u r n o i N.P., 1990 – Pneumatic Punchers. Novosibirsk: IGD SO RAN.

[3] K r i t s k y M.Ya., S y r y a m i n Yu.N., S m o l y a n i t s k y B.N., S k o r k i n N.F., 2003 – Complex of mobile machines and mechanisms for soil strengthening and reinforcement. International Scientific-Practical Con- ference Proceedings “Experience of Construction and Reconstruction of Buildings Founded on Weak Soils”.

Arkhangelsk.

KO£KI I ŒRUBY, TECHNOLOGIE I MASZYNY DLA ICH INSTALACJI

S ³ o w a k l u c z o w e Mocowanie œrub w ska³ach, mechanizacja, wkrêty

S t r e s z c z e n i e

Opracowanie ukazuje efektywnoœæ ko³ków i œrub z elementami ustalaj¹cymi wykonanymi z materia³ów ziarnistych, do zastosowania we wzmocnieniu skarp w masach glebowych. Omówiono interakcjê pomiêdzy instalacjami i geomedium. Autorzy podaj¹ zalecenia odnoœnie zastosowania technologii i maszyn impulsowych do monta¿u omówionych konstrukcji i prezentuj¹ dane uzyskane w próbach.

DOWEL BARS AND ROCKBOLTS, TECHNOLOGIES AND MACHINES FOR THEIR INSTALLATION

K e y w o r d s Rock bolting, mechanization, dowels

A b s t r a c t

The paper proves the efficiency of dowel bars and rockbolts with retainers made of granular materials, to be used for reinforcement of slopes in soil masses. The interaction between the installations and geomedium has been considered. The authors give recommendations on application of the technologies and impulse-forming machines for installation of the discussed structures, and present their test data.