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Journal of Mining Science, Vol. 43, No. 6, 2007

SOIL ANCHORS — THE METHODS AND MACHINES FOR THEIR INSTALLATION

E. P. Rusin, B. N. Smolyanitskii, and S. B. Stazhevskii UDC 621.542, 622.23.05

The application efficiency of anchors with a lock made of granular materials to strengthen soil body slopes is substantiated. Mechanics of interaction between an anchor and the geomedium is considered. The techniques and machines preferable for the anchoring are described, and their test data are reported.

Anchor, dilatancy, soil, granular material, pneumatic puncher

INTRODUCTION

It is known that the most advanced technologies, providing higher stability of pit walls, hillsides, embankments, etc. involve their bar (dowel bar) and anchor strengthening. The efficiency of the bar strengthening (passive method) that prevents slumping of a sliding wedge along potentially hazardous slip area depends on the number of bars. The second approach is referred to an active one as it “bolts” an unstable sliding triangle a resistant mass. This stabilization efficiency depends on the number of anchors and their preliminary tension force restricted by deformation properties of toe. The number of anchors required for reliable stabilization of a certain pit wall or slope, may be less as compared to the number of bars wanted for the similar purposes. Nevertheless, bolting is competitive with the dowel bar strengthening solely when:

— the structure is simple, cheap and sufficiently reliable; — it comes into play immediately after installation; — the technologies employed are simple and do not require much hand work and time for bolting,

operation with scaffolding is possible. At the Institute of Mining SB RAS, the researchers have designed an anchor, meeting the above

requirements and appreciated by both Russian and foreign experts [1 – 3, 4, 5]. The brand new machines and technologies provide high capacity of anchoring [6].

DILATANTIONAL ANCHORS

An action chart of a dilatantional soil anchors and stresses in their structural components under the influence of a pulling force P are shown in Fig. 1. The structure consists of a load-carrying bar (tie rod) 1, toe end 2, grab 3, screw-nut 4, and is installed in borehole 5 driven through a potential sliding wedge 7 and localized deformation band 8; the structure is deepened down for a depth L into stable zone 9. A loose (grained or granulated) material functions as an anchor lock 6.

The dilatancy effect found by Reynolds in 1885, is observed in granular materials under load and exhibits in their volume expansibility. Dilatability of granular materials is sometimes considered as a negative property. Thus, for example, dilatancy developing in grain in storage tanks can cause the multifold growth of stresses and high strains resulting in the complete failure of structures [7, 8].

Institute of Mining, Siberian Branch, Russian Academy of Sciences, E-mail: gmmlab@misd.nsc.ru, Novosibirsk, Russia. Translated from Fiziko-Tekhnicheskie Problemy Razrabotki Poleznykh Iskopaemykh, No. 6, pp. 82-88, November-December, 2008.Original article submitted November 12, 2007.

1062-7391/07/4306-0640 2007 Springer Science + Business Media, Inc.

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Fig. 1

In the present study, the dilatancy effect is considered as a positive one. It is realized under conditions of appreciable constrained deformation imposed by borehole walls 5 on loaded granular material 6. Hence, the force P, which is transferred by tie rod 1 to anchor toe end 2 interacting with lock 6, starts to grow and induces dilatancy and higher normal σ and shear τ stresses in the area of 6, followed by augmentation of the pullout resistance of the structure.

The experimental data in Fig. 2 verify the above statement. Test “boreholes” 5 were “driven” in a photoelastic material (Fig. 2a) and a weak-cohesive course-grained sand deposited layer-by-layer (Fig. 2b). The anchor design and its loading scheme are identical to those in Fig. 1. Fine silica sand was used as anchor lock 6.

The optic picture in Fig. 2a clearly revealed the soil body areas mainly exposed to the force P. Growth of normal stresses at the interface of geomedium and actively dilating lock 6 is accompanied by the wall divergence in a local borehole section. The borehole diameter d increases by d∆2 based on the physico-mechanical properties of a soil mass and the load P (Fig. 2b), thus, promoting a higher carrying capacity of an anchor in certain conditions.

Fig. 2

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So, the laboratory tests showed that the simplest anchor design, together with unequivocally used discrete materials and their basic properties as dilatancy and friction enable creation of a strengthening such that meets all the above requirements.

The soil anchors are distinguished for their specific application scope in a geomedium that exhibits strongly pronounced deformation properties as compared to a rock mass. Herefrom one more condition arises, meeting of which gives an additional opportunity to higher the carrying capacity of the strengthening under discussion by extra consolidation of walls in boreholes intended for the dilatantional anchoring.

ANCHORING TECHNOLOGIES AND MACHINES

The pneumatic percussion machines and the related technologies elaborated by the researchers of the Institute of Mining SB RAS make it possible to fulfill the above stated requirements. Of prime significance are remote pneumatic percussion machines (PUM) designed to drive bars into soil and commercially tested on installation of dowel bar strengthening [6].

The new-generation machines have no analogues in the world. Their specific feature and advantage over other machines is the availability of the through axial channel equipped with a clamp that allows fixing PUM on the driven element passed through the machine and transferring the impact load on this element in any cross section. Moreover, the said element does not lose the longitudinal stability under the influence of dynamic forces, and the long bars or pipes can be efficiently driven into soil.

In the new-developed technology, the pipes act as an instrument working element that provides the borehole making and the anchor toe end placing at a projected mark. From the assembly viewpoint, this overlapping of functions is very efficient, and, in combination with high velocities of driving and removal of bars, it makes PUM a high-performance mobile instrument for the discussed strengthening implementation.

Soil anchoring by PUM (Fig. 3) runs as follows: the working element, an instrument rammer 1 with a tubular cross section and anchor 2 inserted into it are placed in the axial channel of a pneumatic percussion machine 3, and is clamped. The machine is set into a prescribed spatial position with/without a start-up device (Fig. 1) [6]. Then bar 1 together with anchor 2 are impacted into soil down to a preset level (Fig. 3a, b). The rammer is removed by PUM from the soil body. A model of PUM, specially designed installation of strengthening structures under consideration in soil, is switched to a striking mode, and the impact direction is opposite to the initial impact direction.

The borehole driven by tubular bar 1 is air-filled with granular material 4 through a thin hose which is placed into the gap between the anchor tie rod 2 and the internal wall of the bar. The operation is performed either during (Fig. 3c) or after (Fig. 3d) the bar removal. To reduce displacements of the loaded anchor, its lock made of the granular material can be additionally compacted while being formed. The additionally compaction is carried out by the same PUM machine. The anchor is tensioned by turning screw 4 (Fig. 1) manually or by a screw-driver up to a design value.

The soil anchors with diameters of toe end and tight rod of 32 and 6 mm, respectively, were installed by PUM on a proving ground of the Institute of Mining SB RAS for testing in sandy loam soil with 17 % humidity, 1740 kg/m3 density, ~ 21° internal friction angle. Boreholes were filled after a rammer was removed. River sand with natural water content and grains 1mm in size was used as a filling material. It was fed by gravity with no subsequent compaction, thus the anchor performance was estimated under the least favorable conditions.

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Fig. 3

A hydraulic jack with an axial hole was used to tension the structure. The load characteristic of such anchor is presented in Fig. 4. It was tensioned until the tie rod failed. The tension force was as high as ~ 6 kN, the toe end displacement amounted to ~ 0.2 m. Based on the obtained results, the dilatantional anchors for soil bodies can stand rather high loads even with a relatively small toe end diameter. The tests revealed that the pre-compaction of the lock material allows that the structure displacement under pulling-out action is actually diminished by an order as compared to the cited one under the other conditions being the same.

In the analyzed method of the soil anchoring, the introduction of an instrument working element into soil is the most time-consuming operation because the velocity of bar driving by PUMs is rather high, and this operation is combined with the anchor placement on the project level. As for the productivity, the studied method is up to large-scale application requirements.

When driving a working element into soil, the soil body is compacted around the element driven, positively affects the load-bearing capacity and lowers flexibility of the structures. The tests showed that as the borehole diameter increased to a certain value, the compaction of a soil body where the anchor boreholes were driven by the method under discussion, tended to grow [9]. The recently realized performance of the PUM machines permits to drive bars and pipes up to 65 mm in diameter. Unfortunately, more powerful machines now available to be applied for the same purposes, have no the through axial hole yet, hence, the range of use of the method under discussion is slightly limited.

Fig. 4

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It has been found that an alternative technique for driving anchor holes in soil involves the employment of mobile pneumatic punchers developed at the Institute of Mining SB RAS, [10]. The pneumatic punchers eliminate the use of mechanical means intended for the machine handling, and their mobility is comparable to that of PUM. The pneumatic punchers [10] with up to 150 mm diameter of the body, which is a working element, usually function as soil drifting machines. The radius of the compaction zone formed around boreholes driven by these machines, is higher as compared to that for PUMs, thus, it is possible to improve the design parameters of the soil anchors.

The dilatantional anchoring by an pneumatic puncher starts with driving borehole 5 (Fig. 1). Then a anchor is installed into the borehole and lock 6 is formed. The operation can be performed by a sand blaster and by other devices. The described method ensures tight compaction of a material in the lock. The extra compaction of the lock while filling the borehole is expedient in order to reduce flexibility of the structure when under loading.

First commercial tests of the subsurface soil body anchor and their installation method with the use of pneumatic punchers were practiced on temporary strengthening of pit walls at the constructed subway station “Studencheskaya” in Novosibirsk *. The soil contained sandy loams, loamy soil with natural water content of up to 17 %, density of 1500 – 1800 kg/m3, internal friction angle of 16 – 20°. The state of things was complicated by a relatively thin watered lens at the foot of the pit wall to be strengthened.

Boreholes ~ 12 m in depth were driven at ~ 20° to horizon by pneumatic puncher IP-4603 having body diameter of 150 mm, developed at the Institute of Mining, SB AN, USSR. To guide the machine movement U, it was actuated from the special start-up site. The anchor tie rod was made of the reinforcing steel periodic profile 30 mm in diameter. The toe end was a circular steel plate 126 mm in diameter and 30 mm in thickness. River sand 0.5 – 2.0 mm in size was used as a filling material, its humidity was 12 – 15 %. The anchor lock was formed by air-driven mortar-supercharger RN-1.

As the pneumatic puncher velocity the soil body slope was 30 m/h, it took about 30 min to drive a borehole and to remove the machine from the new hole in the reverse operation mode. One anchor was installed in 1.0 – 1.5 h, in total.

The anchor structures were tested by applying the pulling force Р to their tie which was passed through a hollow center guide of the air-driven jack. In Fig. 5 the test data are presented for two anchors with a toe end diameter of 126 mm, installed at subway station “Studencheskaya.” Three specific areas OA, AA1, A1A2 can be distinguished on the curves. The first one presents the structure behavior at the stage of sand compaction in the hole and the progressively increasing anchor resistance to the pulling-out effort.

Fig. 5

* The operation was performed by A. A. Kramadzhyan’s team.

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The rise of the second area is related to the plastic yield of the borehole walls at 60.50≈P kN at the intersection with the watered soil lens. The section A1A2 in both curves is specified by the repeated growth of the force Р. The anchor displacement under the rated pulling force of 100 kN amounted to 35 mm (Fig. 5). The tests made it possible to conclude that the dilatantional anchors and their installation technology are efficient to provide the higher load-bearing capacity and the reliable strengthening of soil bodies even weak and moist, along with the fact that the technology avoids using expensive materials, high labor input and large elaborate apparatuses.

CONCLUSION

The experimental and commercial tests confirmed the efficiency and the prospectivity of the reported research findings for the construction industry. At present the research work aimed at mastering the soil anchors and their installation method, as well as the related calculation procedure and software development is going on in high gear.

REFERENCES

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