Slope Stabilization Using theShotcrete Grid Beam Systemin Japanby Yuji Samaru andKoji SugiyamaCThe Grid Beam system is often used in combination with ground anchors or soil nails where they
are required to stabilize the slope. In this application,a guide pipe is installed in advance at an intersection of beams during formwork fabrication (seeFig. 3). After completing concrete placement byshotcreting, a hole is drilled through the guide pipe
and a ground anchor is inserted and stressed against
the Grid Beam which serves as an anchor pad.
Anchor heads with corrosion protection caps areshown in Fig. 4.

Shotcrete Grid Beam

Shotcrete Characteristics

Basic Structure

The main components of the Grid Beam system
include unique formwork made of steel wire meshand a required amount of standard reinforcing bar,
based on structural calculations. The wire mesh formwork is delivered as folded units. A slope worker
unfolds them (Fig. 2) and fabricates the formworkon the slope, fastening them to the surface using

Fig 1: Grid Beam system of harmonizing with the surrounding environment
36

anchor bolts. Typical dimensions of a beam crosssection are 8 × 24 × 24 in. (200 × 600 × 600 mm),depending on the retaining capacity required. The
typical spacing between two adjacent beams
ranges from 5 to 10 ft (1.5 to 3 m), center to center.

ontinuous grid beam structures made of
shotcrete have been used for over 30 years
in Japan to stabilize natural and man-madecut slopes. This slope stabilization structure, generally called “Grid Beam,” is widely applied in Japan.
It was used in more than 4600 slope stabilizationsites in Japan in the year 2002 alone. The GridBeam system is often used independently to protect
slope surfaces from erosion, or used as a supporting
structure, combined with ground anchors or soil
nails to stabilize the slope against failures. Thearea surrounded by the beams is often revegetatedto improve aesthetics and harmonize the installation
with the surrounding environment (see Fig. 1).
Due to the geometric ﬂexibility of this system, theconcrete beams can easily follow the undulationsof natural slopes and maintain good contact withthe surface.

Ground Anchor and Soil Nail

Portland cement-based mortar is generally usedfor shotcrete in Japan. Wet-mix shotcrete is predominantly used because of its homogeneity inquality and high work efﬁciency. The standard design
strength of Grid Beam shotcrete is normallyin the range of 2175 to 3480 psi (15 to 24 MPa),
slightly less than the requirements for shotcrete
in western countries.

Fig. 2: Installation of Grid Beam formworks
Shotcrete • Spring 2005

Revegetation within Grid Beam Frames

The slope surface surrounded by the beams iseither shotcreted to protect it against erosion or
revegetated by installing plant based materials
using the same shotcrete equipment to improve
aesthetics and environmental integrity. As shownin Fig. 5, the Grid Beam becomes covered withgrasses and shrubs as time goes by.

Design Considerations

The Grid Beam design, including the cross section
dimensions, required amounts of reinforcement,
spacing between beams, and required strength, isdetermined from the following perspective:1) Stability of the target slope (that is, required
retaining capacity to stabilize the target slope);and2) Force applied to an intersection of Grid Beams(that is, tension transferred from ground anchor
or soil nail). Generally, the maximum tension
which can be born by the Grid Beam is in therange of 300 to 500 kN.
Stress is normally analyzed in terms of bending
and shear stresses on the Grid Beam. Deformation and stress intensity of the Grid Beam,when a certain load is applied, is analyzed as acontinuous beam supported at intersections or asa continuous beam attached to the surface ofelastic ground, depending on the properties ofthe slope materials.

Fig. 3: Guide pipes are installed

Case Example—Stabilization
above Tunnel PortalTunnel Portal Site

A shallow sliding failure was anticipatedabove a tunnel portal on the Tateyama Highway,
located in a suburb of Tokyo. Therefore thetarget area was cut and reshaped in advance andstabilized by the Grid Beam system together
with soil nail installation (see Fig. 6 and 7). At
intersections of the concrete Grid Beams, 1157
pieces of soil nail 1 in. (25 mm) in diameter and
16 l ft (5.0 m) long were installed to cover thetarget area of 5442 yd2 (4550 m2). The dimensionof the Grid Beam cross section was 12 x 12 in.
(300 x 300 mm) with reinforcement of 4 steel
reinforcing bars of a diameter 0.6 in. (16 mm).
The standard spacing between two parallel
beams was 6.7 ft (2.0 m). A 20 in. (50 cm) thickplant base layer was installed inside the beam
frame using the same shotcrete equipment. The
total piping length from the plant to the spray
location was 722 ft (220 m) with a verticalinterval of 427 ft (130 m). The required unconﬁned
compressive strength of the shotcrete was 3480 psi(24 MPa). The biggest challenge of this site wasto pump materials over a long distance without
losing suitable plastic properties for shotcrete
Shotcrete • Spring 2005

Fig. 4: Anchor heads and Grid BeamFig. 5: Grid Beam covered with grasses
37

application and, at the same time, to satisfy productquality and strength requirements.

Materials and Mixture Design

thickening agent were added and mixed with portland
cement and ﬁne aggregate in a truck mixer. After
mixing, materials were delivered to the spray nozzle
on the slope using a piston pump and were sprayedStandard mixture proportions used for thissite are shown in Table 1. Portland cement andadmixtures, such as a high-performance water
reducing agent and thickening agent, were used.

Shotcreting Method

Equipment system used for pumping the wet-mix
shotcrete to construct the shotcrete Grid Beams
is shown in Fig. 8. A water reducing agent andFig. 7: After installation

Fig. 6: Before installation

Low friction hose

Truck mixer
Piston pump

Water pump for cleaning

Admixtures

Line switcher

Y-branch

Steel pipe
Pressure regulator

AirCompressor

Fig. 8: Pumping wet-mix shotcrete
Table 1: Standard mixture proportions used to construct Grid Beam

GmaxSlumpSand
Wateraggregatebinderratio (w/c)
ratio

9 in.
—(225 mm)
±10%

38

52.0%

100%

Standard mixture proportions kg/m3
Water
(w)

Cement(c)FineCoarseChemicalaggregate aggregate admixture

257

494

1482

(434 lb/yd3)(833lb/yd3)(2500lb/yd3)—

Thickening
agent

7.4

4.9

(12.5lb/yd3)(8.3lb/yd3)

Shotcrete • Spring 2005

to construct beams. Compressed air was added ata Y-branch 66 linear ft (20 m) before the nozzle.

Procedure

1. Proper operation of each part of the equipment,
gauges, and proper connection of pipes arechecked before operating the piston pump.
2. Cement paste is pumped ﬁrst to lubricate andform a thin layer of the paste inside the pipe.
3. Slump of the material is conﬁrmed to staywithin the range of 9 in. (225 mm) ±10% (seeFig. 9).
4. To obtain optimum filling and compaction
performance, the air pressure is set at 44 to
73 psi (0.3 to 0.5 MPa) at the regulator.
5. The material loses ﬂuidity immediately after
shotcreting so that the slump suddenly becomes
reduced to approximately an 3 in. (80 mm)±1 in. (30 mm) level. This mitigates sloughing(see Fig. 10).

6. After placement, it is a common practice inJapan to ﬁnish the beam surface with a trowel
to provide a better surface texture (see Fig. 11).

Quality Control

During Grid Beam construction, 33 samplingboxes were prepared in the same way as used for
the Grid Beams by shotcreting into a box madeof the same steel mesh formwork as shown inFig. 12. Six test pieces were cored from eachsampling box. Unconﬁned compressive strength
tests were performed on three cores at 7 days andthe other three at 28 days after sampling box
preparation and the results were averaged respectively. The average unconfined compressive
strength of all the cores at 28 days was 5018 psi(34.6 MPa), which well exceeded the design
strength of 3480 psi (24 MPa), with a coefﬁcientof variation of 8.2% (see Fig. 13).

Fig. 9: Slump measurement

Fig. 11: Surface ﬁnishing in progressFig. 10: Shotcrete work in progressFig. 12: Sampling box made of steel mesh formwork

Shotcrete • Spring 20053920
Av: 35 MPa
Design strength: 24 MPaFrequency rate

1612840
20.0

25.0

30.0

35.0

40.0

45.0

50.0

Compressive strength (MPa)Fig. 13: Histogram of strength test results

References (published in Japanese)

JSCE Concrete Committee, “Standard Speciﬁcation of Concrete Structures, Part for Construction,”
Maruzen, Japan, 2002.
JSCE Concrete Committee, “Standard Specification of Concrete Structures, Part for Codes,”Maruzen, Japan, 2002.
JSCE Concrete Committee, “Guideline for Concrete Construction Using Pumps,”
Gihoudo, Japan, 2000.
Raito Kogyo, “Technical Manual of UNILAP Method,” Raito Kogyo Co., Ltd, 2003.
Raito Kogyo, “Plant Manual of UNILAP Method,” Raito Kogyo Co., Ltd, 2000.
Japan Slope Protection Association, “Guideline for Grid Beam Design and Construction,”
Japan Slope Protection Association, 1995.

JSCE fellow Yuji Samaru, PhD, is President ofRaito Kogyo Co., Ltd., Japan, one of the leading
specialist contractors in Japan, specializing in suchareas as slope stabilization, slope greening, andground improvement. He has been making efforts toimprove the status of specialist contractors in theconstruction industry in Japan. Samaru is Presidentof the Japan Anchor Association and of the Japan Slope
Protection Association, the largest association of slope contractors inJapan. He led the development team of the Pumping Delivery Shotcrete
System for Grid Beam construction at Raito Kogyo.
Koji Sugiyama, a former member of the JSCE Concrete
Subcommittee and of the Shotcrete Grid Beam Association, has been extensively involved in technicaldevelopment activities of Raito Kogyo Co., Ltd., andis a corporate ofﬁcer in charge of the Development
Division of the company. He was a key member of thedevelopment team for the Pumping Delivery Shotcrete
System for Grid Beam construction.

40Shotcrete • Spring 2005