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Method Statement – Soil Nails with Guniting
GEOPROFOUND
Method Statement for Installation of Soil Nails with
Guniting for Slope Stabilisation
1.0.
INTRODUCTION
Hillslopes can be stabilized using soil nails with guniting. The soil nails are
designed to work together with guniting. They shall be installed together to
work effectively, whereby the soil nails reinforces the slope and the
guniting, tied down onto the soil nails holds the surface of the slope,
preventing surface failure.
2.0.
MATERIAL USED
The material used for the prefabricated horizontal drains include:
Soil Nails :
a) Y20 reinforcement bars, threaded and joint using couplers.
b) 150x150mm MS plate and lock-nut for the head.
c) Non-shrink cement grout (by adding Sika Intraplast-ZTM)
d) PVC spacers
e) Tremie Grout tube – polypipe ¾” or 1” (removable)
Guniting :
a) A6 BRC
b) Chippings (<10mm) and Sand
c) OPC Cement
d) 50mm PVC pipe, wrapped with geotextile as weepholes
Method Statement – Soil Nails with Guniting
2.1.
GEOPROFOUND
Material Specification - GROUT
Grout shall be mixed from ordinary Portland cement and clean water
supplied from the public mains.
The proportions of grout and the minimum strength of work cubes shall
comply with the following requirements :
The quantities of cement in the mix shall be measured by weight. The
grout shall be free from segregation, slumping and bleeding. Grout shall
be mixed on site and shall be pumped into its final position as soon as
possible.
Two sets of three test cubes shall be taken for one batch of soil nails
grouted for testing purpose. Three test cubes shall be tested at 7 days and
the remaining cubes at 28 days after casting. The characteristic strength
of the grout shall be 21 N/mm2 and 30 N/mm2 at 7 days and 28 days.
2.2
Material Specification – Steel Reinforcement Bar / BRC
The main steel component of a soil nail shall consist of a top anchorage
and an axial steel component, which are designed to transmit the pulling
and shearing forces derived from the sliding forces along the potential
failure surface to the further ground beyond the failure surface.
Axial steel component consisting of hot rolled deformed high tensile bar
with a minimum yield stress of 460 N/mm2. The top segment of the soil
nail bar is a threaded bar to receive the lock nuts and bearing plates.
All steel reinforcement bars and BRC delivered to site shall possess a
valid mill certificate. Three (3) samples of Y20 rebars shall be sent for
tensile testing to confirm the strength.
Method Statement – Soil Nails with Guniting
2.3.
GEOPROFOUND
Material Specification – GUNITING
Shotcreting or guniting is a versatile building technique. It is applied
without forms for horizontal, vertical and overhead surfaces of free shape
in any desired thickness.
Cement used : Ordinary Portland Cement (OPC).
Sand used : Natural sand or crushed stone sand.
Cement : sand : chipping (size 9mm) = 50kg : 165kg : 69kg.
28 days characteristic strength for this mix is 30 N/mm2. The minimum
strength for the gunited surface is 25 N/mm2.
Equipment Required :
o Shotcrete/Gunite machine.
o Air compressor-Minimum 390 cfm.
o Water pump-Air / motor driven.
o Cement Mixer
Method Statement – Soil Nails with Guniting
3.0.
GEOPROFOUND
METHOD OF INSTALLATION
3.1.
CONSTRUCT SCAFFOLDING ON SLOPE
Scaffolding shall be installed safely using GI scaffold pipes of min
2” diameter and secured using an appropriate clamp. For safety
reasons, no tie wires or other methods of securing the GI pipes
shall be used except the approved clamps.
As the slope at Munchy’s is steep (40-600 to horizontal) and high,
proper scaffolding must be used and additional bracings must be
made to ensure its safety. Working platforms shall be installed and
all workers must wear safety belt/harness while working on the
scaffolding.
3.2.
DRILLING & INSTALLATION
a) Prior to drilling, the slope shall be cleared of loose material.
b) Survey & mark the location of the soil nails on the slope with a
peg or spray paint.
c) Using GI pipes or other appropriate supports, lift and position
the drilling rig at the first drill location.
d) Install the drill rod and drill bit and level the machine using an
angle level to the required tilt
e) Drilling is done using an air compressor connected to the drill
rod and bit. Assemble the air compressor and suitable air hose
to the rig.
f) The air compressor will deliver compressed air to the machine
while drilling is done. The drilled soil will be flushed out of the
borehole using the compressed air.
g) Advance the drill bit a little at a time depending on soil
hardness. Adjust the air compressor air flow rate to suit type of
soil, whether dry or wet soil accordingly.
Method Statement – Soil Nails with Guniting
GEOPROFOUND
h) Upon completion of the borehole, withdraw the rods completely.
i) Insert the prefabricated soil nail rebar into the hole. Ensure
spacers are installed on the bars, typically at 2m c/c. For joints,
use an approved coupler to join the rebars.
Fig 1 : Drilling works on a typical slope, showing scaffolding & setup for drilling.
Method Statement – Soil Nails with Guniting
GEOPROFOUND
3.3.
GROUTING WORKS
a)
During the insertion of the rebar for the soil nail, a tremie pipe is
also inserted together till the tip of the rebar.
b)
For grouting works, setup the grouting plant consisting a mixer,
tank and diaphragm pump (powered by air compressor) or piston
pump at the base of the slope.
c)
Mix the cement according to the grout specification.
d)
Filter the mixed cement grout and contain it in a tank.
e)
Pump the grout mix using a pump via the tremie pipe into the soil
nail.
f)
Upon completion of grouting, withdraw the tremie grout pipe.
Fig 2 : Grouting works setup.
Method Statement – Soil Nails with Guniting
3.4.
GEOPROFOUND
GUNITING WORKS
NORMAL DRY-MIX SHOTCRETE/GUNITE OPERATION
a)
Excavate certain depth (if require) and trim face to receive
shotcrete/gunite.
b)
Install steel mesh held down by short steel rods drilled/ hammered
into the slope at approximately 2m c/c.
c)
Shotcrete/gunite shall be batched by volume using a wooden box of
volume equal to one 50kg bag cement.
d)
Mixing shall be by drum mixer for approximately two minutes.
e)
Shotcrete/ Gunite shall be conveyed by dry process through hose.
Pipes may be used for long conveyance distance.
f)
The nozzle man shall hold the nozzle at a distance of 600mm to
1200 from the shotcrete/ gunite surface.
g)
Shotcrete/gunite shall be installed in layer not exceeding 100mm
each and allowed to set before applying the next layer.
The
finished surface shall be rough cast natural shotcrete/ gunite finish.
h)
Curing shall be by water spray at least once a day for 4 days.
Method Statement – Soil Nails with Guniting
Fig 3 : Guniting operation in progress
GEOPROFOUND