172 Ground improvement by deep vibratory methods
of a minimum thickness of 0.30 m. At 150% design load, the single column
settled generally not more than 15 mm with the three column group tests
settling less than 25 mm. At the chosen design load of 300 kN per stone
column, the settlements were only 8 or 8.5 mm, respectively.
4.7.2 Vibro replacement soil improvement
for a double track railway project
The Ipoh–Rawang and Ipoh–Padang Besar double track railway construction projects in Malaysia have been under way since 2001 and involve
about 330 km of new line. The project crosses five states and encompasses
the construction of a new track near, or next to, the existing railway track.
Ground improvement works were necessary for about 32 km, where the
new track crosses over poor soils. Some stretches of the track cut through
valuable farmland. In such areas, stone column installation was restricted
to the dry bottom feed method to avoid contamination of the neighboring
fields by the silt- and clay-loaded process water. The wet vibro replacement
method was used when low headroom conditions below existing road
bridges only allowed the execution of short columns of about 5 m length.
As the soil improvement work had to be carried out as close as 2.5 m from
the existing live track, very stringent safety measures were put in place
to minimize the risks for passing trains and for the workmen and their
rigs. Vibro stone columns were also used to support earth embankments
and reinforced earth walls approaching new bridges crossing the railway
tracks. Figure 4.38 shows a typical situation of a dry bottom feed vibrocat
operating next to the existing track.
Where ground improvement was necessary, the subsoil consisted generally of soft clay deposits to about a depth of 7 m and of firm clay to 10 m.
At that elevation, stiff clay was encountered whose stiffness increased at
15 m depth and beyond considerably. Table 4.11 summarizes the soil conditions in an idealized profile.
The design of the railway line had to fulfill the following performance
criteria:
• A 25 mm maximum settlement with a maximum deflection of
1:1000 of the track over 6 months from the certificate of acceptance
• A minimum factor of safety against slope instability of 1.2 during
construction and of 1.4 during service stage
The vibro stone column scheme that was proposed and adopted consisted of a rectangular grid of 1.0 m diameter stone columns with lengths
of generally 10 m, but in places up to 18 m. The distance between vibro
probes within the grid was 2.25 or 2.5 m, resulting in area replacement
factors a c = 0.16 and 0.13, respectively. In total, an area of over one million
square meter covering unsuitable soil was improved by the vibro replacement method by the end of 2008 using M-type Keller bottom feed depth
of a minimum thickness of 0.30 m. At 150% design load, the single column
settled generally not more than 15 mm with the three column group tests
settling less than 25 mm. At the chosen design load of 300 kN per stone
column, the settlements were only 8 or 8.5 mm, respectively.
4.7.2 Vibro replacement soil improvement
for a double track railway project
The Ipoh–Rawang and Ipoh–Padang Besar double track railway construction projects in Malaysia have been under way since 2001 and involve
about 330 km of new line. The project crosses five states and encompasses
the construction of a new track near, or next to, the existing railway track.
Ground improvement works were necessary for about 32 km, where the
new track crosses over poor soils. Some stretches of the track cut through
valuable farmland. In such areas, stone column installation was restricted
to the dry bottom feed method to avoid contamination of the neighboring
fields by the silt- and clay-loaded process water. The wet vibro replacement
method was used when low headroom conditions below existing road
bridges only allowed the execution of short columns of about 5 m length.
As the soil improvement work had to be carried out as close as 2.5 m from
the existing live track, very stringent safety measures were put in place
to minimize the risks for passing trains and for the workmen and their
rigs. Vibro stone columns were also used to support earth embankments
and reinforced earth walls approaching new bridges crossing the railway
tracks. Figure 4.38 shows a typical situation of a dry bottom feed vibrocat
operating next to the existing track.
Where ground improvement was necessary, the subsoil consisted generally of soft clay deposits to about a depth of 7 m and of firm clay to 10 m.
At that elevation, stiff clay was encountered whose stiffness increased at
15 m depth and beyond considerably. Table 4.11 summarizes the soil conditions in an idealized profile.
The design of the railway line had to fulfill the following performance
criteria:
• A 25 mm maximum settlement with a maximum deflection of
1:1000 of the track over 6 months from the certificate of acceptance
• A minimum factor of safety against slope instability of 1.2 during
construction and of 1.4 during service stage
The vibro stone column scheme that was proposed and adopted consisted of a rectangular grid of 1.0 m diameter stone columns with lengths
of generally 10 m, but in places up to 18 m. The distance between vibro
probes within the grid was 2.25 or 2.5 m, resulting in area replacement
factors a c = 0.16 and 0.13, respectively. In total, an area of over one million
square meter covering unsuitable soil was improved by the vibro replacement method by the end of 2008 using M-type Keller bottom feed depth