ENGINEERING LESSONS EPISODE 16 MAHA METRO MACHINARY COLLAPSED
MAHA METRO MACHINARY COLLAPSED
Place of the Incident : Maha Metro, Pune Metro Project
Date of the incident: Jan 2019
No Fatal Reports
The incident took place as the machine operator could not gauge the sogginess of the soil that caved in. This happened during underground utility checking by the 30 foot piling rig.
which narrowly misses two key flyovers in the area.
The site team missed to place the machine on firm ground, which caved during lifting heavy loads.
In case of Accidents in working place a fire engine is mandatory to rescue them. But the officials did not ensure the presence of a fire engines.
Negligence is the main reason for the incident.
How to avoid this kind of Incidents?
There are a range of risks
associated with crane operation; Crane pads made of fabricated steel are
adopted to distribute the high loading pressure of the crawler crane. The interaction
between the crane pad and soil is complicated, and depends on the pad stiffness
and the strength and stiffness of the underlying soil. Studies on the bearing
capacity and stability of crawler crane working on pads are rarely reported.
Geotechnical bearing capacity of crane pad is
governed by both the shear strength of the soil and the allowable settlement criteria
of the crane.
Movement around site of this type of
crane is easy because the tracks enable them to travel over uneven ground and man
oeuvre into tight working areas.
The working surface must be level and capable
of supporting the bearing pressure exerted by the tracks.
These failures have caused significant damages
to local communities or even loss of life. Crawler cranes are widely used for
longer duration operations, and enjoy quick and routine movement over short
distances.
In general, the design process of crawler
cranes should satisfy the following criteria: Allowable soil stress, Material
allowable stress, and Crane stability.
In order to determine the type of crane pad required, it is necessary first to assess the bearing capacity of the ground. The bearing capacity depends on a number of factors including in-situ geotechnical conditions, sizes of crane tracks, allowable settlements and lifting load conditions. The current design methods of crawler cranes are based on the soil bearing capacity and mat strength.
In practice, an economic pad width will help distribute the load to the soil effectively. It is generally considered that the effective pad width depends on the type of soil the pad is resting on. Firm to hard soil strata cause limited soil contact, i.e., no settlement.
When the pad stiffness is greater than the soil, there is a uniform soil pressure distribution is below the crane pad, provided that the bearing pressure is less than allowable soil stress. The full pad width is in contact with soil, and the settlement controls the design of crane pads.
When soil stiffness is greater than the crane pad, the soil underneath the crane pad experiences an uneven soil pressure distribution. In some extreme cases, weak timber crane pads will bend due to the soil pressure concentration. Crane pad may fail by bearing (crushing at the surface), bending or horizontal shear (splitting longitudinally).
Figure shows the crawler track load distribution on the crane pad. The angle of the load spread for the top soil is taken to be 1 vertical to 1 horizontal (i.e., 1V: 1H).
Pad width based on soil bearing capacity
The required crane pad area can be determined by dividing the crane loading over the allowable ground bearing pressure. Divide this area by the length of the track and we can get the required effective pad width. This pad width is adapted to calculate the bending and shear stresses in the pad based on the assumption of uniform soil pressure distribution. If the actual stresses are less than the allowable stresses, the pad design is acceptable.
Pad width based on pad strength
The effective bearing length of the pad is assumed initially and is then adjusted until the resulting bending or shear stress reaches the corresponding allowable stress. The ground bearing pressure is then calculated assuming the effective bearing length. If the actual pressure is equal to or less than the allowable ground bearing pressure, the design of the pad is acceptable. The design method described herein is iterative. There are available commercial programs for crane designers to check the soil pressure or pad strength to determine the pad width. Geotechnical engineers normally provide the soil bearing pressure before the design of crane pad.
The following inputs were used for the design of the crawler crane pad
The compacted gravel
layer on top of the stockyard bund is 1.2 m, which overlies on a 2.7 m thick
Rock fill.
Assumed maximum allowable
settlement of the crawler crane is 25mm which is equivalent to a tilt of 1/333
for the track spacing of 8.4 m.
The permissible bearing pressure for a crawler crane with 2 m wide tracks is 400kPa based on the shear failure bearing capacity theory. The maximum bearing pressure recommendation is based on a factor of safety of 3 against bearing capacity failure, and a typical settlement tolerance of 25mm.
This permissible pressure is also applicable for a crane with
1.5 m to 3.5 m wide tracks and/or load spread pads. It should be noted that the recommended maximum bearing pressures presented
make no allowance for any simultaneous horizontal load or vertical load
eccentricity, as the presence of either may have a reducing effect on the maximum
bearing pressure recommendations.
The stability of the cranes working on the berm was assessed using limit equilibrium methods for both static and seismic loading conditions. Three loading cases were considered for the analysis as summarized
Loading Case I (LC I) ‐ Normal Operation
Loading Case II (LC II) ‐ Normal + wind + skew
Loads
Loading Case III (LC III) ‐ Abnormal operation
The stability of the berm under the three
loading conditions was calculated by using limit equilibrium methods. Under
normal operation loading case (static), the safety factor of crane foundation berm
is about 1.43, higher than 1.3 for the abnormal loading case. However, it is
just slightly less than 1.5. The safety factors of crane working under
different loading combination cases are summary in Table 4. All safety factors
under seismic loading conditions are higher than 1.1, which are deemed
acceptable.
