Generally the work of foundation is to provide stability to the structure carrying OHE equipment. DA , Upright , Boom ,Mast, TTC is main part of OHE structures. Type of foundation in OHE are decided through these equipment’s load and momentum. These structures are subjected to various types of direct & indirect load & momentum.
- It is important to embed the mast 1350 mm into the concrete. In cases where needed, achieve this by adding a super block (SD) with dimensions matching the top of the foundation.
- The type of foundation should be decided based on the FBM code.
- The length of the mast below the rail level should be at least 1850 mm for regulated Overhead Equipment (OHE) and 1750 mm for unregulated OHE.
- It’s important to have a 150 mm thick layer of concrete below the mast’s base.
- The top of foundation should be 50-100 mm above the surrounding ground level.
The type of foundation is determined based on soil pressure/bearing pressure
The following allowable bearing pressure are generally adopted for different kinds of soils
| Sr. no | Type of Soil | Bearing Pressure |
| 1 | Average good soil | 11000 kg/m2 |
| 2 | New bank and bad soils | 5500 kg/m2 |
| 3 | Moorum soil in cuttings | 22000 kg/m2 |
| 4 | Dry black cotton soil | 16500 kg/m2 |
|
5 |
Wet black cotton soil | 8000 kg/m2 |
Types of Foundation in OHE
1) Side Bearing Foundation (B-Type)
Side bearing foundation is of parallelized shape. Generally these are used for stability of B-series mast/structure (B-150,B-200& B-250).
Side bearing foundation is used in OHE where the specified conditions are fulfilled –
- Soil bearing capacity is 11,000 or 21,500 Kgf/m2
- The shoulder width should not be less than 300 cm.
However, in the conditions of overlap inter-mast and inside curve, it should not be less than 550cm.
- Good consolidated embankment is necessary for side bearing foundations
- In case of side bearing foundation both side pressure as well the base pressure are supposed to resist the overturning moment and provide stability to OHE mast/Structure.
2) Side Gravity Foundation (BG-Type)
Side Gravity foundation is of trapezoidal shape. Generally these are used for stability of B-series mast/structure (B-150,B-200& B-250).
Side gravity foundation is used in OHE where the specified conditions are fulfilled –
- Soil bearing capacity is 8000 or 11000 Kgf./m2 or acceptable shoulder width (300cm or 550cm in case of overlap/Curve) is not available.
- Foundation should not be exposed.
3) Pure Gravity Foundation(G-Type, P-Type)
Pure Gravity Foundation have been designed for following conditions-
- These foundation may be used where soil surrounding the foundations is loose and cannot exert passive pressure on the foundations.
- G-Type foundation are used where soil bearing capacity is 5500 , 8000 and 11000 Kgf./m2 .
- P-Type foundation are used for portals whaea soil bearing capacity is 8250 and 11000 Kgf./m2 .
4) New Pure Gravity Foundation(NG-Type)
New Pure Gravity foundation is of trapezoidal shape
Side bearing foundation is used in OHE where the specified conditions are fulfilled –
- Soil bearing capacity is 5500, 8000 or 11000 Kgf./m2 or acceptable shoulder width (300cm or 550cm in case of overlap/Curve) is not available.
- it should be ensured that foundation is not exposed.
5) Foundation in Black Cotton Soil(BC Type)
In case of black cotton soil, we can used WBC (Wet black cotton) & NBC type of foundation.
WBC foundation is done in wet black soil condition whereas NBC foundation in dry black soil condition.
When we have doubt regarding classification of BC soil as to dry or wet, it is preferable to make NBC type foundation.
Depth should not be less than 3 m in case of black cotton foundation.
Volume chart of WBC & NBC foundation is show in figure.
6) OHE Anchor Foundation
There are mainly two types of anchor foundation in OHE.
- Anchor foundation in normal soil
- Anchor foundation in black cotton soil.
Volume chart of anchor foundation in normal soil shown in figure
Volume chart of anchor foundation in black cotton soil shown in figure
In case of loos soil, you can follow to black cotton foundation’s volume chart.
Draft Mast Anchor
Dwarfs mast anchor are used in in following conditions-
- If normal anchors infringing with the movement of staff and passengers.
- normal guy-rod infringes with the schedule of dimensions due to insufficient track canters.
Generally, RSJ(200x150mm) mast of 4.3 m length is used as dwarf mast.
Volume Chart
Foundation’s type is depend upon FBM(Foundation Bending Moment) Code. Foundation Bending Moment Code is determined by the mast employment schedules or by actual calculations.
Selection of the type and size of OHE foundation is done from the volume chart (Drg. No. ETI/C/0058) on the basis of following terms:-
- FBM code
- Type of soil
- Bearing capacity of soil
- Shoulder width
- The extent of projection above ground level.
What is FBM Code
The “FBM code” refers to a three-digit code that explain the vertical load and bending movement for a foundation.
The first digit (from left side) denoted to the vertical load and next two digit denoted to magnitude of bending moment.
| First digit of FBM code as per Vertical load | ||
| Sr. No | Vertical Load | Symbol |
| 1 | Upto 1000kg | 1 |
| 2 | 1000kg to 2000kg | 2 |
| 3 | 2000kg to 3500 kg | 3 |
The next two digits represents bending moment in hundreds.
For example-
Let us consider, we have following value at location no:-54/08
1)Bending Moment=4800KG, 2) Vertical load – 1890kg
Vertical load is in range of 1000 to 2000 kg , so first digit will be 2.
Bending movement is 4800 , so next two digit will be 48.
That’s why FBM Code will be ‘’248’’.
In the volume chart , 248 code not available , so we will taken next higher code. That’s why we will consider FBM Code 250.
Now, we will decided type of foundation at 250 FBM code.
- If acceptable shoulder width is available and bearing capacity of soil is between 11000 to 21500 Kgf/m2, then we can do B-7 type of foundation.
- If acceptable shoulder width is not available and bearing capacity of soil is between 8000 to 11000 Kgf/m2, then we can do B-7 type of foundation.
