CW Milestones Part-6 - Backfilling and top slab pouring

Once the proper curing of the foundation pad and columns has been achieved, backfilling of excavated area has to be started. At instances (as per project demand and mentioned in DEDs) the backfilling is also required for whole site up to a certain level. This is normally required, when EGL at site is lower than its surrounding. Or if there is risk of flooding during some part of year due to excessive rains etc.

In some rollout projects where BTS and Generator pads are under tower, the top slab or pad is supported by columns and is cast monolithically on beams connecting all the columns. And in some projects, there is separate pad for BTS/Equipment and Generator/Fuel Tank. In later case the, the supporting wall (either concrete or bricks) for pads, has to be raised before backfilling.

  

During backfilling proper soil compaction is required to be achieved. Soil compaction is method to mechanically increase the density of soil. If the proper care would have been taken during back filling, the top surface settlement may occur within one year, and sometimes it happens after first rain. Remember, doing it wrong ways costs much more than if it was done right. Some good information about backfilling can be found on this link.

Following are the key points to be considered during back filling and top slab pouring

Following the curing, backfilling should be started with the approved and suitable backfill material up to the required height. The backfilling layer should not exceed 200mm and to be compacted to required level and density.

  

1. Properly backfilled layer

2. Reversible plate type compactors are normally recommended
on telecom construction projects.

Sand replacement test would be applied to every subsequent compacted layer to check the required compaction value, i.e. 95% in the tower area & 90% in the compound area.
Density of Backfill material should be ≥18KN/m3 .

3 Soil Replacement test is being carraid out

  

The steel for tie beams, top slab and remaining columns area would be fastened and all the three components would be poured in one go to have monolithic structure.

Prior to concreting of top slab, tie beams and columns, following should be checked:

Check list before concreting the top slab

• Temporary Bench Mark position.
• Required compaction beneath top slab and tie beams to withstand concrete weight.
• Positioning of anchor bolts.
• Size and length of re-bars.
• Re-bars spacing.
• Verticality of columns and tie beams formwork.
• Re-bars condition. (No rust bends & cracks etc.)
• Steel binding.
• Sizes and depths of tie beams.
• Application of admixture (if required).
• Concrete cover (PCC spacers) in both directions and their positioning.
• Placing of chairs (to maintain required distance between main and temperature steel).
• Proper formwork placing.
• Size and quantity of coarse and fine aggregates.
• Availability of required cement bags.
• Availability of concrete vibrators, wheel barrows, trowels and other tools.
• Application of shuttering oil.
• Ambient temperature. (Should be below 30 degree Celsius.)
• Availability of water for concreting.
• Availability of cubes for testing.
• Availability of slump checking cone.
• Condition and working of mixer machine.



4 Reinforcemnet for tob slab alongwith
tower template tied with tower anchor bolts

• Arrangements to prevent leakage/ seepage during concreting.

  

Once the above checks are checked and found okay, concrete is ready to be poured. The materials, as per mix ratio, are mixed in the mixing drum with specified water content for a certain period of time (at least for 75 seconds). The mix is then shifted to the wheel barrows and then to the main concreting area.