MARKING & EXCAVATION OF BUILDINGS - METHODOLOGY

MARKING AND SETTING OUT

1.         The setting out process must provide an accurate base for the subsequent construction process.  You must be aware of the potential errors attributable to the instruments you are using and the techniques required minimizing these effects.

2.         The degree of accuracy required will depend upon the nature of the work being carried out and it is important that you recognize what can be termed “Economy of Accuracy”.  There is no merit in setting out the line of a road for topsoil strip to 5 mm when 150 mm would be quite sufficient.  It is obvious, however, that it is not acceptable to set out a structure to an accuracy of 20 mm when

prefabricated units are being used that have a tolerance of 10 mm or less.

3.         Work from the whole to the part and constantly check the correctness of your own work and that of others that is based on your information. Pegs can be moved, sight rails altered or used with the wrong size of traveler, etc., with dire results if this is not done.

4.         Use detailed dimensions, or calculated ones, wherever possible and avoid scaling from the plan unless all else fails, but be sure to check overall dimensions against the site.

5.         Always establish permanent control marks outside the area of operations at the very earliest stage, or much detailed work may have to be repeated and overall control of the project may be lost as works proceed.

6.         The most important check is “proving” the site drawings, in principle this involves checking that the structures will fit into the site in the positions shown on the drawings.  Make checks to existing and proposed roads, other buildings and existing permanent features.

7.         Make sure that site personnel understand the significance and importance of all marks and pegs.

8.         Closing a traverse in one direction say anti-clockwise and coming back to the point of origin (clockwise) is the correct way to check the work. Closing error should be within permissible limits.

B.         EXCAVATION FOR BASEMENT AND FOOTING

1.         Establish Temporary Bench Marks (TBM) all round the site. Locations are decided based on visibility and permanency of TBMs. Write the Reduced Levels (RL) on the TBM. TBM should be related to an OBM (Ordinance Bench Mark) taken from the Survey of India authorities.

2.         Take grid levels at 5-metre interval before the start of mass excavation. If the terrain is sloping or rolling, closer grids of 3m may be required for greater accuracy.

3.         Excavate in a phased manner after proper setting out of works.

4.         Keep dewatering pumps available and discharge point accessible. Keep necessary information regarding water table, periods of rainfall and flooding of site and so on.

5.         Shore slopes or step cut the sides as you excavate below 1.2 metres to ensure stability of sides. 

6.         The combination of machinery used in terms of output and efficiency should be matched. This will ensure minimum idling time of equipment.

7.         Avoid excavation after sunset. Due to poor visibility errors may occur, even under artificial lights. Again it is not safe to work under poor lighting conditions.

8.         Depth of excavation is to be constantly monitored with dumpy level in relation to the TBM located around the site

9.         Plan your works to balance cutting and filling. Re-handling of excavated earth is costly and should be minimized.

10.       Structural consultant should inspect the site after the required depth of excavation is achieved i.e., the bearing stratum is exposed. Immediately after inspection the bottom level of excavation is sealed with blinding.

11.       Hard rock removal can be done by compressed air jackhammer, wedging, chiseling etc. Do not resort to blasting as much as possible.

12.       If blasting is necessary the work should be entrusted to a certified, license holding contractor. All necessary statutory clearances should be obtained. Controlled blasting should be resorted to. Put up adequate precautionary signage during the period of blasting. All blasting works should be completed before commencing RCC works.

13.       Proper arrangement for dewatering should be made whenever the excavation crosses the existing water table level.

14.       Ascertain location of permanent main services like UG cable OFC, water supply, telecom lines, sewage etc. Maintain and protect these lines throughout the phase of excavation and backfilling

C.        FILLING AND COMPACTION

1.         Approved material shall be used for backfilling. Soil with liquid limit exceeding 65% and for plasticity index exceeding 35% cannot be used. High clay soil is not preferred for backfilling.

2.         Imported fill shall be selected - graded hard granular fill with 100% free stones larger than 100mm, up to 50% passing 5mm mesh and not more than 20% passing a 75 micron sieve.  

3.         Compaction is done in layers of 150mm. Chose the compaction equipment based on the nature of soil

TYPE OF COMPACTION –

                                             COHESIVE SOIL              WELL GRADED GRANULAR      UNIFORMLY GRADED SOIL

          PLANT                                                                                          SOIL

Smooth wheeled roller       Suitable                               Suitable                                           Unsuitable

Pneumatic tyred roller                Suitable                                   Suitable                                            Unsuitable

Vibratory roller                                       Unsuitable                     Suitable                                           Suitable

Vibratory compactor               Unsuitable                         Suitable                                           Suitable

4.         Soil with organic material, construction debris, susceptible for spontaneous combustion, soft clay, and moisture content greater than optimum value to be avoided as backfill material.

5.         Insitu field densities of compacted material should be 95% of maximum dry density or as specified by the consultant. Test should be conducted 10m centre-to-centre and before the anti-termite treatment is done. Sand replacement method can be used to ascertain insitu field density.

6.         If removal and replacement of the weak strata with strong compact material is necessary, check the compatibility of the proposed fill material with the existing chemical ground conditions.

7.         The checks that need to be made before work starts include the stability and levels of the original ground or foundations at the base or top of embankments, selection of suitable fill material and the stability and profiles of side slopes.

8.         The stability of slopes to cuttings and embankments (in all weather conditions) is critical to safety on site.  If in doubt, the Project-in-charge must be consulted and the decisions taken to be recorded.

9.         Know your soil conditions before starting any large-scale earth moving operations.

10.       Unit of measurement of back fill material should be clear before the work starts.

11.       For volumetric calculations cater for 30% quantity more of loose backfill to compacted volume.

INSPECTION METHODOLOGY FOR QUALITY ASSURANCE

1.         First check if the approved site plan exists 

2.         The checks that need to be made before work starts include the stability and levels of the original ground or foundations at the base or top of embankments, selection of suitable fill material and the stability and profiles of side slopes.

3.         Marking in conformance as per drawings. The centerline should be checked and certified by a marking supervisor from another site.

4.         Ensure that correct area is excavated properly including the depth, length and breadth of the site using dumpy levels as per drawings. Check of levels (random) using a dumpy level.

5.         Where ever the depth of the trenches is more than a meter, barricading all around the excavated site at a distance of one meter from the circumference. 

6.         If the slope of the excavated earth has a gradient more than 60 degrees, than ensure that the stepping /terracing is done to avoid any caving in of earth due to landslides.   

7.         Check up if the ground has been chemically treated for termites, or not.  

8.         All access routes to the sites have to be free and approachable at any point of time.  

9.         Visual examination of site and check for water table report.  Check whether dewatering procedures have been applied on site