Plinth

Plinth is the part of the superstructure between the top of the foundation at the finished ground level and the floor level of the building. The Plinth is technically from where the superstructure starts and the substructure ends and it is a line that differentiates the both. Plinth is where you introduce or lay the damp proof course which is an additional protection against dampness of a building or structure.

The plinth is one of the most underappreciated, and yet most important, elements of a building construction. The plinth height is provided essentially to protect the superstructure from dampness which may seep in due to direct contact from the ground. Plinth height also prevents storm water/overflowing drainage from directly entering the building in case of heavy rainfall.

Other important uses of plinth
  • To carry the dead weight of the masonry wall and transmit the load of the superstructure to the foundation.
  • To act as a retaining wall to keep the filling portion below the raised floor or the building.
  • To protect the building from damp or moisture penetration into it.
  • It enhances the architectural appearance of the building.
  • Helps in reducing the differential settlement in the structure.
Ideal height of plinth
  • As per Byelaws, the plinth should not be less than 45cm and generally plinth level is provided at about 45cm above finished ground level.
  • Plinth level should be decided by considering the top of the road level, the former should be kept at least 15cm higher. (Water level is used to take the road level). It is done to prevent the water to enter into the house during rainy season.
  • If the construction site is near to a water body and more prone to flood during hide tides, the height of the plinth should be raised to 60 – 70 cm.
  • If you construct the plinth too high above the road level, then the adjacent parking area level will also increase causing difficulty for parking. The height should be such that, only 3-4 stairs are required to reach plinth.

Type of plinth

When North Paravur Taluk is concerned, the most widely adopted material for plinth construction is Random Rubble masonry where the underneath foundation is constructed with the rubble. It is observed that, the method used while constructing plinth with Rubble is DRY masonry, where the stones are placed closer to the ones already laid and above one another to reach the desired height and width (usually 45 cm height and 40 cm width). In dry masonry, cement mortar is not used between the joints and the stones are laid in such a manner to avoid large voids. The voids are filled and packed with stone chips as well as cement mortar in not less than 1:6 cement: sand ratio. To provide a uniform and flat surface on top of the rubble, a mortar layer is also laid. Over the years, the method of constructing plinth with rubble has changed as nowadays providing a plinth beam of min 10 cm is provided on top of the rubble to increase the structural strength. During plinth filling, utmost care should be given to the plinth so that the packing of the rubble does not get affected. No voids should be present in the plinth in order to avoid the seepage of water through the voids. The exterior side of the plinth should be plastered which will act as a finishing as well as arrest water seepage if any.

Materials commonly used for plinth construction

1. Plinth RCC beam

Plinth constructed with RCC is gaining popularity due to the structural strength it provides, the availability of materials in abundance and easiness in loading and unloading materials to the site. The plinth beam is a reinforced concrete beam built between the wall and its foundation. The plinth beam is provided to prevent the extension or cracking of the foundation cracks in the wall above when the foundation suffers from laying. It is mandatory to provide plinth beam in areas that are prone to earthquake. While placing the concrete into the formwork, the concrete should be compacted well enough to avoid honeycombing. Minimum dimension of plinth beam is 20cm whereas its width should match the width of final course of the foundation. Plinth beam can be constructed to the desired height of the plinth, but steel bars should be provided according to the size of the beam.

Formwork for Plinth Beam

Formwork used for plinth beam construction should be properly installed and adequately secured prior to concrete placement. The concrete needs to be compacted sufficiently to prevent steel bars from aggressive elements.

Steel bars specification

It is recommended to provide two bars with minimum diameter of 12mm at the bottom of the beam. Similarly, two bars with minimum diameter of 12mm shall be provided at the top of the plinth beam. Reinforcement bars should be protected by 25mm concrete cover. As far as stirrups are concerned, stirrup diameter should be at least 8mm and a spacing of 15cm should be sufficient.

2. Brick work masonry

The least-expensive option for basement construction involves the use of cement concrete bricks or laterite brick masonry. Steel rebar of 6mm diameter can be generally used to reinforce the strength of these walls. Concrete block walls tend to be more susceptible to water leaks. In addition to the potential for leaks along floor and wall joints, water can sometimes seep through the mortar that holds the individual blocks together. Plinth constructed with laterite bricks are not preferred nowadays due to the high water absorption characteristics of the brick. If the plinth is being constructed with brick masonry, it is necessary to construct a plinth beam of minimum 10cm above the brickwork with four numbers of 12mm diameters steel bar and 8mm diameter rings. Provision of plinth beam will help in the transfer of loads to the foundation.

Backfilling and plinth filling and its procedure

The terms backfilling and plinth filling is used for filling soil in the foundation and the plinth respectively. However, both the process are not merely dumping soil to fill in the voids. It requires skills and knowledge of the process, understanding of soil conditions, and extensive experience in the field. In many construction sites, it has been found out that no proper procedures are followed by the construction workers while carrying out filling earth in foundation and plinth. In any building construction, the process of replacing or reusing the soil that is removed during building construction to strengthen and support a structure’s foundation or any other structural member and also filling excavated plinth or trenches is known as backfilling whereas plinth filling is done to fill the area within the plinth. The plinth is filled with excavated material from the plinth or using filling material which is procured from outside. The factors which affect both the process includes choosing the right material, compaction and period of filling.

Materials that can be used for filling in North Paravur

  • Construction and demolition waste (C&D)

    C&D covers all the waste that has been developed during the demolition of any building and any building activities including excavation of land related with construction activities. Demolition wastes include earth, bricks, plaster, timber, concrete, plastics etc. Waste materials including excavated earth bricks, plaster and concrete can be used as the first layer for plinth filling whereas timber, plastics and other materials shall be avoided. The waste materials should be broken to small pieces before filling and proper compaction should be adopted in order to avoid voids in between. C&D should not be used for filling the whole area of plinth.

  • Quarry waste

    Quarry waste is a byproduct of the crushing process which is a concentrated material to use as a fine aggregate in the construction field. In quarrying activities, the rock has been crushed into various sizes; the dust generated during the process is called quarry dust and it is formed as waste. This waste material can be used as filling material as a replacement for river sand. While using quarry waste as a filling material, utmost care should be given by removing larger particles from the waste as it will affect the compaction and results in less bonding

  • Red earth filling

    Red earth also called Murom or Moorum is widely distributed throughout the world in the regions with high rainfall. Red earth is either excavated during piling works or scooped out during levelling of land by razing hillocks or mining hillocks. But, now mining is restricted so red earth is mostly not available.

  • Clayey sand

    Clay is a by-product of the manufacturing of M-Sand which is a material used for filling practices in the construction field. Clay is fine particles less than 150microns of size. This material cannot be used for any type of masonry construction, plastering and concrete. The disposal of this bi-product is an environmental issue; hence filling inside basement with clay will reduce environmental impact and favourably contribute to Green Concept.

  • Commercial By-Products

    The places where suitable natural materials cannot be found, materials such as furnace slag or fly ash can be used as a backfill material if such products are locally available.

    Fly ash and furnace slag are not available at North Paravur

Procedures to be followed in backfilling
  • The foundation should be cured for at least five to seven days before back filling; otherwise it can result in cracks in the foundation.
  • If there is water in the area, it should be pumped out and all grass, loose stones, rubbish of all kinds should be removed from the ground before the process
  • Select the type of material suitable for backfilling. A mixture of various materials such as rocks, soil and stone are commonly used. Try to use the refilling material from the excavated earth. The material shall be cleaned of all rubbishes, vegetation etc.
  • Begin backfilling at the corners and be sure that the distribution of the soil is even so as to provide lateral support for the walls
  • Compress the soil and stone using the rolling compactor. You must repeat the same backfill procedure until the entire area is completely filled.
  • Filling should be done in layers, each layer being of 15cm to 20cm.
  • Each layer is watered and compacted with heavy rammers of wooden logs or steel.
Procedures for plinth filling
  • The material for filling shall be free from lumps, roots, vegetation, harmful salts/chemicals and organic materials etc.
  • The Plinth shall be cleared of all the shoring and formwork material or trench
  • The entire space between the substructure i.e. foundation and side of excavation shall be filled back to the original surface level in layer not exceeding 250 mm in thickness.
  • For Plinth filling it shall start from the lowest level in the horizontal layer not exceeding 250 mm in depth.
  • Each layer should be compacted by ramming with iron rammer of 7-10 Kg weight.
  • Filling shall be adequately watered and ramped for achieving maximum compaction
  • To prevent future sink age and damage to the floor it is necessary, that the plinth area is flooded with water for about 24 hours to enable earth filling to consolidate thoroughly. In this case, earth filling should be allowed to dry and then rammed and consolidated, before the flooring is laid.
  • Where there is black cotton soil, it shall be removed to a depth of 60 cm as it is liable to absorb moisture and expand thus damaging the floor. The top 30 cm immediately below the floor shall be sand or cinder.
Don’ts
  • Using black cotton soil for filling in plinth and footing pits
  • Damaging the drainage pipes laid on the ground while filling
  • Filling the plinth without proper curing of the basement
  • Plinth filling after the construction of superstructure
Sources and rate of each material in North Paravur (As on June 2020)
Sl.no. Filling Material Source Avg. rate per load
1. Quarry waste Manakkappadi 3900
2. Demolition waste Manakkappadi 1800
3. Clay Material park 1800
Manakkappadi, Cherai, Anachal, Perumpadanna
Main source:-
Manjapra, Chullil, Chembarkki, Kizhakkambalam
4. Red earth Ashtamichira, Mala, Chalakka, Kizhakkambalam 2700
(Table 6: The availability of different plinth filling material in North Paravur in Paravur Taluk and its rate as on August 2020)