The Taj Mahal is a perfectly symmetrical monument representing the finest architectural and artistic achievements through excellent craftsmanship and perfect harmony in the design. Its foundation, in particular, was a herculean task to build as in those days clayey soil was usually avoided for construction due to its unpredictable nature.
The mausoleum is located on the southern bank of the river Yamuna in the city of Agra, India. It was constructed by the emperor Shah Jahan as a homage to his partner Mumtaz Mahal. The construction began in the year 1632 and was completed in 1653.
The Taj Mahal is one of the seven wonders of the world and was recognized as a world heritage site by the United Nations Educational, Scientific, and Cultural Organization (UNESCO) in 1983.
Owing to the age of the structure, initial drawings used for the construction of the Taj Mahal are no longer available. Also, with time, conditions around the site have altered significantly. Therefore, the exact construction details of the foundation are not known. However, the information provided in this article is based on previous research studies.
The structure still stands tall and intact even after so many centuries essentially because of the construction technique adopted for its foundation. This article explains the design and construction sequence of the foundation along with the materials used in its construction.
1. Location of Taj Mahal
The monument exhibits exemplary designs of Persian and earlier Mughal architecture. To add to the aesthetics, a moonlight garden and elegant trees were integrated into the design. The fusion of these elements made it an ideal site for the construction of the monument. The site is near to the point where the river makes a sharp bend far from the monolith, ensuring enough supply of water for the gardens.
Figure-3 illustrates the sloping landscape on the riverside with Mehtab Bagh on the other side.
2. Geology of Taj Mahal
The following points describe the geology of the Taj Mahal:
- Clay of low to intermediate compressibility (CL- CI) is available from reduced level (RL) 149 to 133 m.
- The non-plastic to silty sand (SP- SM) is available from RL 133 to 123.8 m.
- A thicker clay layer of intermediate compressibility is available from RL 123.8 to 60.2 m.
- Fine to medium-grained quartzitic sandstone is available below 60.2 m.
- The depth of the groundwater table is located at 5.25 m.
Based upon the consolidation test result, 99.4 % of the total settlement had taken place during the construction of the Taj Mahal.
The observation well closest to the Taj Mahal is the Shahganj Prithvi Nath well in the Bichpuri area of Agra. Figure-5 shows the groundwater information for four different seasons throughout the year.
3. Foundation of Taj Mahal
The information of the superstructure should be known before constructing the foundation of any structure. The superstructure of the Taj Mahal is huge and loads due to its weight are very high. The following points describe the superstructure of the Taj Mahal:
- The main structure of the Taj Mahal is made up of white marble.
- Its height above the garden level is 74.4 m.
- Weight of the dome is 12,000 tons.
- Inner diameter of the dome is 17.6 m with a thickness of 4.3 m.
- Pressure intensity due to dome of the Taj Mahal is around 218 KPa (evenly dispersed load) and 397 KPa (concentrated load).
- Pressure intensity due to the walls is around 766 KPa.
3.1 Foundation Details
The loads acting on the foundation due to the superstructure were very high. Therefore, special attention was given towards the construction of the foundation. The techniques and details for the construction of foundation are discussed below:
- The excavation for the foundation of the Taj Mahal was very deep. Researchers have figured out the precise depth of the foundation, to be 17.5 m, using the ground penetration radar method.
- After the excavation, the well foundation was constructed over the excavated stratum.
- In addition, the wells were filled with stone debris. Strong masonry of stone and lime were used as filling material.
- Furthermore, a structural system was built on top of the wells. This system consists of piers and arches. Arches were connected to piers so that the superstructure load can be distributed evenly on the well foundation.
- After that, a big sandstone base was created on top of the structural system.
3.2 Materials Used in the Foundation
Bricks, stones, mortar, iron, and wood were used in the foundation construction of the Taj Mahal.
Bricks were extensively used in the construction of domes and arches. For preparing bricks, clay and sand were mixed, shaped, and baked at the site of the construction. Mughal paintings of the construction work show the brick kilns set up on the worksite of Taj Mahal.
The common brick size of 203.2 mm × 190.5 mm × 44.5 mm was brought down to 177.8 mm × 114.3 mm × 25.4 mm to make the bricks lighter and bricklaying faster.
Mortar is a vital element for foundation construction. The significance of mortar is rather apparent from Mughal-era paintings of building and construction.
For the foundation of the Taj Mahal, a unique water-resistant mortar called “Sarooj” was prepared. The basic contents of Sarooj were lime, clay, and sand. In addition, ash obtained from different sources such as wood and animal dung was added in the Sarooj mortar. Further, local ingredients such as jaggery, pulse, sugar bubbles, gram flour, resin, jute, and gum were also added. This helped in manufacturing a strong bonding agent. Local ingredients and fibers made the Sarooj mortar impenetrable.
The use of iron in the foundation was limited to the external works, such as iron nails used to secure the stones to the links of the vertical arches. Iron nails were also used in splitting the stones.
Mughal houses utilized wood in various parts of the foundation, such as circular braces in the wells. The parts made of wood were used on the riverside wall of the foundation to reduce the river’s scouring. Mostly, Sagwan wood (teak) and bamboo were used for this function. Bamboo was used to make large scaffolding structures, while Sagwan wood was used to build wheels to support the core of the well foundation.
4. Construction Sequence of Taj Mahal's Foundation
The Taj Mahal’s massive weight needed a very robust foundation, as any soil displacement would have been disastrous for the superstructure. A possible construction sequence of its foundation is discussed in two segments.
4.1 Construction of Wells
The following points describe a possible construction sequence of well foundation:
- Prior to construction, the builders used their surveying tools to find any modifications to the site. Measurements were done using bamboo and jute ropes.
- The first construction operation was the excavation for the well foundation.
- During excavation, the walls of the well were secured with wood. This process continued till workers reached a strong ground bed.
- Wood axles with spokes were used for the bracing of the wells. The bracing was provided at a suitable interval along the well core. Figure-10 illustrates this process.
- Further, the well core was filled with stone debris and bounded with leak-proof mortar. Figure-11(a) illustrates this process.
- In a similar style, a series of wells were constructed close by so that the base of the foundation can be set upon them.
- Lastly, the debris of stone blended with lime mortar and sand were filled in the area between the wells. Figure-11(b) demonstrates this process.
4.2 Construction of Piers and Arches
Arches and piers were constructed so that the load of the superstructure can be distributed evenly on the well foundation. A possible construction sequence for piers and arches is discussed below:
- After the construction of the well foundation, the next step was to raise the piers. The bricks mixed with lime mortar were used for constructing the piers. Moreover, Sarooj mortar was used to make the piers waterproof. Figure-11(c) illustrates this process.
- The next step involved the construction of arches and linking them to the piers. Figure-11(d) illustrates this process.
- Following this, the area between arches was filled with sand, mortar, and debris of stones. Sarooj mortar was extensively used for this purpose. Figure-11(e) illustrates this process.
- The resulting arches enabled a well-balanced distribution of the load to the series of wells located underneath it.
- To avoid any kind of differential settlement, the construction of a big red sandstone base was carried out on top of the arches.
- Since the entire structure was constructed on a natural slope, the thickness of the sandstone base was provided as 13 m on the riverside and 1.8 m on the garden side. Figure-11(f) illustrates the final version of the Taj Mahal’s foundation.
The parts made of wood were used on the riverside wall of the well foundation to reduce the river's scouring. Mostly, Sagwan wood (teak) and bamboo were utilized for this purpose.
Various kinds of stones were used to construct the Taj Mahal's foundation. Stones were used in the walls of well foundation. Also, stone debris was used to fill the areas around the piers, wells, and arches. Red sandstone was used for constructing the foundation base.
Sang-I-Gwalior (yellow and grey sandstone) and Sang-I-Surkh (red sandstone) were used in the Taj Mahal's foundation.