Rock Cut Monuments - Engineering Perspective

 

INTRODUCTION:

With over fifteen hundred rock-cut caves and temples in India, I have been interested in finding peer-reviewed articles on the geotechnical aspects and structural engineering techniques used to develop these massive and impressive structures. I was not successful in locating peer-reviewed material that discusses in detail the engineering and construction methods used to carve out these grand monuments; however, I found information on the design of underground caverns. From a study of these journals, I could understand the complexity of designing structures like the Kailash temple at Ellora and the Ajanta caves. The design of these monuments required the application of complex engineering principles.

STUDY OF ROCK FORMATION

In the modern era, the excavation of underground structures like caverns and tunnels begins with the study of the type of rock formation. The formations are classified into two main types.

1. WEAK FORMATIONS. These are the formations of shale, clay, limestone, etc. Weak formations are prone to crumbling. The Badami Caves are an example of weak formation, as these are sandstone structures.

2. HEAVILY JOINTED FORMATION OR HARD ROCK FORMATION. Granite, diorite, and basalt are examples of heavily jointed formations. Natural hard rock cliffs and formations have numerous cracks and discontinuities that break into many smaller blocks. The types of failures experienced in hard rock formations are gravity-controlled falling, sliding of blocks and collapse of rock columns. Mahabalipuram has granite structures, while Ajanta and Ellora caves are carved out of basalt rock.

TYPES OF FAILURES IN ROCK EXCAVATION:

Any underground excavation faces problems from failures in roofs and walls, from the collapse or crumbling of either the roof or the wall. Modern underground cavern design involves complex engineering calculations that primarily rely on geotechnical data, rock mechanics principles, and numerical modelling to ensure stability and safety. Key calculations include assessing in situ stresses, determining rock mass strength, predicting failure modes, and designing support systems. 

The stability of the excavation depends on the strength of the rock mass that surrounds the excavation.  Stress induced in the structure being excavated is a function of the excavation shape and the in-situ stress that existed before excavation. The strength of the rock formation is often not uniform and varies.

ANCIENT SANSKRIT TEXTS ON BUILDING PLANNING & CONSTRUCTION

In ancient times, Sanskrit treatises like the Silpa Shastra, the Vastu shastra, the South Indian Agama text and texts like the Samarangana Sutradhara provided the calculations and methodology required to complete complex constructions, including rock-cut monuments. The documents discuss in detail the construction of towns, villages, public houses, residential areas and also deal with the construction of the temples. These documents provide instructions on how a building, such as a temple, must be developed from the blueprint phase to the final consecration phase. The documents also provide complicated geometrical calculations, units of ancient measurement and conversion from one unit of measurement to another.

 

THE ANCIENT ENGINEER

The master architect/engineer who developed the plans and oversaw the construction was called a Stapathi. The ancient treatises have dedicated chapters that discuss in detail the qualification of a Stapathi. The documents also mention that the Stapathi, who is engaged to oversee and plan the construction, must have sufficient practical experience. The Stapathi must be proficient in engineering, designing machines, carpentry, arithmetic, astronomy, geology, and astrology. The ancient texts have details about building machines that were used to manipulate / harness energy from the five elements. We normally assume that the massive stone structures were carved with the help of a hammer and a chisel, but the ancient documents specify the design of many machines that could be used to manipulate the construction material.

The design and construction of monuments like the Kailash Temple at Ellora were carried out by a team of professionals who were quite proficient in engineering and geotechnical sciences. The Samarangana Sutradhara, for example, discusses the study of the site, soil verification and testing before commencing construction. The site for the rock-cut temples and caves was carefully chosen and studied before construction could begin. These complex structures were definitely not built by primitive artisans who arbitrarily chose a location and started to chisel away at the rock. There was careful planning and study of the rock layer at every phase of the construction. The strength of jointed rock masses depends on the interlocking blocks that exist between individual pieces. Damage to the interlocking blocks during excavation results in instability. Hence, the construction could not have proceeded without studying the exposed structure at various phases.

KAILASH TEMPLE - ELLORA

For this article, I am focusing on the Kailash temple, a monolithic structure at Ellora. The basalt cliff is part of the Deccan plateau, which was formed by layers of volcanic lava. The cooling of lava created a network of interconnected fractures, cracks and columns in the basalt. Modern excavation uses rigorous structural monitoring and reinforcement. The ancient engineers from this time period also followed such procedures, as evidenced by the partially built vihara in cave 24 at Ajanta.

ANCIENT BLUEPRINT

The building of the Kailash temple started with a proper plan (blueprint). The Vastu Shastra texts define blueprint types that follow a grid system for planning the structure and the space within the temple. The method of making the blueprint is called padavinyasa system. Depending on the scale and complexity of the planned temple, the plan for the temple is drawn on an 8X8, 9X9 or 10X10 intricate grid. A detailed plan of the temple with ventilation holes, drains and arches on doorways was mapped on a grid-based plan, which allowed for designing a symmetrical layout.

STRUCTURAL DESIGN & SEISMIC RESILIENCE

The texts on constructing temple like the Vastu Shastra or the Samarangana Sutradhara, also classify the temples based on the type of spires; curvilinear shikara style, gopuram style, etc. The temple tower on the vertical axis was modelled after mountains like Mount Meru, Mount Mandhara, Mount Kailasha, etc. The number of storeys on the temple tower was based on the mountain model selected for the temple tower. The tapered pyramidal structure of the temple tower facilitates downward load transfer and minimises lateral thrust. The lateral thrust is the horizontal force generated by building elements like arches, walls, etc., that push outwards against their support or adjacent soil. For example, arches convert vertical loading into diagonal forces that exert horizontal thrust at their base. The stepped tiered structure of the tower minimised vertical loading. The temple has a low centre of gravity because of its tapered pyramidal structure with a wide base. This design, along with load-bearing components like beams that were carved out of the same monolithic rock, makes the temple exhibit inherent seismic resilience.

TOP-DOWN CONSTRUCTION METHOD

The construction team used a top-down approach.  At first, a narrow tunnel was drilled closer to the roof. From there, the team excavated downward and outward. The advantage of this method was that scaffolding was not needed.

At each layer, the team had to stop, study the strength of the rock and then either continue with their work or make in-situ modifications.  The Kailash temple has a network of staircases and interconnecting corridors. The load-bearing pillars and beams have been carved out of the rock. Taking into consideration the staircases and corridors, the top-down construction method with in-situ alterations to avoid faults in the rock was a very complicated process. When even a 2D drawing of this temple is difficult to create, imagine an intricately carved structure that was excavated one hundred feet into the hill. The design also incorporated ventilation systems, a drainage system for drainage of rainwater, and temperature control through passive cooling. There was no room for error. Even with in-situ modifications, the overall design with passageways, doors, staircases, vents and drains had to comply with the original plan. With free-standing structures, it is relatively easy to make corrections and alterations, whereas with carving a monolithic structure like the Kailash Temple, there is no room for error because the design is literally set in stone!

 

REFERENCES:

Methodological Elements of Indian Temples – Laxmi Singh

Practical Rock Engineering – Evert Hoek

Building Materials of the Hindu Temples Part II GRANITES and other ROCKS - Dr Uday Dokras, PhD, Sweden, Srishti Dokras, Architect

Temple engineering and seismic design in ancient Indian architecture: An inquiry into structural intelligence and earthquake resilience - Sikhasree Ray and Santigopal Jana