Why you should consult a geotechnical engineer before building your house
Although geotechnical engineers cannot prevent earthquakes directly, they play a crucial role in minimising their destructive impact by analysing soil condition to inform the design of earthquake-resistant structures
Recently, in Bashundhara Residential Area's N Block, a construction project was underway for a new house next to an already existing one built two years ago.
As the workers started cutting the soil to lay the foundation, something unexpected happened: Sand began to flow from the neighbouring property, and soon, water started seeping through the sand.
The garage's car parking area next door began to break down, though the building itself stood firm, thanks to the piles supporting it.
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The residents of the existing building panicked and quickly filed a complaint with RAJUK. Neither they, nor the workers or the developer, were aware of the complexities hidden beneath the ground.
In fact, the architectural design of a house often takes top priority, but the foundation that supports it—the very soil on which it stands—demands thorough analysis and consultation with a geotechnical engineer. Yet, it frequently gets overlooked.
Without this expertise, even the most beautifully designed buildings, or the neighbouring buildings, can face severe issues down the line.
This is especially true for Dhaka, where the terrain is far from uniform.
In rapidly expanding areas like Purbachal, where the soil is firm and resembles the Barind Tract, foundations can be laid as shallow as 4-5 feet. However, in nearby Jolshiri, where wetlands have been filled with sand or clay, the situation is entirely different.
There, the ground had to be filled with either clay or sand up to a depth of 20 to 30 meters before the land could be sold as plots.
This issue isn't limited to just one area—much of Dhaka consists of hundreds of localities with similar soil conditions, as many were once wetlands or low-lying areas, transformed due to the city's rapid but unplanned expansion.
"In such areas, piles have to go as deep as 60-70 feet into the ground. Even then, if the soil is sandy, it will move during an earthquake unless soil improvement techniques are applied," said Dr Mehedi Ahmed Ansary, Professor of Civil Engineering in BUET.
According to the academic specialising in geotechnical engineering, it's precisely why it is very important to understand the type of soil you're building on.
Sand, for example, has large grains that allow water to pass through easily, making it more prone to movement. Clay, on the other hand, settles over time but is denser.
Understanding these differences is important as they affect how soil holds weight, drains water, settles over time, and behaves in earthquakes, which is essential for building safe structures.
It is the responsibility of a geotechnical engineer to evaluate these characteristics and ensure the proper foundation type for your specific plot, preventing future disasters.
Although geotechnical engineers cannot prevent earthquakes directly, they play a crucial role in minimising their destructive impact by analysing soil conditions, to inform the design of earthquake-resistant structures.
"Through soil tests, engineers assess the risk of soil liquefaction, where saturated soils lose strength during shaking, leading to building collapse. By identifying such hazards, they guide foundation designs to mitigate these risks," said Shafiqur Rahman Shafi, Assistant Engineer at Education Engineering Department.
He further explained that based on soil stability and seismic hazard, engineers recommend structural systems like Intermediate Moment Frames (IMF) for moderate seismic zones or Special Moment Frames (SMF) for high-seismic regions, which offer varying degrees of flexibility and energy dissipation, ensuring structures remain resilient during earthquakes.
Consulting a geotechnical engineer before building a house can be essential for various other reasons as well.
Over time, buildings can experience differential settlement, where one part of the structure sinks more than the other due to uneven soil compaction or changes in moisture content.
This often leads to cracks in walls and floors, making the structure unsafe.
"People often come to us with problems like these because they didn't consult us first. It's always easier—and cheaper—to prevent these issues than to fix them later," Dr Ansary said.
Another important consideration is water drainage and retention. The permeability of soil significantly influences how water is absorbed and drained.
Without comprehensive analysis, your home may experience waterlogging issues, especially during the heavy rains witnessed in various parts of the country this monsoon. Insufficient drainage can undermine the soil's stability beneath a structure, leading to long-term damage.
Geotechnical engineers suggest solutions, such as retaining walls or soil grading, to ensure water is directed away from the foundation, keeping it safe and intact.
Constructing on or near a slope adds another layer of complexities. Ensuring slope stability is crucial to avoid landslides or soil erosion, which can gradually or suddenly inflict serious damage on a structure.
Geotechnical engineers assess the slope's angle, soil cohesion, and various other factors to determine the safety of the site for construction.
In some areas, one may encounter expansive soil, particularly those with high clay content. These soils expand when wet and shrink when dry, putting stress on the building's foundation. Without proper testing, building on expansive soil can cause significant structural damage over time.
A geotechnical engineer can identify such problematic soil types and recommend stabilisation techniques or alternative foundation systems to counter these effects.
In spite of numerous advantages, consulting a geotechnical engineer isn't likely to increase one's overall budget significantly, because still it's really very cheap as compared to architectural charges.
According to Dr Ansary, while architects may seek charges ranging from Tk50-200 per square foot, geotechnical and structural engineers often demand as little as Tk5-10 per square foot.
However, the role of the latter is just as crucial as that of the former. "It's easy to get carried away by architectural plans and interior designs, but the true strength of your home lies in its foundation," Dr Ansary emphasised.
Also, consulting a geotechnical engineer can at times save you money instead.
"Many people don't invest in reliable soil testing, often opting for quick tests that cost as little as Tk5,000. However, spending Tk15,000 to 20,000 on proper soil testing can reduce foundation costs by lakhs of taka," said Shafi.
It's because without accurate soil data, structural engineers may overestimate the foundation requirements, leading to unnecessary costs in pile size and number.
For example, if the soil's bearing capacity is unknown or assumed to be low, engineers will design for a more expensive foundation.
"By investing a little more in testing, you can give your engineer the information they need to optimise the foundation and save you money," Shafi concluded.
