The magnitude 7.7 earthquake in Myanmar on 28 March 2025, whose shaking was felt in high-rise buildings in Bangkok, has brought the question of "the ground under our building" back into focus. Many people were surprised that an earthquake with its epicenter more than a thousand kilometers away could make Bangkok towers sway noticeably. A large part of the answer lies not only in the building itself but in the soil layers beneath it. This article explains how soil layers decide a building's fate during an earthquake, and how soil investigation addresses the problem.

Mechanism 1: Why Does Soft Soil Amplify Shaking (Site Amplification)?

Seismic waves travel through bedrock at high speed, but when they enter soft soil layers near the ground surface they slow down and their amplitude increases — the same principle as ocean waves rising higher as they reach shallow water. The thicker the soft soil, the stronger the amplification. Importantly, the vibration period of the soil column may match the natural period of tall buildings, causing resonance, so buildings within certain height ranges shake especially hard.

This is why Bangkok — a city sitting on a basin of thick soft clay (read more on Bangkok soft clay and foundation design) — feels distant earthquakes more strongly than stiff-soil areas. It is also why the earthquake Ministerial Regulation B.E. 2564 (2021) places Bangkok and its vicinity in Zone 2 even though they are hundreds of kilometers from any fault, and why DPT standard 1301/1302-61 specifies seismic forces for the Bangkok basin separately. Two buildings designed identically, one on stiff soil and the other on a soft soil basin, will receive different forces from the same earthquake.

Mechanism 2: How Does Liquefaction Happen?

Liquefaction occurs in saturated loose to medium-dense sand layers (below the groundwater table) when they are shaken rapidly and repeatedly. The mechanism can be summarized in 3 steps:

  1. Shaking makes the sand grains try to pack more densely — like shaking a bottle of sand and watching it settle
  2. But the pore water cannot drain away fast enough — the shaking is so rapid that the squeezed water has nowhere to go, so pore water pressure rises sharply
  3. Once the water pressure is so high that the grains barely touch — the soil loses almost all its strength, behaves like a thick liquid and can no longer carry the building

The consequences: whole buildings sink or tilt, a sand–water mixture spurts up through cracks as "sand volcanoes" (Sand Boils), the ground cracks and moves sideways (Lateral Spreading), and buried pipes or tanks float up. Damage of this kind has been seen in major earthquakes worldwide, and it is one of the main reasons buildings whose structure is still sound become unusable because their foundations failed.

Liquefaction in a saturated loose sand layer: comparison before and during an earthquake The left panel shows conditions before the earthquake: a building on a shallow foundation above a loose sand layer below the groundwater table. The close-up shows sand grains in contact, forming a load-carrying skeleton, and the water level in the piezometer tube equals the normal groundwater level. The right panel shows conditions during the earthquake: the ground is shaken repeatedly, pore water cannot drain in time, and excess pore pressure rises until the water in the piezometer shoots above the ground surface. The close-up shows sand grains separated and floating in water; the soil loses strength, and the building sinks and tilts, with sand boils at the surface. The bottom strip summarizes three conditions: loose to medium-dense sand, saturated below the groundwater table, and shaking that is strong and long enough. Liquefaction: before vs during an earthquake Not to scale Before quake Sand skeleton carries the building normally GW level Saturated loose sand Grains in contact Piezometer During quake — liquefaction High excess pore pressure, soil loses strength Saturated loose sand Sand boil (Sand Boil) Building sinks + tilts Δu Grains float apart Cyclic shaking 1 Loose–medium sand Grain skeleton collapses when shaken 2 Saturated Below groundwater table 3 Shaken hard and long enough Water can't drain, pressure spikes
Before the earthquake, sand grains in the saturated sand layer are in contact and carry the building normally. When shaken repeatedly, pore water cannot drain in time and water pressure rises until the grains barely touch; the soil loses strength and behaves like a liquid, so the building sinks or tilts.

Which Areas Are at Risk of Liquefaction?

  • Shallow saturated loose sand layers, e.g. river floodplains, recently deposited sand sediments, and reclaimed land or sand fill that was not properly compacted
  • Areas with shallow groundwater — the higher the groundwater table, the closer to the surface the sand is saturated (see more on how the groundwater level affects foundations)
  • Areas near active faults, especially the North and West of Thailand, which the B.E. 2564 (2021) Ministerial Regulation places in Zone 3, the highest-risk zone

Pure clay does not liquefy the way sand does, but it suffers from wave amplification under the first mechanism instead — a different problem that needs the same soil data.

How Do Engineers Screen Liquefaction Risk from Soil Boring Results?

The widely used method is the Simplified Procedure summarized in Youd et al. (2001). The principle is to compare two values in each sand layer. The first is the CSR (Cyclic Stress Ratio), the cyclic shear stress the earthquake imposes on the soil. The second is the CRR (Cyclic Resistance Ratio), the soil's resistance calculated from the SPT N value corrected to (N1)60cs. The factor of safety is then FS = CRR ÷ CSR; if it falls below the criterion set by the engineer, that layer is considered at risk.

Sand layers with (N1)60cs of 30 or more are considered dense enough not to liquefy (Youd et al., 2001). This screening requires N values at every depth interval, fines content from grain size analysis, and the groundwater level from the borehole. Data from a single borehole can therefore answer both the Site Class question and the liquefaction question.

The 3D view below combines both mechanisms in a single soil section. The left side shows thick soft clay amplifying waves until a tall building sways; the right side shows saturated loose riverside sand liquefying. The box on the right summarizes the soil data engineers need and the relevant regulations.

3D model of soil layers and earthquakes: wave amplification in thick soft clay and liquefaction in saturated loose riverside sand Conceptual 3D view: a soil block resting on bedrock, with seismic waves traveling up from the bedrock. On the left is a thick soft clay basin; waves increase in amplitude as they pass through the soft soil, and a tall building on piles sways. On the right, near a river, is a saturated loose sand layer below a shallow groundwater table; a house on a shallow foundation settles and tilts, sand spurts up to the surface, and the ground cracks and spreads laterally toward the riverbank. In the middle is a borehole with SPT values recorded to about 30 m depth. The box on the right summarizes the soil data used: Site Class from the top 30 m of soil, liquefaction screening by soil type, saturation and depth, comparison of CSR with CRR from SPT values, and the relevant laws and standards. 3D model: the soil under a building governs earthquake damage Not to scale 1 Thick soft soil amplifies waves Tall building shakes harder 2 Sand boil Saturated sand liquefies 3 Soil flows to bank Lateral Spreading 4 SPT borehole ~30 m Site Class + liquefaction screening 5 Saturated loose sand Liquefiable layer Soil data needed Design shaking Site Class from top 30 m (average N, su or Vs30) Liquefaction screening Loose–medium sand Saturated, below GWL Shallow, shaken hard enough FS = CRR ÷ CSR CRR from SPT (N1)60 (N1)60cs ≥ 30: no liquefaction (Youd et al., 2001) Thai laws + standards Seismic Min. Reg. 2021 DPT 1301/1302-61 BKK + vicinity = Zone 2 Conceptual view, not to scale
3D model — ① thick soft clay amplifies waves from the bedrock and a tall building on piles sways ② a sand–water mixture spurts up as sand boils ③ soil flows toward the riverbank (Lateral Spreading) ④ an SPT borehole about 30 m deep provides data for Site Class classification and liquefaction screening ⑤ saturated loose sand beneath the settled, tilted house (Image: SPN Soil Engineering)

Which Laws and Standards Govern This?

The main law is the Ministerial Regulation prescribing the load-bearing, resistance and durability of buildings and the ground supporting buildings in resisting earthquake vibration, B.E. 2564 (2021), which designates 43 controlled provinces divided into 3 zones. Bangkok and its vicinity are in Zone 2 because their thick soft clay amplifies waves from distant sources, while the North and West, close to active faults, are in Zone 3. Which buildings are covered depends on the zone, building type and size (see the list of provinces and building types in earthquake-resistant building design under the 2021 Ministerial Regulation). The calculation method follows DPT standard 1301/1302-61, the seismic-resistant building design standard of the Department of Public Works and Town & Country Planning.

The key link to soil investigation is that calculating seismic forces requires the Site Class of the building site, classified from soil properties over roughly the top 30 m: the average SPT N-value, undrained shear strength (su) or average shear wave velocity (Vs30) — data obtained directly from soil investigation. A building on soft soil is assigned higher design forces than the same building on stiff soil.

📌 Note: This article is a general overview only and is not legal advice or project-specific engineering advice. The requirements actually in force depend on the latest Ministerial Regulations and standards. Please consult the design engineer or the local authority.

How Does Soil Investigation Help with Earthquake Preparedness?

A single soil investigation provides several layers of data that can be used directly to assess earthquake risk:

← Scroll to see the full table →

Data from the soil investigationUsed for
Soil profile and SPT N-values over the top ~30 mClassify the Site Class to set the design seismic forces
Location of loose sand layers + groundwater levelPreliminary liquefaction risk screening
Grain size analysis and density resultsAssess the liquefaction potential of each layer
Depth of stiff soil / dense sand layersDesign piles to transfer load to layers not at risk of losing strength

Note that the depth of data required (~30 m) is often deeper than investigations for ordinary foundations of small buildings. When deciding the number and depth of boreholes, discuss with the design engineer from the outset whether the data will also be needed for Site Class classification.

If a risk is found, engineers have options at several levels, from changing the foundation system and using deep piles through the at-risk layers into dense soil, to ground improvement — and every option starts with correct soil data.

Summary: The Soil Under a Building Determines the Forces It Receives

In the same earthquake, two buildings that differ only in "the soil underneath" may suffer vastly different damage, both from soft-soil wave amplification and from the liquefaction risk of saturated sand layers. A soil investigation to about 30 m is therefore not just about bearing capacity under normal conditions; it is the starting data for Site Class classification and liquefaction screening under the B.E. 2564 (2021) Ministerial Regulation and DPT standard 1301/1302-61, especially for buildings in monitored zones and tall buildings on soft soil basins (see SPN's soil investigation and testing services).