Why Does Soft Soil Make Earthquakes Feel Stronger?

Site amplification is the phenomenon in which soft soil layers amplify the intensity of seismic waves as they travel up to the ground surface, so buildings on soft soil shake harder than buildings on rock at the same distance from the epicenter.

The cause is that when seismic waves travel from rock into soft clay, their velocity drops and their amplitude increases. Part of the energy is "trapped" in the soft layer and reverberates back and forth. In addition, the natural period of a thick soft soil deposit is long and may be close to the natural period of tall buildings, which can cause resonance and further increase the shaking.

This is why, even though Bangkok has no major fault within the area, earthquakes on distant faults can still be clearly felt in the city's high-rise buildings: the Bangkok soft clay, tens of meters thick, amplifies the shaking. Understanding the soil profile beneath a building is therefore the starting point of earthquake-resistant design.

How Do Earthquakes Affect Foundations?

The effects of an earthquake on a foundation are the forces and movements imposed on the foundation and the surrounding soil during shaking. These differ from normal service conditions, where loads are mainly vertical compression. The key effects are:

  • Lateral force — an earthquake generates horizontal inertial forces that are transferred from the building down to the foundations and piles, so piles must resist lateral bending and shear, not just axial compression.
  • Liquefaction — in saturated sand layers, shaking can cause the soil to lose strength temporarily and behave like a liquid; foundations that rely on that layer may settle or tilt.
  • Differential settlement — when the soil under different footings responds differently to shaking, uneven settlement may occur and damage the structure.

For this reason, designing an earthquake-resistant foundation is not just about making it carry more load; the soil and structure must be considered together under seismic shaking.

What Does Earthquake-Resistant Foundation Design Consider?

Earthquake-resistant foundation design means designing the foundation and pile system to carry both normal service loads and earthquake-induced forces, in line with the actual soil conditions. Engineers consider 4 main factors:

  • Site Class — assessed from the properties of roughly the top 30 meters of soil to show how strongly the ground amplifies shaking, which affects the seismic forces used in design.
  • Liquefaction risk — analyze whether there are saturated sand layers at risk of liquefying; if so, mitigation must be considered, such as piles extending through the at-risk layer or ground improvement.
  • Lateral load on piles — design piles and footings to resist seismic bending and shear, not just vertical load capacity (the lateral capacity of piles can be verified in the field with a Lateral Load Test)
  • Structural continuity and ductility — connect footings, grade beams and piles to work together as a system, distributing forces and reducing differential settlement.

Pile foundations bearing on a stiff layer are a common choice in soft-soil areas because they carry loads through the soft layer and partly reduce the effects of site amplification. But the piles themselves must also be designed for lateral loads and the behavior of the surrounding soil — driving piles deeper does not automatically make a building safe.

Soft clay amplifies earthquake shaking: a building on a shallow foundation compared with a building on piles reaching dense sand The left image is a soil section: a deep stiff layer at the bottom, then dense sand, and soft clay at the surface. Seismic waves travel up from below and their amplitude grows as they pass through the soft clay. The left building sits on a shallow foundation in soft soil, so it rocks and settles unevenly. The right building sits on piles whose tips reach the dense sand, carrying load through the soft layer, but the piles must still be designed for lateral bending and shear. The right image is a conceptual graph comparing ground-surface shaking on a stiff layer, which is small, with shaking on soft clay, which is stronger and lasts longer. The strip at the bottom summarizes the four factors engineers consider in design. Soft soil amplifies shaking: shallow footing vs piles Not to scale Stronger shaking at the surface ! Footing on soft soil Rocking · uneven settlement Piles to dense sand but must resist lateral load Soft clay (amplifies shaking) Dense sand / stiff soil Deep stiff layer / rock Bending + shear Surface shaking (concept) On stiff ground / rock Small, brief shaking On soft clay Strong, long-period, longer Time → 1 Site Class Top ~30 m of soil 2 Liquefaction Saturated sand at risk 3 Pile lateral load Bending + shear 4 Continuity Grade beams tie footings
Soft clay amplifies the amplitude of seismic waves as they travel up to the surface, so shallow foundations on soft soil risk rocking and settlement, while piles that reach the dense sand layer carry loads through the soft layer to the bearing stratum — but they must still be designed for lateral loads.

For existing buildings, seismic retrofit is possible in many cases, but the appropriate approach must be assessed case by case by the responsible engineer, based on the structural condition, soil data and level of risk.

Why Is Soil Investigation Needed Before Seismic Design?

Soil investigation (soil boring) is the process of collecting data on the soil layers beneath a construction site. It is an indispensable basis for earthquake-resistant design, because a building's behavior during an earthquake depends directly on the soil beneath its foundations.

Soil investigation data used in seismic design fall into 4 groups:

  • Thickness and sequence of soft soil layers
  • Soil strength values, such as the SPT N-value and shear strength
  • GW level
  • Shear wave velocity of the soil layers, for projects that require higher precision

These data are used to classify soil layers by SPT value, determine the Site Class and assess liquefaction risk.

3D model of a pile foundation system for a building on soft clay, showing the 4 factors engineers consider in seismic design and the data used from a soil investigation borehole (conceptual) A conceptual 3D view of a soil block cut away to show a section: soft clay at the top, a saturated sand layer below it, and a stiff soil layer at the bottom, with a blue dashed line showing the groundwater level. On the ground surface is a two-storey building frame on six footings, all connected by grade beams in both directions. The front section shows three footings, each with two piles passing through the soft clay and saturated sand to embed their tips in the stiff layer. On the left of the section, an orange wavy line shows seismic waves traveling up from the stiff layer; on entering the soft clay, wave velocity drops and amplitude increases. Orange arrows at each floor level show the building's lateral inertial forces. Number 1 is a soil investigation borehole with a drilling tripod, using data from roughly the top 30 meters of soil to determine the Site Class. Number 2 is the saturated sand layer whose liquefaction risk must be assessed. Number 3 is the piles, which must resist lateral bending and shear, with an orange dashed line showing their deflected shape. Number 4 is the grade beams tying the footings together to work as one and reduce differential settlement. The box on the right summarizes all four factors and lists the soil investigation data used: thickness and sequence of soil layers, SPT N-value and shear strength, groundwater level, and shear wave velocity for projects requiring higher precision, with reference to the DPT 1301/1302-61 standard. 3D model: pile foundation on soft soil, earthquake-resistant design Not to scale 1 2 3 4 Seismic lateral force Building inertia goes to footings Building on piled footings Footings tied by grade beams Soil borehole GW level Soft clay Saturated sand Firm layer Seismic waves from stiff layer In soft soil: speed down, amplitude up Pile tips embedded in stiff soil Section through front footing row · conceptual, not a construction drawing What engineers consider 1 Site Class From top ~30 m of soil 2 Liquefaction risk Any saturated sand? 3 Pile lateral load Resists bending + shear 4 Foundation continuity Grade beams tie footings Soil investigation data Layer thickness & sequence SPT N-value · shear strength GW level Shear wave velocity (detail) Ref. DPT 1301/1302-61 Design from real soil data only
3D model of a pile foundation system on soft soil — numbers 1–4 are what engineers consider: Site Class from borehole data, liquefaction risk of the saturated sand layer, lateral load on the piles, and grade beams tying the footings together. Conceptual illustration, not a construction drawing.

Designing for earthquakes without real soil data is guesswork about how much the ground amplifies shaking, risking both an unsafe design and an unnecessarily expensive one. Soil investigation is therefore a worthwhile investment compared with the damage that could occur. This links directly to earthquake risk and soil liquefaction and the particular characteristics of Bangkok soft clay.

Which Seismic Design Standards Apply?

Earthquake-resistant design in Thailand refers to the standards of the Department of Public Works and Town & Country Planning (DPT), in particular DPT 1301/1302-61, the standard for seismic-resistant building design. It sets out how to assess design seismic forces, including accounting for soil effects through Site Class. Read a summary of the key points in Seismic Building Design Standard DPT 1301/1302-61.

The key principle is that design seismic forces depend not only on earthquake intensity and building location, but also on the type of soil beneath the foundations. Soft-soil sites therefore usually have to be designed for higher seismic forces than stiff-soil sites, other conditions being equal. This makes soil data from a site investigation an essential input for design to the standard.

For ground improvement to reduce risk on soft-soil sites, there are several methods to choose from depending on site conditions; read more in Soft Soil Improvement Methods. If you need a team to analyze the soil profile and design foundations, see our geotechnical design service.

Summary

Earthquake-resistant foundation design on soft soil is not just about bigger footings or deeper piles. It is about understanding that soft soil amplifies shaking, assessing liquefaction risk, and designing footings and piles to carry both gravity and lateral loads in accordance with DPT 1301/1302-61. At the heart of every step is real soil data from a site investigation — the basis for a design that is both safe and cost-effective.

📌 Disclaimer

This article is for general information only and is not design advice for any particular project. Foundation, pile and seismic retrofit design must be carried out by a licensed engineer based on soil investigation data and analysis specific to that site.