What Is Ground Improvement and When Is It Used?

Ground improvement is a group of geotechnical engineering techniques that improve soil properties to meet a specific goal, such as increasing bearing capacity, reducing settlement, accelerating consolidation or reducing liquefaction risk. The concept differs from piling: piles "bypass" soft soil by transferring load down to a firm layer, whereas ground improvement "fixes" the soil itself so it can carry the load.

Situations that commonly call for ground improvement include large-area fills on Bangkok soft clay such as roads, embankments, storage yards and industrial estates; projects where the existing soil bearing capacity is insufficient for shallow foundations; and areas of loose saturated sand at risk of earthquake-induced liquefaction

What Are the 8 Main Ground Improvement Methods?

There are 8 commonly used ground improvement methods, arranged in 4 groups according to the mechanism that improves the soil. Each group suits different soil types and goals.

Group 1 — Soil Replacement

1. Soil Replacement (Excavation & Replacement) — the soft layer is excavated and backfilled with selected material compacted in layers. It is the most straightforward method and the easiest to quality-control. Suitable when the soft layer is shallow (generally no more than 2–4 m); if the soft layer is thicker, the volume of excavation and fill becomes so large that it is uneconomical.

Group 2 — Accelerated Consolidation

2. Preloading (Surcharge) — fill or surcharge material is placed on the area in advance so that the soft clay consolidates and settles first; the load is then removed and the actual construction proceeds. Material cost is low but it takes a long time, because water drains out of clay very slowly.

3. PVD with Preloading (Prefabricated Vertical Drain) — vertical drain strips are installed into the soft clay on a closely spaced grid, giving pore water a shorter path out. The shorter drainage path cuts consolidation that would normally take years down to months. It is the standard method for large-area fills on the soft soils of Central Thailand, such as roads, airports and ports. During the work, instrumentation such as settlement plates and piezometers must be installed to track settlement and pore pressure until targets are met. For an in-depth look at the principle and procedure, see PVD + Preloading: Accelerating Consolidation of Soft Soil

Group 3 — Mechanical Densification

4. Dynamic Compaction — a heavy weight (tens of tonnes) is lifted and dropped repeatedly on the ground surface in a grid pattern. The impact densifies the soil to a depth of several meters. Suitable for loose sand, old fill and construction-debris fill areas; not suitable for saturated clay, and vibration affecting adjacent buildings must be controlled.

5. Vibro Compaction (Vibroflotation) — a vibrating probe is lowered into loose sand with water jetting; the vibration rearranges the sand grains into a denser state over the full depth. It is effective only in sands and gravels with a low fines content. Its key strength is directly reducing liquefaction risk.

Group 4 — Reinforcement / Admixture

6. Stone Column (Vibro Replacement) — a vibrating probe penetrates the soft soil and crushed stone is compacted to form columns in the ground. The columns both share the load with the soil and act as vertical drains. Suitable for soft to medium clay and for reducing settlement of embankments, tanks and factories.

7. Deep Soil Mixing (Cement Column) — cement is mixed in situ with soft soil using a mixing head to form columns, achieving strength several times that of the original soil. Very widely used in Bangkok soft clay for embankment support, excavation support and foundations for light to medium structures. Read more in Deep Soil Mixing / Cement Column

8. Jet Grouting — very high-pressure grout jets cut and mix the soil into soil-cement columns (soilcrete). Position and shape can be controlled precisely, and the work can be done in tight spaces or beneath existing structures, making it the option of choice for foundation repair, water cut-off and tunneling, at the cost of a high price per unit volume.

4 main groups of ground improvement methods: soil replacement, accelerated consolidation, mechanical densification and reinforcement with added materials A four-card diagram. Card one, soil replacement: the shallow soft layer is excavated and backfilled with selected material compacted in layers; suitable for shallow soft layers of about 2 to 4 m. Card two, accelerated consolidation: PVD strips are installed in soft clay and surcharge fill is placed; water flows up along the strips; suitable for thick soft clay. Card three, densification: Dynamic Compaction drops a heavy weight on the ground surface in a grid, densifying loose sand or fill to several meters deep, but with strong vibration. Card four, reinforcement: cement columns and stone columns extend down to the firm layer, adding strength and aiding drainage. Each card lists other methods in the same group. Soft soil improvement: 4 main groups Not to scale Group 1 · Replacement Backfill, compact in layers Soil Replacement For: shallow soft layer ~2–4 m Simple, easy to verify Depth-limited Group 2 · Consolidation Surcharge PVD strips PVD + Preloading For: thick soft clay Cheap but needs time + Preloading only Group 3 · Densification Denser soil, several m deep Dynamic Compaction For: loose sand, fill Strong vibration, avoid bldgs + Vibro Compaction Group 4 · Reinforcement DSM Stone DSM Stone Columns to firm layer DSM · Stone Column For: soft clay Columns down to firm layer + Jet Grouting
Soft soil improvement methods fall into 4 main groups: soil replacement (excavating a shallow soft layer and backfilling in compacted layers), accelerated consolidation (PVD with preloading), mechanical densification (Dynamic Compaction for loose sand and fill) and reinforcement with added materials (DSM cement columns and stone columns) — each group suits different soil types and goals

Comparison Table of the 8 Methods: Which Soil Does Each Suit?

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MethodSuitable soilMain goalRemarks
Soil ReplacementShallow soft layer ~2–4 mRemove the soft layerSimple and easy to verify, but depth-limited
PreloadingSoft clayInduce settlement in advanceCheap but slow; requires waiting time
PVD + PreloadingThick soft clayAccelerate consolidationStandard for large-area fills on soft soil
Dynamic CompactionLoose sand, fillIncrease densityStrong vibration; avoid near buildings
Vibro CompactionLoose sand–gravelIncrease density, reduce liquefactionIneffective in clay
Stone ColumnSoft–medium clayIncrease strength + drainageShares load with the existing soil
Deep Soil MixingSoft clayIncrease strength as columnsVery popular in Central Thailand soft soil
Jet GroutingWide range of soilsLocal strengthening / water cut-offPrecise, works in tight spaces; high cost per point

This table is a preliminary comparison guide only. The actual depth and effectiveness of each method depend on ground conditions, equipment and the design of each project.

How to Choose a Ground Improvement Method

The starting question is not "which method is best" but "what is our soil, and what do we need to fix?" The typical decision sequence is:

  1. Know the soil first — a soil investigation determines soil type, soft-layer thickness, SPT N-Value and groundwater level. This is the baseline data for every method
  2. Use soil type as the first filter — soft clay points toward Preloading/PVD, Stone Column or Deep Soil Mixing, while loose sand points toward Dynamic Compaction or Vibro Compaction
  3. Consider time and area — can you wait several months (Preloading)? Is the area large or localized? Are there adjacent buildings where vibration is not allowed?
  4. Compare cost with a piled alternative — in some cases piles alone may be more economical, particularly for heavy, concentrated structural loads. A geotechnical engineer will compare the two approaches using actual soil data

This design work is part of SPN's Geotechnical Design service, which covers settlement analysis, ground improvement method selection and design integrated with the foundations.

How Do You Know Ground Improvement Has Succeeded?

Ground improvement is considered successful when post-improvement test results meet the criteria set by the designer — it is not finished simply because the work is done. The 4 most common verification tests are:

  • Repeat boring/SPT after improvement — directly compare N-Values before and after
  • Field Density Test — checks the density of compacted fill layer by layer
  • Plate Bearing Test — checks the actual bearing capacity of the improved ground (ASTM D1195/D1196 and DPT 1253-62)
  • Settlement Plate + Piezometer — tracks settlement and pore pressure during preloading to confirm whether design targets have been reached (see our instrumentation installation service)

The figure below summarizes which soil each method is used in and where on site each of the 4 verification tests is performed.

3D model of a ground improvement area showing 4 methods on a cut-away soil block, with the verification test location for each method (conceptual) A conceptual 3D view of a soil block cut away to show its cross-section. The left and middle are soft clay, the right is loose sand, and the bottom is a dense soil layer. The area is divided into four zones from left to right. Zone one is excavation and replacement where the soft layer is shallow; the section shows compacted fill in layers, and a technician performs a Field Density Test on the surface, number 1. Zone two is PVD with surcharge: a fill embankment sits on the surface, the section shows blue PVD strips extending down to the dense layer, a settlement plate at the base of the embankment with its rod protruding above the fill, and a piezometer tip in the clay, number 2. Zone three is DSM cement columns alternating with stone columns, extending down to the dense layer; on the surface is a Plate Bearing Test setup using kentledge as the reaction, number 3. Zone four is Dynamic Compaction on loose sand: a crane drops a weight onto the surface, forming craters in a grid; below the craters the sand is densified, and a boring rig performs a repeat SPT after improvement, number 4. The right-hand panel summarizes the four verification tests: Field Density Test checks the density of compacted fill layer by layer; settlement plates and piezometers track settlement and pore pressure during surcharging; the Plate Bearing Test checks the bearing capacity of the improved ground; and repeat boring/SPT compares N-Values before and after. It concludes that soil type is the first filter: soft clay points toward PVD, Stone Column and DSM, while loose sand points toward Dynamic or Vibro Compaction. The bottom strip shows the four-step method selection sequence. 3D model: ground improvement methods and where to verify Not to scale Replace PVD + surcharge DSM · stone col. Dynamic Compaction 1 2 3 4 Dense / stiff stratum Soft clay Loose sand A Know the soil Soil boring · SPT · GWL B Soil type: first filter Soft clay or loose sand C Time and area Can wait? · Near buildings? D Compare with piles Engineer uses real soil data How to verify results 1 Field Density Test Fill density, layer by layer 2 Instruments during surcharge Settlement Plate + Piezometer 3 Plate Bearing Test Bearing of improved ground 4 Repeat boring / SPT Compare N before–after Soil type is the first filter Soft clay PVD · Stone Column · DSM Loose sand Dynamic / Vibro Compaction Every method needs verification Conceptual, not actual layout
3D model of ground improvement and verification points — soft clay is treated by excavation and replacement (where the soft layer is shallow), PVD with surcharge, or DSM/stone columns, while loose sand is treated by Dynamic Compaction. Numbers 1–4 are the verification tests for each method. Conceptual figure, not an actual layout.

Summary

Ground improvement has 8 main methods in 4 groups: soil replacement, accelerated consolidation, mechanical densification and reinforcement with added materials. Soft clay points toward PVD + Preloading, Stone Column or Deep Soil Mixing, while loose sand points toward Dynamic Compaction or Vibro Compaction. Every project starts the same way — a soil investigation to understand what problem is being solved — and always ends with verification testing.

📌 Disclaimer

This article provides general knowledge only and is not design advice for any specific project. Actual ground improvement method selection and design must be carried out by a licensed engineer based on soil investigation data for that site.