Negative skin friction in Bangkok soft clay: soil layers, soil settlement compared with pile settlement, and axial force in the pile The left panel shows an example soil profile: new fill from 0 to 3 m, soft Bangkok clay from 3 to 14 m, medium stiff clay from 14 to 20 m, and dense sand below 20 m. The pile carries the building load with its toe in the dense sand. Above the neutral plane, red downward arrows show negative skin friction dragging the pile down; below the neutral plane, green upward arrows show positive skin friction, with end bearing at the toe. The middle panel shows settlement with depth: soil settlement is greatest at the surface and decreases with depth, while the pile settles almost uniformly along its length; the two lines cross at the neutral plane. The right panel shows axial force in the pile, starting from the building load at the pile head, increasing to a maximum at the neutral plane, then decreasing to the toe load. The strip at the bottom shows mitigation measures: founding the pile toe in dense sand, coating the pile shaft with bitumen, and preloading so the soil settles before construction. Negative skin friction: soil settles faster than pile Not to scale Example profile (Bangkok) 0 -3 m -14 m -20 m -27 m Building load New fill Soft clay Medium stiff clay Dense sand NSF ↓ Pos. ↑ Settlement Larger → Soil Pile Soil > pile Pile > soil Axial force Q Larger → Q from bldg. Q max Q at toe Neutral pl. (Neutral Plane) 1 Toe in dense sand Firm bearing layer 2 Bitumen coating Reduces NSF 3 Preloading Soil settles before build
Negative skin friction develops when the soil around a pile settles faster than the pile itself, turning the soil from a load-carrying support into an added load. The axial force in the pile therefore peaks at the neutral plane, not at the pile head (Diagram: SPN Soil Engineering)

What Is Negative Skin Friction?

Under normal conditions, the soil around a pile helps carry load through friction on the pile shaft, known as positive skin friction. But when the soil around the pile settles faster than the pile itself, the direction of friction reverses, and the soil changes from a load-carrying support into an extra load on the pile. This phenomenon is called negative skin friction, or downdrag.

Two consequences follow at the same time: the load the pile actually carries is greater than the building load the engineer calculated, and the accumulated settlement at the pile head increases over time, even though no new load has been added to the building.

Key point

Negative skin friction is not caused by a defective pile but by the behavior of the surrounding soil. A pile of perfectly good quality can still experience negative skin friction if it stands in an area where the ground is settling.

Why Are Bangkok and Its Vicinity Especially at Risk?

Soil cross-section of the Chao Phraya plain, showing thick soft clay above stiff clay and sand layers
The soft clay of the Chao Phraya plain is tens of meters thick, a basic condition that makes negative skin friction likely (Illustration: north–south soil cross-section of the Chao Phraya plain)

Bangkok lies on the Chao Phraya plain, which has a continuous layer of soft clay (Soft Bangkok Clay) tens of meters thick above the sand layer where pile toes are founded. This soft layer consolidates readily under surcharge loads or when groundwater levels change, which is the basic condition for negative skin friction.

InSAR satellite data for 2014–2024 show that Bangkok subsided by an average of about 0.3 cm/year (Oh et al., 2026), with most of the inner city subsiding no more than about 1 cm/year (Ahmed et al., 2024). Some areas, however, such as around Ramkhamhaeng, are still subsiding 2–3 cm/year (Department of Mineral Resources, 2025). These rates are much lower than in the era of heavy groundwater pumping, but they still mean the ground around piles keeps moving down, and in areas that still use groundwater or have extensive new filling, local settlement rates may exceed the average. These values are area-wide land subsidence; settlement of the soil around an individual building must be assessed from that site's soil data.

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Cause of negative skin frictionWhen it occursSigns on siteHow to check
New fill on soft soilA few months to several years after fillingGround around the building settles below its original level; pipes and front steps are left hangingBoring log showing fill and soft clay thickness, together with consolidation test results
Groundwater drawdownContinuous, long-termA wide area settles together, not just one buildingGroundwater level data from boreholes and observation wells
Stockpiles or adjacent roadsDuring and after constructionUneven settlement toward the loaded side; diagonal wall cracksGround surcharge layout together with soil investigation results
Old fill still consolidatingExtensions built on old fillExtension settles separately from the original building; joints tear openSite filling history, with boreholes comparing the fill and the original soil
Ground improvement or excavation nearbyDuring construction of an adjacent projectSettlement accelerates over a short period, linked to nearby activityInstrumentation such as settlement plates and piezometers

Where Is the Neutral Plane, and Why Does It Matter?

The neutral plane is the depth at which soil movement and pile movement are exactly equal. Above it, the soil settles faster than the pile, producing negative skin friction; below it, the pile moves down more than the soil, so friction returns to normal positive skin friction.

The neutral plane matters because it tells you two things: the depth of the maximum force in the pile and how much the pile head will settle, since pile-head settlement equals the soil settlement at the neutral plane plus the elastic shortening of the pile itself. The location of the neutral plane depends on the thickness of the soft layer, the pile toe resistance and the building load, and must be analyzed from each site's soil data by the responsible engineer. This concept underlies the Unified Design approach for piles proposed by Fellenius, which checks both the structural force in the pile at the neutral plane and settlement together.

A long pile founded in sand can still settle

A pile with its toe in dense sand settles little, but it must still carry the extra load from negative skin friction along the whole length passing through the soft layer. This force must be included in checking the structural capacity of the pile section, not just the geotechnical capacity.

The figures below model a building on piles in an area where the ground is still settling, as a 2D section with a settlement-versus-depth graph and a 3D model, showing that the neutral plane is where the soil and pile settlement lines cross, and where the monitoring instruments are installed.

2D model (side section) of a building on piles in a subsiding area, with a graph of soil and pile settlement versus depth The left side is a section through a building on piles passing through new fill, soft clay and stiff clay down to dense sand. The ground around the building has settled below its original level, leaving a gap under the floor and a broken pipe at the building entry. Red arrows above the neutral plane show the soil dragging the pile down; green arrows below it show the soil helping carry load. Monitoring instruments include a settlement plate at the base of the fill, a piezometer in the soft clay and a groundwater observation well. The right side is a graph of settlement versus depth: the soil settlement line is greatest at the ground surface and decreases with depth, while the pile settlement line is nearly constant; the two lines cross at the neutral plane. Side section + settlement graph (2D) Not to scale Settlement → 0 Soil decreases with depth Pile nearly uniform Soil > pile = negative SF Neutral pl. Original GL New fill Soft clay Soft Bangkok Clay Stiff clay Dense sand layer Pile end bearing Pipe broken at bldg. joint Observed on site Gap under slab 1 2 3 Monitoring 1 Settlement Plate 2 Piezometer (pore pressure) 3 Groundwater obs. well
2D model — left: the ground around the building has settled, leaving a gap under the floor and a broken pipe. Above the neutral plane the soil drags the pile down (red arrows); below it the soil helps carry load (green arrows) · right: soil settlement is greatest at the surface and decreases with depth, while the pile settles almost uniformly along its length; the two lines cross at the neutral plane. The graph is conceptual, not design values (Image: SPN Soil Engineering)
3D model of a building on piles in a subsiding area, showing a soil cut-away, the neutral plane and the direction of friction along the piles 3D view with one corner of the soil cut away. The building stands on 4 piles passing through new fill, soft clay and stiff clay down to dense sand. The ground around the building has settled below the floor, leaving a gap under the floor and a bent, broken pipe at the building connection. An orange dashed line around the cut faces marks the neutral plane. Red arrows above the plane show the soil dragging the piles down; green arrows below it show the soil helping carry load. In front, a settlement plate at the base of the fill and a piezometer in the soft clay monitor settlement. 3D model (cut-away) Not to scale Building on piles Floor does not settle Gap under slab Ground around settles Kinked pipe Settlement plate at base of fill Fill + soft clay keeps settling over time Neutral pl. Neutral Plane Dense sand layer Pile toes founded here Piezometer Pore pressure in clay Soil drags pile down Negative SF Soil carries load Positive SF 4 piles Transparent view
3D model — one corner of the soil cut away. The building stands on 4 piles through fill and soft clay down to dense sand. The orange dashed line on the cut faces marks the neutral plane. The settlement plate and piezometer are used to confirm whether soil settlement is approaching a steady state (Image: SPN Soil Engineering)

Why Do Piles That Passed a PDA or Static Load Test Still Settle?

Static load test site using a kentledge reaction system per ASTM D1143
A static load test gives the load–settlement relationship on the test day, while an instrumented test can also show which soil layers the friction comes from (Photo: SPN site)

The short answer is that pile testing measures load capacity on the test day, not how many more years the soil around the pile will keep settling. Both PDA and static load tests take hours to a few days, while negative skin friction builds up over months and years as the clay consolidates.

  • PDA (dynamic load test) gives load capacity from stress waves during driving, a momentary condition that usually reflects the soil at that time and does not cover the long-term behavior of soft soil
  • Static load test gives a more accurate load–settlement relationship, but the measured settlement is due to the test load only and excludes future settlement of the surrounding soil
  • Seismic integrity test checks the integrity of the pile shaft and is not directly related to load capacity or settlement

So a passing test result and future settlement are two different questions. What answers the long-term settlement question is soil profile data from soil investigation, laboratory consolidation test results and monitoring of actual settlement after construction. For projects that need to know which soil layers the friction comes from, an instrumented static load test, which measures force along the depth of the pile, is used.

How Can the Risk of Negative Skin Friction Be Assessed?

The assessment relies mainly on soil data, not on pile test results alone. Engineers combine the following information:

  1. Soil investigation deep enough — the full thickness of the soft layer and the depth of the dense sand must be seen; if boreholes are too shallow, the thickness of the layer causing negative skin friction cannot be assessed
  2. Consolidation test — shows how much more and how fast the clay layer will settle under additional load
  3. Groundwater level data — a falling water level increases effective stress in the soil and accelerates settlement of soft layers
  4. Site filling history — recently filled areas keep settling; both the fill thickness and how long ago it was placed should be known
  5. Instrumentation and monitoring — e.g. a settlement plate to track soil settlement and a piezometer to track pore water pressure, used to confirm whether settlement is approaching a steady state

How Can the Effects of Negative Skin Friction Be Reduced?

The approaches below are general information. Their selection and any numerical values must be based on analysis by the design engineer responsible for each project.

  • Include negative skin friction in the pile capacity check — the most direct approach, considering that the pile must carry both the building load and the downdrag from the soil
  • Reduce shaft friction through the soft layer — e.g. coating the pile with a friction-reducing material or using a sleeve over that section only, which must be specified at the design stage and quality-controlled during installation
  • Minimize ground surcharge — reduce fill thickness around the building or use lightweight fill to reduce the settlement trigger at the source
  • Accelerate settlement before construction — preloading combined with ground drainage such as PVD is an approach used on large projects with time for site preparation
  • Separate the ground floor from the soil — use a slab that transfers load to beams and piles instead of resting directly on the ground, so the floor does not settle with the moving soil
What to do before designing the foundation

Carry out a soil investigation covering the full depth of the soft layer, take samples for consolidation testing, and pass the complete data set to the design engineer. Complete data from the start costs far less than fixing settlement problems after the building is finished.

Summary: A Passing Pile Test Does Not Mean No Settlement

Negative skin friction develops when the soil around a pile settles faster than the pile, turning the soil into extra load that drags the pile down instead of helping carry it. Bangkok and its vicinity are at high risk because of the thick soft clay, ongoing land subsidence (about 0.3 cm/year on average, with some areas still subsiding 2–3 cm/year) and new filling that triggers consolidation.

A passing pile test confirms only the load capacity on the test day; it does not answer the question of long-term settlement. Risk assessment requires sufficiently deep soil investigation data, consolidation test results and monitoring of actual settlement after construction. If you need soil data for foundation design, the SPN engineering team can advise and plan the soil investigation.