Pile driving rig and hammer driving a concrete pile on a construction site, while the supervisor records the blow count
Hammer and pile head during driving on site — the values the supervisor records while driving are the source of both the blow count and the Last 10 Blows (Photo: SPN Soil Engineering)

What Is Blow Count and How Is It Counted?

Blow count is the number of times the hammer strikes the pile head to drive the pile down one specified interval. It is usually recorded as "blows per 30 centimeters" or per foot over the full driving depth.

Counting is straightforward. Before driving, the pile is marked at equal intervals along its length. During driving, the site supervisor counts how many blows it takes for the pile to pass each marked interval and writes it in the Pile Driving Record for that pile.

This value shows how hard or easy driving is through each soil layer. When the pile passes through soft soil, the blow count is low (easy penetration); when it enters stiff soil or sand, the blow count rises clearly. Engineers therefore use a blow count vs depth plot to see whether the pile has reached the bearing layer.

Do not confuse this with the SPT N-value in soil investigation, which is also counted as blows per 30 cm but comes from driving a soil sampler in a borehole, not from driving a pile. Read more in How to read SPT N-values

  • Consistently low blow count = the pile is still in soft soil and may need further driving
  • Rapidly rising blow count = the pile is entering stiff soil
  • Total blow count per pile = used to assess the driving effort and the risk of pile head damage from a large number of blows

What Is Last 10 Blows (Last Ten Blows) and How Is It Measured?

Last 10 Blows (Last Ten Blows) is the total penetration of the pile under the final 10 blows, reported in "millimeters per 10 blows" (mm/10 blows). It is measured at the end of driving to decide whether the pile is firm enough to stop driving.

This value is directly related to the "set", the penetration per blow (mm/blow). Dividing the Last 10 Blows by 10 gives the average set at the end of driving. The smaller the Last 10 Blows (the pile barely moves), the more it indicates that the pile toe is resting on a highly resistant layer.

On site, a mark is made on the pile in line with a fixed reference timber, 10 blows are delivered with the same hammer and drop height, and the distance the mark has moved down is measured in millimeters.

  • Small Last 10 Blows = little penetration; the soil at the pile toe is dense
  • Large Last 10 Blows = the pile still penetrates steadily; the soil is not yet firm enough
  • Usually recorded together with the rebound (the bounce-back of the pile head after impact), which is one of the variables in driving formulas such as the Hiley Formula

The figure below combines both values in one view. The left side shows the marking on the pile and detail A, the method for measuring the Last 10 Blows. The right side is a blow count vs depth chart, where the bars lengthen quickly as the pile enters sand or stiff soil.

2D diagram of pile driving with a drop hammer and interval marks on the pile, measurement of penetration under the final 10 blows, and an illustrative blow count vs depth chart Two-panel diagram. The left panel is a side view of a pile driving rig with leads, a drop hammer that is raised and released to strike the pile head, and a helmet and cushion on the pile head. The pile carries red interval marks at equal spacing, e.g. every 30 centimeters, used to count the blows for each interval. Beside the pile is a fixed reference timber that does not move with the pile. Below the ground surface are a soft clay layer and a sand or stiff soil layer at the pile toe. Detail A shows the Last 10 Blows measurement: a mark is made on the pile in line with the reference before driving, 10 blows are delivered with the same hammer and drop height, and the distance the mark moves down is measured in millimeters per 10 blows; dividing by 10 gives the average set per blow. The right panel is an illustrative bar chart with no real values, showing the blow count per depth interval from the ground surface to the pile toe. In the soft clay the bars are short because driving is easy; on entering sand or stiff soil the bars lengthen quickly. The final orange bar is the end-of-driving interval where the Last 10 Blows is measured, and driving stops when the criterion set by the design engineer is reached. The bottom strip summarizes that blow count is in blows per 30 centimeters, where a high value means hard driving; Last 10 Blows is in millimeters per 10 blows, where a low value means little penetration; and the actual bearing capacity must be confirmed by PDA per DPT 1252-51. Principle: blow count over depth vs Last 10 Blows Not to scale Pile driving + interval marks Log blows per mark, then end-of-drive set Blow count vs depth chart Illustrative · assumed values, not real data Soft clay Sand / stiff soil A Drop hammer Raised, dropped on pile head Helmet + cushion Takes impact at head Interval marks on pile e.g. every 30 cm · blows/interval Fixed reference line Ref. timber, not moving A — Measure Last 10 Blows Before: mark at ref. 10 blows After: mark moves down Penetration = L10B Unit: mm/10 blows ÷ 10 = set (mm/blow) Blow count (blows/30 cm) → Ground Pile toe ← Depth Soft clay Low blow count · easy Sand / stiff soil Clearly harder driving Stop End = Last 10 Blows (see A) Stop at the criterion set by the design engineer 1 Blow Count blows/30 cm · high = hard driving 2 Last 10 Blow mm/10 blows · low = little set 3 Confirm capacity PDA per DPT 1252-51
Principle of blow count and Last 10 Blows — interval marks on the pile (e.g. every 30 cm) are used to count blows over the full depth; at the end, the penetration under 10 blows is measured against a fixed reference (mm/10 blows). The chart is illustrative, with assumed values, not actual driving results (Image: SPN Soil Engineering)

How Many Last 10 Blows Before Driving Stops, and Who Sets It?

There is no single Last 10 Blows figure that applies to every project. The design engineer sets the target value for each project based on the pile type and size, the soil layers and the hammer weight used. The site supervisor stops driving when the measured value reaches that criterion.

The Last 10 Blows is used as the criterion because it represents the actual condition at the end of driving. Bearing capacity cannot be measured directly on a piling site during driving, so engineers use the penetration per number of blows (set) as a proxy, based on pile driving formulas such as the Hiley Formula or ENR.

The principle is that part of the hammer energy is converted into work that pushes the pile down (set). If the pile barely moves any more, the soil resistance is high, which relates to the expected bearing capacity.

If the pile reaches the criterion well above the design toe level, do not decide on site; notify the engineer to review first. See the guidance in Piles that refuse before design depth

📌 Important note

The exact set criterion must be specified by the responsible project engineer on a case-by-case basis. This article provides general knowledge and is not a design value for your project.

Hammer drop onto the pile head in a Dynamic Load Test to measure actual energy and resistance
Hammer drop onto the pile head in a Dynamic Load Test — measuring actual energy and resistance instead of inferring them from the set (Photo: SPN Soil Engineering)

How Do Blow Count and Last 10 Blows Differ?

The two values differ in what they count. Blow count counts the number of blows per fixed penetration (e.g. 30 cm), while Last 10 Blows measures the penetration per fixed number of blows (10). The table below summarizes the definition, measuring interval and how to interpret high and low values.

← Scroll to see the full table →

Comparison of blow count and Last 10 Blows
TopicBlow CountLast 10 Blow
DefinitionNumber of blows per penetration interval (blows/30 cm)Total penetration under the final 10 blows (mm/10 blows)
Measuring intervalOver the full driving depthOnly at the end, before stopping
UnitBlows per distanceDistance (mm) per 10 blows
Used forReading soil layers and driving progressDeciding the termination criterion
A high value meansStiff soil, hard driving (often entering the bearing layer)The pile still penetrates a lot (not yet firm enough)
A low value meansSoft soil, easy drivingLittle penetration, dense soil at the toe (near the termination criterion)

The most common point of confusion is the "direction" of the values. A higher blow count means harder driving and stiffer soil, while a lower Last 10 Blows means less penetration and denser soil at the pile toe. However, abnormally high or low values are not always good — for example, the toe may be hung up on a thin hard layer or the hammer may be too small — so they should be read together with the soil investigation and pile test results.

The 3D view below shows how values are recorded on site, from marking intervals on the pile to measuring the Last 10 Blows against a reference timber. The panel on the right lists the steps in order and stresses that the recorded values are not yet the bearing capacity.

3D model of a pile driving site with a drop hammer, interval marks on the pile, a fixed reference line, and a site supervisor recording the blow count and Last 10 Blows Conceptual 3D view of a pile driving site. A soil block is cut away at the front corner to show a soft clay layer on top and a sand or stiff soil layer below. A square precast concrete pile stands at the cut-away corner with its toe in the sand layer. The pile carries red interval marks at equal spacing, e.g. every 30 centimeters. Behind the pile is a rig with a base on the ground, guy cables and vertical leads; the hammer runs along the leads above the helmet on the pile head. To the right of the pile is a reference timber fixed in the ground with a horizontal straightedge touching the pile face, used to mark and measure penetration. A site supervisor stands nearby holding a record sheet. The panel on the right lists the recording sequence: one, mark intervals on the pile before driving; two, count the blows for each marked interval over the full depth as the blow count; three, set up a fixed reference line and mark the pile in line with it; four, deliver 10 blows with the same hammer and drop height and measure the penetration in millimeters per 10 blows; five, compare with the termination criterion set by the design engineer. A warning box states that blow count and Last 10 Blows are not bearing capacity values and must be confirmed by a Dynamic Load Test (PDA) per DPT 1252-51 or a Static Load Test. 3D model: recording pile driving on site Not to scale Drop hammer Dropped from same height 1 Interval marks e.g. every 30 cm 3 Fixed reference line Mark, measure set Rig + leads Hammer runs on vertical leads 2 Supervisor records Blow count per mark Soft clay Easy driving · low count Sand / stiff soil layer Hard driving · criterion met On-site recording steps 1 Mark intervals before driving e.g. every 30 cm, full length 2 Log blow count per mark Blows per interval, full depth 3 Set a fixed reference line Mark the pile in line with it 4 10 blows, then measure set Same hammer/drop · mm/10 blows 5 Compare with stop criterion As set by the design engineer Blow count ≠ bearing capacity Only a preliminary site indicator Confirm by PDA per DPT 1252-51 or Static Load Test Conceptual Measuring method and criteria per project specifications
3D model of recording blow count and Last 10 Blows on site — blows are counted for every marked interval, then the penetration of the final 10 blows is measured against a fixed reference line. The termination criterion is as set by the design engineer (conceptual image, not to scale · Image: SPN Soil Engineering)

The Termination Criterion Is Met — Why Is a PDA Test Still Needed?

Because the Last 10 Blows and blow count are only "preliminary indicators", not direct measurements of bearing capacity. Pile driving formulas that convert the set into bearing capacity rest on several assumptions, such as hammer efficiency, pile head condition and energy losses, so they carry errors.

Key limitations to be aware of:

  • Driving formulas do not separate end bearing from skin friction
  • They say nothing about pile integrity, such as cracks, voids or necking
  • Some soils, such as Bangkok soft clay, show strength regain (soil setup), so the actual capacity changes with time after driving (read Restrike Test and Soil Setup)
  • The values depend on hammer energy, cushion condition and how carefully the site supervisor records them

For these reasons, the termination criterion must be confirmed by testing the actual bearing capacity. The method commonly used for driven piles is the Dynamic Load Test (PDA) per DPT 1252-51 and ASTM D4945, which measures actual force and velocity with sensors on the pile head, followed by CAPWAP analysis.

The results separate end bearing from skin friction and are more reliable than reading the set alone. Where bearing capacity must be measured directly by applying real load, a Static Load Test is used. To plan your testing, see our pile testing services.

Dynamic Load Test (PDA) sensors installed on a pile head to confirm actual bearing capacity
Installing strain sensors on the pile head before a PDA test per DPT 1252-51 (Photo: SPN Soil Engineering)

Summary

Blow count (blows/30 cm) shows the driving progress and soil layers throughout driving, while Last 10 Blows (mm/10 blows) is used to decide when to stop driving at the end, according to the criterion set by the design engineer. Both are good tools for site control, but they are preliminary indicators that must be confirmed with a Dynamic Load Test (PDA) per DPT 1252-51 before you can be confident the pile carries its design load.

Another point to plan alongside is test timing, because concrete piles must wait until the concrete has sufficient age and strength. Read more in Concrete age and pile testing and see how many piles need testing in How many piles to test

📌 Note on pricing

Dynamic Load Test (PDA) starts from about THB 5,500/pile. This price excludes travel and equipment mobilization, which are charged extra according to the distance to the site.