Who Decides How Many Piles to Test?

The design engineer decides, by stating it in the project specification and construction drawings. Before looking at any numbers, understand the order of precedence of the documents that actually govern:

  1. Project specification and construction drawings — the design engineer sets the number of tests and test methods; these are contractually binding
  2. Owner agency requirements — government agencies, state enterprises and industrial estates often have their own criteria
  3. Reference standards — e.g. DPT 1251 (Static Load Test), DPT 1252-51 (Dynamic Load Test), DPT 1551-51 (Seismic/Integrity Test) and ASTM standards. These standards mainly define the "test method"; the number of piles is left to the designer's judgment

So the most accurate answer to "how many piles must be tested?" is: check your project specification. The figures in this article reflect common practice and are for preliminary budgeting only.

An SPN engineer marks the locations of selected test piles on an actual site, spreading test points across the whole building footprint
Selecting and marking the piles to be tested on an actual site — test locations spread across the whole building footprint (Image: SPN Soil Engineering)

How Many Piles per Test Method? Common Practice

Short answer: Seismic Integrity Testing is usually done on all or almost all piles; PDA on a sample of about 1–2% of all piles (at least 1 pile, depending on each project's specification); and Static Load Testing on fewer piles than PDA, but at least 1 pile per pile type/size.

The table below shows common practice in project specifications, for preliminary budgeting — not a mandatory requirement. The actual number follows the specification and the design engineer. (New to these methods? Read How many pile testing methods are there? and What is a Seismic Integrity Test? first.)

← Scroll to see the full table →

Test typeCommon quantity in project specifications*Reason
Seismic Integrity Test
(DPT 1551-51 / ASTM D5882-16, withdrawn in 2025)		All piles (100%) or almost all, especially bored pilesLow cost per pile, fast; screens for defects across the whole project
Dynamic Load Test (PDA)
(DPT 1252-51 / ASTM D4945)		Sampled as a proportion of all piles (often ~1–2%, and at least 1 pile)Confirms bearing capacity at lower cost than a static test
Static Load Test
(DPT 1251 / ASTM D1143)		Fewer piles than PDA (often at least 1 pile per pile type/size)High cost; used as the primary reference test

* Figures reflect common practice, not mandatory requirements — the actual number follows the project specification and the design engineer.

The figure below turns the table into an example foundation plan: blue pile heads are Seismic tests on every pile, orange circles are PDA samples spread across different corners of the plan, and navy boxes are static tests on representative piles of each size. On the right is an example calculation with assumed values: 600 piles × 1–2% gives about 6–12 PDA tests.

Conceptual foundation plan showing how many piles each test method usually covers: Seismic on all or almost all piles, PDA sampled as a proportion, and static tests on representative piles, with an example calculation using assumed values A two-panel diagram arranged left and right. The left panel is a conceptual foundation plan showing only part of a building, with five vertical and three horizontal column grid lines. Footings along the edge of the plan are small two-pile foundations; footings under the three main columns in the middle are large four-pile foundations. Every pile head is shaded blue, meaning Seismic Integrity Testing on all or almost all piles. Four piles spread across different corners of the plan are circled in orange, meaning Dynamic Load Testing (PDA) sampled as a proportion. Two piles are framed by navy squares, meaning Static Load Tests on representative piles: one is a small pile at the edge of the plan and the other a large pile under a main column, so that every size is covered. A note says the number of piles in the figure is not to scale. The right panel is an example calculation with assumed values: the project has 450 small piles and 150 large piles, 600 in total. Seismic tests all or almost all piles, about 600 piles. PDA at the common level of about 1 to 2 percent gives about 6 to 12 piles, at least 1 pile and covering both sizes. Static Load Testing covers fewer piles than PDA, at least 1 pile per pile type or size, so at least 2 piles. A note box states that the figures reflect common practice and are not mandatory. A bottom bar sums up three points: check the project specification first; a minimum of at least 1 pile per pile type or size; and the design engineer sets the actual number and locations. How many? All → proportion → representative Not to scale Foundation plan (conceptual) Part of plan · pile count not to scale 1 2 3 4 5 A B C 3 main footings · L piles Seismic All / almost all PDA Proportional sample Static Representative, each size Pile size Small (S) · Large (L) Example (assumed values) 450 S piles + 150 L piles Total N = 600 piles Seismic Integrity Test All (100%) or almost all → approx. 600 piles Dynamic Load Test (PDA) 600 × ~1–2% Min. 1 pile · covers S and L → approx. 6–12 piles Static Load Test Fewer than PDA ≥ 1 per type/size (S + L) → at least 2 piles Common practice, not mandatory Actual number per project spec and the design engineer 1 Check the spec first For preliminary budgeting 2 Minimum 1 pile per pile type/size 3 Designer decides actual number & locations
Conceptual foundation plan: Seismic Integrity Testing on all or almost all piles, PDA sampled at about 1–2% (at least 1 pile), and Static Load Testing on representative piles, at least 1 per pile type/size — the example calculation uses an assumed N = 600 piles. Figures reflect common practice, not mandatory requirements; the actual number and locations are set by the design engineer.

What Factors Do Engineers Use to Set the Number of Test Piles?

There are four main factors: pile type, building size and importance, variability of the soil strata, and the results of the first tests. The higher the risk, the more piles are usually tested.

1. Pile type

  • Bored piles — cast in place, so the concrete below ground cannot be seen; the integrity risk is higher, so Seismic testing of every pile is usually specified
  • Driven piles — factory-made under quality control, but at risk of cracking during driving; Seismic testing is usually done on a sample, together with blow count monitoring during driving

2. Building size and importance

High-rise buildings, public buildings or heavily loaded structures usually specify a higher testing proportion than ordinary houses, and often require at least 1 Static Load Test.

3. Variability of the soil strata

If the soil investigation shows that the strata across the site vary greatly, the engineer may increase the number of test piles and spread the test locations to cover every zone. To understand where soil data comes from, see Why soil investigation is needed before construction.

4. Results of the first tests

If abnormal results are found — for example a Seismic test shows a necking signal, or PDA gives a capacity lower than the design value — the engineer usually orders additional test piles around the problem pile to define the extent of the problem.

📋 Seismic shows integrity · PDA/Static show capacity

Put simply, the Seismic Integrity Test answers "is the pile shaft sound?", while PDA and Static Load Tests answer "can it really carry the design load?". Most projects therefore use both groups together rather than choosing one or the other.

Which Piles Should Be Tested? Principles for Distributing Test Locations

The piles to choose are those that represent the whole project, and those at the highest risk. What matters more than "how many" is "choosing the right piles". The principles engineers use to select test piles:

  • Spread them across the whole building footprint and every soil zone — do not cluster them in one corner
  • Choose heavily loaded piles, or piles under the main columns of the structure
  • Include piles with abnormal construction records, e.g. hard driving, concrete slump/drop, or a pile tip that did not reach the design bearing layer
  • Cover every pile type/size used in the project

The model below combines all four principles in one plan: the selected piles are spread across Zone A and Zone B, which have different soil strata; they include piles under main columns that carry heavy loads, a red-flagged pile whose tip did not reach the design layer, and representative piles of both sizes.

Conceptual 3D model showing the principles for selecting piles for PDA and Static Load Tests in a pile layout, after every pile has been screened by Seismic Integrity Testing A conceptual 3D view of a foundation area after piling is complete, on a soil block cut away to show soft clay over sand. The plan is divided into Zone A on the left and Zone B on the right. Zone B has a thicker soft layer, so the front-row piles in Zone B are longer. Piles are grouped by column position: footings along the edge of the plan are groups of two small piles, while the middle of the plan has two groups of four large piles under the heavily loaded main columns, shown with large downward load arrows. Every pile head is shaded blue, meaning integrity has been screened by Seismic Integrity Testing on all or almost all piles. Piles circled in orange are those selected for PDA: the back-left corner pile, a pile in Zone B, a pile under a main column, and a front-row pile with a red flag because of an abnormal construction record; the soil section shows that this pile's tip stops in the soft layer and does not reach the design sand layer. Piles framed by navy squares are representative piles for Static Load Testing: one is a large pile under a main column and the other a small pile in Zone B, so that every size is covered. Each point has a number label one to four linked to the box on the right, which sums up the selection principles: one, spread across the whole plan and every zone, not clustered in one corner; two, choose heavily loaded piles or piles under main columns; three, include piles with abnormal records, such as hard driving, concrete slump or a tip that did not reach the design soil layer; four, cover every pile type and size. This is followed by the cost-effective sequence: Seismic screening first, then select high-risk or heavily loaded piles for PDA, and do static tests only as the specification requires, plus a note that the actual number and locations are set by the design engineer. The figure is conceptual, not an actual pile layout. 3D model: which piles to test Not to scale Pile layout after piling 2 zones by soil · 2 pile sizes Zone A Zone B Main column, heavy load 2 1 1 4 4 2 3 Abnormal (red flag) Tip short of design layer Zone B: thicker soft soil so piles are longer Soft clay Sand Blue head Seismic all/almost all Orange ring Selected for PDA Navy box Static, representative Red flag Abnormal construction record Which piles to test? After Seismic screening of the project 1 Spread across plan · all zones Not clustered in one corner 2 Heavily loaded piles Under main structural columns 3 Abnormal records Hard driving · concrete drop Tip short of design layer 4 Every pile type/size used in the project (S and L) Cost-effective sequence ① Seismic screens the whole project ② High-risk/heavy-load → PDA ③ Static only as spec requires Early results abnormal? Test more around it to find extent Actual number/locations Set by design engineer Conceptual · not an actual pile layout
Conceptual 3D model: after Seismic screening of the whole project, the engineer selects piles for PDA and static testing, spread across every zone, focusing on piles under heavily loaded main columns, piles with abnormal records, and representative piles of every size — the figure is not an actual pile layout; the actual number and locations are set by the design engineer.

How Do You Plan the Number of Tests Cost-Effectively?

The most cost-effective approach is to test in sequence, not to run every method on every pile. An approach that meets the specification without costs spiraling:

  1. Start with Seismic Integrity Testing to screen integrity across the whole project
  2. Select high-risk or heavily loaded piles for PDA
  3. Keep Static Load Tests as the reference test, only as many as the specification requires

Indicative prices per pile for preliminary budgeting: Seismic Integrity Test about THB 100–150/pile, PDA about THB 5,500/pile, and Static Load Test about THB 50,000–150,000+/pile. These prices exclude travel and equipment mobilization, which are charged extra according to the distance to the site. See prices for each method in Pile testing prices 2026, and schedule tests to suit concrete age in How many days of concrete age before a pile can be tested?

⚠️ Do not cut the number of tests to save money without consulting an engineer

Reducing the number of test piles below what the specification requires may cause the work to fail acceptance, or let problem piles go unnoticed. Repairing a foundation later costs many times more than testing. If you want to make the test plan more economical, always have the design engineer review it.

📌 Note

This article is provided as engineering knowledge. The number of piles to test, the sampling proportion and the selection of test piles should follow the project specification and the judgment of a licensed engineer. Proportion ranges (e.g. ~1–2% for PDA) reflect common practice, not mandatory requirements.

SPN Soil Engineering provides every pile testing method — Seismic Integrity Test, PDA and Static Load Test — nationwide in all 77 provinces. See details on the pile testing services page.