A static load test measures pile bearing capacity directly, because it applies a real load and measures the real settlement. The heart of the setup is the reaction system, which must be designed to carry more than the maximum test load. This article compares both systems for engineers and contractors who need to plan the site work.

Why Does a Static Load Test Need a Reaction System?

A reaction system is the structure that holds the hydraulic jack in place so that the jack can actually push down on the pile head.

A static load test uses a hydraulic jack to push the pile head down with a specified load, but the jack needs a reaction force pushing back from above; otherwise the jack would lift itself instead of pushing the pile down. The reaction system is what braces the jack so it can transfer the load into the test pile.

The reaction system must always be designed to carry more than the maximum test load. The maximum test load is usually set as a proportion of the design load according to the project specification.

The static loading procedure follows ASTM D1143/D1143M-20e1 and DPT 1251 (Department of Public Works and Town & Country Planning), which set out the incremental loading steps, load holding and settlement recording.

The diagram below compares the load paths of both systems. Follow the orange arrows (jack force P), which push the beam up and the pile down at the same time. On the kentledge side, this force is resisted by the dead weight W (blue arrow); on the reaction pile side, it is resisted by the uplift force T in the anchor piles (red arrows).

Load path of the reaction system in a static load test, comparing kentledge (dead weight) and reaction piles (anchor piles) in side section Two side-by-side panels showing conceptual side sections, not to scale. The left panel is the kentledge system: three tiers of kentledge blocks sit on a main beam spanning two cribbing stacks of timber sleepers on the ground. A hydraulic jack and load cell stand on the head of the test pile beneath the middle of the beam. When the jack pushes the beam up with force P, the dead weight W holds the beam down, so an equal force pushes the pile down, and the remainder, W minus P, is transferred to the ground through the cribbing. The dead weight must therefore always exceed the maximum test load. A dimension line shows the distance between the test pile and the cribbing, which must follow the project specification. The point to watch is the stability of the kentledge platform on soft ground. The right panel is the reaction pile system: a main reaction beam spans above the test pile, with its ends tied to two anchor piles by tie bars. When the jack pushes the beam up, the beam pulls on the tie bars and pulls the anchor piles up as uplift force T. The soil around the anchor piles resists the uplift with downward friction, while the test pile is pushed down and the soil resists upward. A dimension line shows the spacing between the test pile and the anchor piles, which must be sufficient to limit interaction. The points to watch are the transfer of uplift force and the interaction distance. Both panels have a reference beam with dial gauges, with its supports outside the zone of influence. A legend of arrow colors is below the diagram. The bottom strip summarizes three points: both systems must provide a reaction greater than the maximum test load; a 200 t kentledge test needs more than 200 t of dead weight; and a reaction pile system uses 2 to 4 anchor piles whose total uplift capacity exceeds the test load. Load path: Kentledge vs Reaction Pile Not to scale A Kentledge (dead weight) Real weight holds beam vs jack B Reaction Pile (anchor piles) Anchor piles take uplift, not weight Kentledge W > max test load Main beam Cribbing (sleepers) P Jack + Load Cell P on pile W − P Per spec Test pile Ref. beam + Dial Gauge Legs off cribbing Watch: platform stability / soft ground Main reaction beam T T Beam pulls tie bars → lifts anchors Total uplift > max test load P Jack + Load Cell P Adequate Test pile Anchor Anchor Soil resists Watch: uplift transfer + interaction P jack force W dead weight T anchor uplift Soil resistance W − P to cribbing 1 Both systems Reaction > max test load 2 Kentledge 200 t test → weight > 200 t 3 Reaction Pile Total uplift (2–4 piles) > test load
Load path of the reaction system: (A) Kentledge — the jack pushes the beam up with force P, the dead weight W holds the beam down, and the remainder (W − P) goes to the ground through the cribbing, so the dead weight must exceed the maximum test load; (B) Reaction Pile — the beam pulls the anchor piles up as uplift force T, resisted by the soil around the anchor piles — conceptual diagram, not to scale

How Does Kentledge Work and How Much Dead Weight Is Needed?

Kentledge is a reaction system that uses real stacked weight, such as concrete blocks, steel or weight plates, placed on a platform straddling the pile head. The total dead weight must always exceed the maximum test load.

When the jack pushes up, the entire stacked weight acts as the reaction, pushing the jack back down onto the test pile. For example, for a test at 200 t, more than 200 t of dead weight must be stacked to provide a safety margin as specified by the design engineer.

The platform rests on supports such as cribbing stacks of timber sleepers, which must sit on ground able to carry the load securely and be placed away from the test pile at the distance set by the project specification, so that the platform weight does not load the soil around the test pile and distort the results.

Diagram of a kentledge system with dead weight stacked on a platform above the pile head for a static load test
Kentledge system — dead weight stacked on a platform straddling the test pile head acts as the reaction for the hydraulic jack (Image: SPN Soil Engineering)

Advantages of Kentledge

  • Straightforward, with no need for additional anchor piles; suitable where there are no surrounding piles to use as anchors
  • Suitable when a single pile must be tested without disturbing adjacent piles
  • The reaction is constant and predictable, because it is real weight that can be weighed

Limitations of Kentledge

  • Large amounts of dead weight must be transported to site; the higher the test load, the more time and transport cost
  • Requires a large area for the platform and the stacked weight
  • Platform stability is a concern if the ground is soft or the stack is unbalanced; safety must be strictly controlled
  • Not suited to very high test loads, because stacking several hundred tonnes is difficult
Kentledge platform with concrete blocks stacked above the test pile head, with a crane lifting the weights on an actual site
Stacking the dead weight requires a crane and space around the platform, which is the main cost of a kentledge system (Image: SPN Soil Engineering)

How Do Reaction Piles Work and How Many Are Needed?

A reaction pile (or anchor pile) system provides the reaction through anchor piles, typically 2–4 piles around the test pile, with a main reaction beam spanning across and connecting the anchor pile heads.

When the jack pushes up, the beam transfers the force to pull the anchor piles upward as uplift. The anchor piles resisting that uplift become the reaction that lets the jack push the test pile down. The number and capacity of the anchor piles must be designed so that their total uplift capacity exceeds the maximum test load.

Anchor piles work in tension on the same principle as a pile uplift (tension) test; their uplift resistance comes mainly from friction between the pile shaft and the surrounding soil.

Diagram of a reaction pile system showing anchor piles, reaction beam and reference beam in a static load test
Reaction pile system — a reaction beam spans between the anchor piles to create the reaction for the jack to push down on the test pile (Image: SPN Soil Engineering)

Advantages of Reaction Piles

  • Handles higher test loads better than kentledge, so it is common for large-diameter bored piles
  • No need to transport large amounts of dead weight to site
  • Uses less site area for the same test load
Steel reaction beam assembly and anchor pile head reinforcement of a reaction pile system on a confined construction site
A reaction pile system works in tight spaces, because no room is needed for stacking dead weight around the test point (Image: SPN Soil Engineering)

Limitations of Reaction Piles

  • Requires additional anchor piles to be designed and installed, adding cost and time if no existing piles are available
  • The spacing between the anchor piles and the test pile must be sufficient to limit interaction that affects the test results
  • The reaction beam and anchorage design is more complex and must be calculated by the responsible engineer
Reaction pile system set up on an actual site, with steel reaction beams and a hydraulic jack above the test pile head
Static load test setup with a reaction pile system on an actual site (Image: SPN Soil Engineering)

What Is the Difference Between Kentledge and Reaction Piles?

Kentledge and reaction piles differ in the source of the reaction: kentledge uses real stacked weight, while reaction piles use the uplift resistance of anchor piles. This difference affects the achievable test load, site area, transport and safety issues, as shown in the table below.

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Comparison of kentledge and reaction pile reaction systems
TopicKentledge (dead weight)Reaction Pile (anchor piles)
How the reaction is providedReal weight stacked on a platformUplift of anchor piles + reaction beam
Suitable test loadLow to mediumMedium to very high
Site areaNeeds a large areaLess for the same load
TransportLarge amount of dead weightNo dead weight to transport
Additional piles neededNot requiredRequires 2–4 anchor piles
Design complexityLowerHigher
Main safety issueStability of the kentledge platformUplift transfer and interaction distance

Kentledge or Reaction Piles: Which Should You Choose?

Choose reaction piles when the test load is very high, the site is confined, or existing piles can be used as anchors. Kentledge suits open sites with moderate test loads. The decision depends on 3 main factors: test load, site area and availability of anchor piles.

  • Test load: if the test load is very high, reaction piles usually have the advantage, because stacking several hundred tonnes of dead weight is difficult.
  • Site area: a confined site favors reaction piles, while for an open site with a moderate test load, kentledge is more convenient.
  • Availability of anchor piles: if there are existing piles that can serve as anchors, reaction piles are more economical; if there are none at all, stacking weight may be more cost-effective.
Concrete kentledge blocks stacked waiting for setup beside a foundation excavation, showing the site area a kentledge system needs
Actual site conditions are decisive — space for the dead weight and truck access influence the choice of system as much as the test load does (Image: SPN Soil Engineering)

The choice of system and the detailed design should be made by the responsible engineer based on the actual project conditions, not fixed from an article. For an overview of the test method, see What Is a Static Pile Load Test?, and if you are still deciding between test methods, see Dynamic vs Static Load Test: Which Should You Choose?

If you choose reaction piles, the site will have the components shown in the 3D view below. Note that the reference beam (G) has its supports outside the anchor pile line, and the front cutaway shows the anchor piles being pulled up (red arrows) and resisted by the soil (green arrows). The right panel is a checklist for selecting the system and before testing.

3D model of a static load test setup with a reaction pile system: 4 anchor piles, main beam, cross beams, tie bars, hydraulic jack, independent reference beams, hydraulic pump and datalogger, with a checklist for selecting and checking the system Conceptual 3D view of a static load test site with reaction piles, on a block of ground cut away at the front to show the soil layers. The test pile is in the center, surrounded by four anchor piles at the corners of a square. A hydraulic jack and load cell sit on a steel plate on the test pile head and push up beneath the main beam spanning over the pile head. The ends of the main beam bear on two cross beams, each spanning between two anchor piles and tied down to the anchor pile heads with tie bars and top plates. Red arrows above the tie bars show the uplift carried by the anchor piles. The front cutaway through the two front anchor piles shows the anchor pile shafts extending into the soil; the red arrows in the piles are the uplift force and the green arrows beside the piles are the soil friction resisting the uplift. A dimension line shows the spacing between the test pile and the anchor pile line, which must be sufficient. Two reference beams run parallel to the main beam on both sides of the test pile, with supports outside the anchor pile line, and four dial gauges measure the settlement of the pile head. A hydraulic pump is connected by hose to the jack, and a datalogger receiving signal cables from the load cell and gauges stands on the right, away from the test setup. The right panel is a checklist in two groups. The first group is when reaction piles are suitable: very high test load, confined site, and existing piles available as anchors. The second group is what must be checked before testing: the total uplift capacity of the anchor piles must exceed the maximum test load, the spacing between anchor piles and the test pile must be sufficient to limit interaction, and the reaction beam and tie bars must be designed by the responsible engineer. Note: conceptual view; the number of anchor piles is typically 2 to 4, as calculated by the engineer. 3D model: Reaction pile test setup (4 anchor piles) Not to scale Adequate Soil resists Uplift A B C D E F G H I A Test pile head + plate B Hydraulic jack + load cell C Main beam D Cross beam ×2 E Tie bars + top plates F Anchor pile ×4 G Indep. ref. beam ×2 + dial gauge H Hydraulic pump → jack I Datalogger ← load cell/gauges Red = uplift · Green = soil Reaction pile checklist Suitable when Very high test load Stacking 100s of t is hard Confined site No space for dead weight Existing piles as anchors None = install new (time + cost) Check before testing Total uplift > test load Anchor no./capacity per calc. Anchor–test pile spacing Enough to limit interaction Reaction beam + tie bars Designed by the engineer Independent ref. beam Legs outside anchor line Conceptual Typically 2–4 anchor piles Actual no. and spacing per the project engineer's calc.
3D model of a reaction pile test setup: the jack and load cell push on the main beam, which transfers the force through the cross beams and tie bars to pull on 4 anchor piles; independent reference beams with dial gauges measure settlement — conceptual view; the number of anchor piles is typically 2–4, as calculated by the engineer

Once the Setup Is Complete, How Is the Test Run and Read?

Once the reaction system is ready, the load is applied in increments, and the settlement of the pile head is measured at each increment. The results are then plotted as a load-settlement curve to assess bearing capacity against the criterion set in the specification, such as the Davisson offset.

Before loading begins, the pile head must be prepared smooth and level, and the jack set on the pile axis. See details in Pile Head Preparation Before Testing

Hydraulic jack and dial gauges measuring pile head settlement under the kentledge platform during a static load test
Hydraulic jack and dial gauges mounted on the reference beam to read pile head settlement at each load increment (Image: SPN Soil Engineering)

Detailed guidance on reading the results is in the article How to Read Static Load Test Results

SPN Soil Engineering provides pile testing services, including static load tests, nationwide across all 77 provinces of Thailand. Reports are certified by a licensed civil engineer (Council of Engineers Thailand) and can be submitted to the relevant authorities for building permits.

How Much Does a Static Load Test Cost per Pile?

A static load test costs approximately THB 50,000–150,000+ per pile, depending on the test load and the reaction system chosen, because the main cost lies in preparing the reaction system, not the test itself.

This price does not include travel and equipment mobilization, which are charged extra based on the distance to the site. For figures that match your project, contact us for a free quotation within 24 hours.

📌 Note

This article is provided as general engineering knowledge. The choice of reaction system, number of anchor piles, required dead weight, spacing between anchor piles and the test pile, and safety margin must follow the project's specific requirements and the calculations of a licensed responsible engineer. It is not design advice specific to any particular project.