A test result showing that a pile carries 100 t tells you whether the pile passes the criteria, but not which soil layers that 100 t comes from. Without knowing the split between the skin friction of each soil layer and the force at the toe, the designer can hardly judge whether changing the pile length would gain enough capacity to be worthwhile. Instrumenting the test pile answers this question with real field data.
What Is an Instrumented Static Load Test?
An instrumented static load test is an axial static pile load test to ASTM D1143/D1143M-20e1 and DPT 1251 with strain-measuring instruments embedded in the pile shaft at several levels along its depth. The loading procedure, the holding of each load increment and the settlement readings at the pile head all still follow the standard in every respect. What you gain is a second data set: how much force remains in the pile shaft at each level at every load increment.
How Does It Differ from a Conventional Static Load Test?
The difference lies in the data obtained, not in how the load is applied. A conventional test measures only the applied load and the settlement at the pile head, while an instrumented test also reads the force in the pile shaft at several levels along its depth.
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| Issue | Conventional static load test | Instrumented Static Load Test |
|---|---|---|
| What is measured | Applied load and settlement at the pile head | Adds strain in the pile shaft at several levels along the depth |
| Output | Total capacity of the whole pile | Skin friction per soil layer, and end bearing at the toe |
| Advance preparation required | None; existing piles can be tested | Instruments must be embedded before the pile is cast or manufactured |
| Applicable piles | Test piles or working piles | Mainly test piles planned from the outset |
| Benefit to design | Confirms whether capacity meets the criteria | Allows soil parameters and pile lengths to be refined for the whole project |
In short, a conventional test answers "does it pass?", while an instrumented test answers "why does it pass?" This data can be used to refine the design of the remaining piles across the whole project. See the basics of static testing in Static Pile Load Test
What Do Strain Gauges in a Pile Measure, and How Are Readings Converted to Force?
A strain gauge measures the strain of the pile material at the location where it is embedded, which is then converted into force in the pile shaft. When load is applied to the pile head, the pile shortens slightly; the shortening divided by the original length is the strain. The conversion to force uses the basic relationship for elastic materials.
Force in the pile at a given level = measured strain × elastic modulus of the pile × pile cross-sectional area
The point to watch is the elastic modulus of the concrete: if the wrong value is used, the whole set of calculated forces will be wrong. In practice, a pair of strain gauges is therefore usually installed at the top level, above ground, because at that level the force in the pile still equals the load read from the load cell on the pile head. Knowing both force and strain at the same level lets you back-calculate the actual modulus of that particular pile, which is then applied to the lower gauges.
Strain gauges in concrete piles are usually vibrating wire sister bar gauges tied parallel to the main reinforcement before concreting. At each level they are installed as a diametrically opposite pair, and the average of the pair is used to cancel out bending effects that may arise from slight load eccentricity.
The figures below show the complete test setup as a 2D drawing (side section) and a 3D model. Note that the loading system is exactly the same as a conventional static load test; what is added is pairs of strain gauges at 5 levels in the pile, from SG-1 above ground for modulus calibration down to SG-5 near the pile toe.
How Are Skin Friction per Soil Layer and End Bearing Separated?
Look at how much the force in the pile shaft decreases between two adjacent levels. The force lost is the load that the soil in that interval carries through skin friction, and the force still remaining at the lowest level is the load that reaches the pile toe.
Average unit skin friction of an interval = (force at upper level − force at lower level) ÷ (pile perimeter × length of that interval)
Repeat this interval by interval down to the lowest gauge. The force still remaining at the lowest level is an estimate of the end bearing at the pile toe. The caveat is that the lowest gauge must be reasonably close to the pile toe; otherwise the value will still include the skin friction of the remaining length.
The figure below is a conceptual example (assumed values, not actual test results) of a Ø0.60 m bored pile, 24 m long, instrumented at 5 levels. Look at the solid line at the 300 t test load, where the force decreases to 286, 226, 157 and 83 t with depth, giving a total skin friction of 217 t and 83 t remaining at the pile toe.
Example calculation from the figure: in the medium sand interval at 13–19 m depth, the force decreases by 226 − 157 = 69 t. The surface area of this interval is 1.885 × 6.0 ≈ 11.3 m², so the unit skin friction is about 69 ÷ 11.3 ≈ 6.1 t/m², or roughly 60 kPa, matching the right-hand bar in the figure.
How Do You Read a Load Transfer Curve?
A load transfer curve is a plot of the force in the pile shaft against depth, with one line for each test load increment. Engineers look at 3 things.
- The slope of the line in each depth interval. Where the line slopes steeply (force drops a lot over a short distance), the soil layer provides high skin friction. Where the line is nearly vertical, the layer provides almost no friction, such as very soft clay or a sleeved interval that eliminates friction. In the example figure, the force drops by only 14 t through the 0–6 m soft clay.
- The force remaining at the pile toe. If very little remains, the pile works almost entirely in skin friction. Increasing the length will only help if the deeper soil layers can still provide additional friction.
- Behavior as the load increases. Soil skin friction usually reaches its peak at a displacement of only a few millimeters, while end bearing needs more displacement to fully mobilize. At high load steps, the share of force at the pile toe therefore usually keeps increasing. In the example figure, at 150 t the force reaching the toe is 29 t (about 19%), while at 300 t it increases to 83 t (about 28%).
The pile-head settlement curve is still interpreted with the usual criteria, such as the Davisson criterion, explained in How to read static load test results
What Must Be Prepared Before Casting the Test Pile?
The decision to instrument must be made before the pile is cast or manufactured. This is the most important limitation of this test, because the strain gauges must be tied to the reinforcement cage and cast into the concrete, with the signal cables run up to the pile head. Advance preparation includes:
- Choose a test pile location where the soil profile is representative of the project area, based on the soil investigation results
- Set the strain gauge levels to straddle the boundaries of the soil layers of interest, with the top level above ground for modulus calibration
- Protect the signal cables while the reinforcement cage is lowered and the concrete is poured, which is when instruments are most often damaged
- Record initial readings of all gauges before loading begins, to use as a reference
- Wait for the concrete to reach its specified strength before testing, on the same principle as a conventional static load test
Once a pile has been cast or manufactured, instruments can no longer be added inside it. An instrumented test must therefore be in the plan from the design stage; it is not something you can decide to add later when testing.
When Is an Instrumented Static Load Test Worth It?
It is worth it when the load transfer data can be used to refine the design of a large number of piles, or when the soil conditions are highly uncertain. Instrumentation adds both preparation time and cost, so it is not done on every project. Cases where it is usually worthwhile:
- Projects with a large number of piles. Data from a few test piles may be used to refine the length of all the remaining piles in the project, which may outweigh the added testing cost.
- Complex soil conditions or limited data, for example areas with thick soft clay interbedded with sand layers, where the design parameters are highly uncertain.
- Long or heavily loaded piles, where an error in estimating the split between skin friction and end bearing has a major effect on safety or construction cost.
- Work that must confirm design assumptions, for example projects where the designer uses skin friction values higher than those commonly used and needs field data to confirm them.
For general work that only needs to confirm that piles carry their design load, a conventional static load test or a Dynamic Load Test (PDA) to ASTM D4945 and DPT 1252-51 is usually sufficient. For choosing the number of piles and test method, see How many piles need to be tested
In terms of cost, the static load test portion is approximately THB 50,000–150,000+ per pile, depending on the test load and loading system, while strain gauges and installation are charged extra according to the number of gauge levels. Contact us for a free quotation within 24 hours. These prices exclude travel and equipment mobilization costs, which are charged extra according to the distance to the site.
SPN provides static pile load testing, both conventional and with strain gauges, with reports certified by a civil engineer licensed by the Council of Engineers Thailand (COE). See details at pile testing services
How Does It Differ from a Bi-Directional (O-cell) Test?
The difference is how the load is applied. An instrumented SLT pushes down from the pile head, while a bi-directional load test to ASTM D8169 uses a jack embedded in the pile that pushes up and down simultaneously. Both methods can separate skin friction from the force at the pile toe, and both must be prepared before the pile is cast.
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| Issue | Instrumented SLT (ASTM D1143) | Bi-Directional / O-cell (ASTM D8169) |
|---|---|---|
| Loading method | Pushes down from the pile head | Jack inside the pile pushes up and down simultaneously |
| Reaction system | Requires kentledge or anchor piles | Uses the pile itself as reaction; no kentledge needed |
| Site area required | Large, because a kentledge system must be set up | Clearly smaller |
| Force distribution with depth | Yes, from strain gauges at each level | Gives the forces above and below the jack directly; separating forces per soil layer requires additional strain gauges |
| Must be prepared before casting | Yes | Yes |
Details of the bi-directional method are in Bi-Directional (O-cell) Load Test
Summary
An instrumented static load test is a static load test to ASTM D1143 and DPT 1251 with strain gauges embedded in the pile at several levels, making it possible to separate which soil layers the measured capacity comes from through skin friction and how much remains at the pile toe. This data turns the test result from a pass/fail answer into information that can be used to refine pile design for the whole project. The trade-off is that the instruments must be planned and installed before the pile is cast, and the cost is higher than a conventional test, so it suits projects with a large number of piles or uncertain soil conditions.
This content is general knowledge for understanding, not design advice for any specific project. The number and location of instruments, the soil parameters to be adopted, and the interpretation of results must be handled by the project's responsible engineer holding a COE license, together with the actual soil data and construction drawings.