This article walks through a real job: what the results look like, how to read them, and why a pile length figure alone is not enough. It has to be overlaid on the soil boring results before it can answer the design engineer's questions. For the principles of the test method, see What is the Parallel Seismic Test?
What Was the Brief for This Project?
The brief was to obtain 2 pieces of information for the design engineer before calculations began: how long the existing piles are and what soil layers lie beneath the site. The project was a structural investigation to provide data for building renovation of an existing factory that had been in use for many years. The owner wanted to renovate the building, but no construction drawings or pile driving records remained.
SPN carried out both parts under one project so that the pile and soil data share the same reference level. The scope of work had 2 parts:
- Parallel Seismic tests on 2 piles (TP#1 and TP#2) inside the building, tested on 24 September 2026, with the method referenced to ASTM D8381/D8381M-21
- 1 soil boring (BH-1) to 30.45 m inside the building, drilled on 15–16 September 2026, with laboratory soil testing
How Were the Pipes Installed and the Parallel Seismic Test Run on Site?
Parallel Seismic requires a pipe parallel to the pile for lowering a sensor, and the foundation is struck to measure the time the wave takes to reach each depth. On this job, a hole was drilled and a 2-inch PVC pipe was installed next to each pile and filled with water as the wave-transmitting medium. The foundation was then struck with a hammer while the receiver (hydrophone) was pulled up approximately 0.5 m per hammer blow.
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| Item | TP#1 | TP#2 |
|---|---|---|
| Pipe distance from pile | 1.20 m | 1.40 m |
| Pipe depth (from pipe top) | 29.15 m | 30.80 m |
| Pipe size | 2-inch PVC, water-filled | |
| Sensor pull-up interval | Approx. 0.5 m per hammer blow | |
| Impact point | On the foundation of the pile under test | |
Two figures are worth noting:
- The pipes extend about 5–8 m below the pile toes found, because seeing the change in slope of the graph requires several data points below the toe. If a pipe is so shallow that it ends right near the pile toe, the result cannot be concluded.
- Pipe distances of 1.20–1.40 m are within the criteria stated in the report: no more than 2.5 m, and the pipe must not be inclined.
The section below brings every element of this job together in one figure, from the impact point on the foundation, the PVC pipe beside the pile and the sensor positions, to the soil layers from boring BH-1. Depths are drawn to true scale relative to the factory floor, so you can see at a glance how far the pipe extends below the pile toe and where the toe sits within the soil profile.
- Hammer blow on the foundation sends a wave down the existing pile
- Water-filled 2-inch PVC pipe 1.20 m from the pile, 29.15 m long from the pipe top
- Hydrophone pulled up approximately 0.5 m per hammer blow
- Alongside the pile the wave travels in concrete at ~4,000 m/s
- Below the pile toe the wave must pass through soil at ~1,500 m/s, so the graph changes slope
- Pile toe TP#1 ≈ 23.35 m (TP#2 ≈ 22.60 m) at the layer boundary at 23.0 m
- Boring BH-1, 30.45 m deep SPT-N = 8 at 22.5 m → 20 at 24.0 m
- Data acquisition unit on the factory floor, recording wave arrival time at every depth
How Are Parallel Seismic Results Read, and What Pile Lengths Were Found?
The pile toes on this job are 23.35 m (TP#1) and 22.60 m (TP#2) below the factory floor, read from the point where the graph changes slope. The Parallel Seismic result is a graph of wave arrival time at the sensor versus depth. While the sensor is alongside the pile, the wave travels down through the concrete, which is fast (this job used an analysis value of 4,000 m/s), so the data points line up along a steep straight line. Once the sensor goes below the pile toe, the wave must travel through the slower soil (about 1,500 m/s), so the slope of the graph changes. The point where the slope changes is the pile toe level.
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| Pile | Pile toe depth from PVC pipe top | Pile toe depth from factory floor |
|---|---|---|
| TP#1 | 23.75 m | 23.35 m |
| TP#2 | 23.00 m | 22.60 m |
Why Are There 2 Values, and How Important Is the Reference Level?
The PVC pipe top is 0.40 m above the factory floor, so the report gives 2 sets of values. The value for practical use is the one referenced to the factory floor, because it is the same level the designer uses, and boring BH-1 was also started from the factory floor.
A 0.40 m discrepancy looks small, but on this job the pile toes are very close to a layer boundary. If the levels are compared incorrectly, the conclusion about which layer the toe sits in could change immediately.
Another point to understand is that the reported results are "approximate depths". The resolution depends on the sensor pull-up interval (here every 0.5 m), the pipe distance and how the lines are fitted to find the break in the graph. The 2 piles in the same building differ in length by 0.75 m, which is normal for driven piles in older buildings. Testing more than 1 pile therefore helps reveal the range of actual lengths.
What Does Soil Boring BH-1 Tell Us?
Boring BH-1 found soft clay about 21.5 m thick before reaching stiff clay at a depth of 23.0 m. The hole was drilled by rotary wash boring, with undisturbed samples taken by thin-walled tube (Shelby tube) per ASTM D1587 in the soft clay and SPT per ASTM D1586 in the stiff soils, at 1.5 m intervals. The groundwater level was measured at −1.00 m. The soil layers are summarized as follows:
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| Depth (m) | Soil description | Key test values |
|---|---|---|
| 0.00–1.50 | Fill materials | — |
| 1.50–23.00 | Very soft to medium stiff gray clay (CH) | Su 0.67–2.63 t/m² · natural moisture content up to about 96% |
| 23.00–30.45 | Very stiff to hard clay and sandy CLAY (CH, CL) | SPT-N 20–55 blows/ft |
This soil profile is the Bangkok soft clay commonly found across Samut Prakan. To learn to read a boring log fluently, see How to read a boring log and Interpreting SPT N-values
What Do the Two Results Show When Overlaid?
When the pile lengths are plotted on the boring log using the same reference level, it is clear that the toes of both piles (22.60–23.35 m) sit right at the layer boundary at 23.0 m. The SPT sample at 22.5 m is still medium stiff clay (N = 8), while at 24.0 m it is very stiff clay (N = 20).
This conclusion matters to the designer, because pile toe resistance depends on the soil layer the toe rests in. A pile whose toe reaches the stiff layer and one whose toe is still in medium stiff clay may have clearly different capacities, even though their lengths differ by less than 1 m.
This data set is the starting input for the design engineer's further assessment, for example choosing conservative values for piles whose toes have not reached the stiff layer, or considering whether more data are needed. Deciding how much new load the existing foundation can carry, and whether foundation strengthening is required, is a matter for the judgment of the engineer responsible for the project.
Parallel Seismic mainly tells you the "length" of a pile. It does not tell you the load capacity, cross-section size or integrity of the pile shaft. To find those out, other methods are needed, such as exposing the pile head or a Seismic Integrity Test where the pile head is accessible.
Lessons from This Job for Anyone Renovating an Existing Building
This job offers 5 lessons for gathering existing foundation data before renovating a building:
- Always pair Parallel Seismic with soil boring — pile length alone does not tell you what the toe rests on
- Drill the boring deeper than the expected pile toe — this job drilled to 30.45 m, revealing both the layer boundary and the stiff layer below
- Install pipes several meters below the pile toe, straight, parallel and no more than about 2.5 m from the pile
- Use one reference level for all data — the pipe top, building floor and borehole collar must be converted to the same level before comparison
- Test more than 1 pile — piles in the same building can differ in length; on this job by 0.75 m
If you are planning to inspect an old building before renovation or extension, see the overview in Old buildings before extension: what structural checks are needed?, or see SPN's non-destructive structural testing (NDE) and soil investigation services
Fees for Parallel Seismic and soil boring depend on the number of piles, the pipe and borehole depths, and working conditions inside the building, so they are estimated per project. Request a free quotation within 24 hours on the SPN contact page. Prices exclude travel and equipment mobilization costs, which are charged according to the distance to the site.
The figures and photos in this article come from an actual SPN test job, without naming the building owner. The Parallel Seismic graph and sections in the article are conceptual model diagrams created from the project values, not the original graphs, and the conclusions in the article are explanatory, not foundation design results.