Soil Testing Before You Build: What the Report Tells Your Engineer

Why the ground matters so much here

Much of Bangladesh is built on young river deposits: layers of soft clay, silt, and loose sand, often with groundwater close to the surface. In growing areas around Dhaka and Gazipur, many plots are on land that was filled to raise it above flood level, sometimes only a few years before construction. Fill that has not settled, or a soft layer a few metres down, can make a building settle unevenly and crack.

These conditions can change from one plot to the next. That is why the foundation of a multi-storey building should be designed from data for its own site.

What happens on site

A soil investigation is planned around the building, not just the plot. The engineer sets how many boreholes are needed and how deep they go, based on the footprint, the number of floors, and the expected loads. On site, the investigation team then:

1. 01

   Drills boreholes

   Boreholes are drilled at planned locations, spread across the footprint so that variations in the ground can be seen.

2. 02

   Runs standard penetration tests

   At regular depth intervals, typically about every 1.5 metres, a standard penetration test (SPT) counts the hammer blows needed to drive a sampler into the soil. This blow count, the N-value, shows how dense or stiff each layer is.

3. 03

   Collects samples

   Disturbed and undisturbed samples are taken from each layer and labelled for the laboratory.

4. 04

   Records groundwater

   The groundwater level is noted, because it affects bearing capacity, excavation, and any basement.

The samples are then tested in a laboratory to classify the soil and measure properties such as grain size, moisture content, unit weight, strength, and, for clays, compressibility.

How many boreholes, and how deep?

No single number suits every building. Too few boreholes can miss a soft pocket. Boreholes that are too shallow can stop above the layer that controls settlement. As general principles:

• More than one borehole is normal, even on a small plot, so that variation across the site can be seen.
• Boreholes should go well below the depth that the foundation will stress. For a taller, heavier building, that is deeper.
• If piles are likely, boreholes need to go beyond the expected pile tip.

The site investigation requirements of the Bangladesh National Building Code (BNBC 2020) set the baseline. The engineer adjusts the plan for the particular building and site.

Reading the report

A soil report can look intimidating, but its main parts are straightforward:

Borelogs

A drawn column for each borehole showing the soil layers, their depths, the SPT N-values, and the groundwater level. Low N-values mean soft clay or loose sand.

Laboratory results

Tables of the tested properties for each sample, used to classify the soil and calculate strength and settlement.

Bearing capacity

The pressure the ground can safely carry at a given depth, used to size shallow foundations.

Settlement estimate

How much the building is expected to settle, and whether it will settle evenly.

Recommendations

The suggested foundation type and depth and, for piles, their expected capacity.

From report to foundation

The structural design takes the report's values and turns them into a foundation: isolated or combined footings where firm ground is close to the surface, a mat where loads need to be spread over the whole footprint, or piles where competent soil is deep. The choice affects cost, construction time, and the risk to neighbouring buildings during excavation, so it should be made from evidence.

Common mistakes to avoid

• Using the neighbour's soil report, or an old report prepared for a different building.
• Ordering the cheapest investigation without asking how many boreholes, how deep, and which laboratory tests are included.
• Not telling the investigator the final number of floors, so the boreholes stop too shallow.
• Testing land that is still being filled, so the report does not describe the ground the building will stand on.
• Starting the structural design before the report arrives, and then having to redesign the foundation.

Our soil investigation and foundation analysis service plans the boreholes from the building, not from a standard package.