Grain size analysis (sieve + hydrometer) for geotechnical projects in Cork

On a recent project down by the Marina, a contractor hit a lens of fine silt right where the borehole log showed gravelly sand. The foundation design assumed free-draining material, but the grain size distribution told a different story—one that required a complete rethink of the drainage strategy. That is exactly why a full grain size analysis combining sieve and hydrometer is non-negotiable in Cork, where the glacial and alluvial deposits can flip from coarse to fine within a few metres. Our laboratory runs the complete curve from 63 mm down to the 2-micron clay fraction, following IS EN ISO 17892-4:2016, so the design team gets a single, coherent dataset that feeds directly into permeability estimates, frost susceptibility checks, and soil classification per the Irish Annex of Eurocode 7. For deep excavations in the Lee valley, we often pair the hydrometer results with Atterberg limits to nail down the plasticity range of the silty clays that dominate the city centre quaternary sequence.

A single grain size curve from 63 mm cobbles down to 2-micron clay particles removes the guesswork from Cork's layered glacial soils.

Our approach and scope

The setup we bring to Cork jobs splits into two distinct stages. First, the mechanical sieve stack—a Retsch AS 200 with 200 mm diameter sieves—handles the coarse fraction from 63 mm down to 63 microns, using a shaking amplitude calibrated for the angular sandstone gravels typical of the Glanmire area. For the fines passing the 63-micron sieve, we switch to a sedimentation hydrometer, usually a BS 1377:2 compliant model, measuring density at timed intervals in a controlled-temperature bath. The hydrometer bath sits at a steady 20 °C, and we run duplicate readings at 0.5, 1, 2, 4, 8, 15, 30, 60, 120, and 1440 minutes—enough data points to build a reliable Stoke's Law curve. Cork's groundwater, often slightly acidic from the underlying limestone, can flocculate clays, so we add a sodium hexametaphosphate dispersant as standard. The combined sieve-plus-hydrometer report includes the full particle size distribution chart, coefficients of uniformity and curvature, and the USCS classification group symbol alongside the IS EN ISO equivalent.

Site-specific factors

Comparing two sites just three kilometres apart in Cork tells you everything about why skipping the full hydrometer curve is a gamble. Over in Blackrock, the limestone till is dense and granular—you might get away with a sieve-only analysis and still design a reasonable shallow footing. But move north to the Kiln River floodplain near Mayfield, and the soft estuarine silts can carry 30–40% clay fraction, much of it below 10 microns. If you classify that material by sieve alone, it looks like a silt with low plasticity, but the hydrometer reveals a tail of active smectite that swells with moisture and drops the effective friction angle by several degrees. That difference changes everything: bearing capacity, settlement rate, even the choice between a raft and a stone column ground improvement scheme. Cork's post-glacial marine clays are notorious for this—uniform to the naked eye, dangerously variable under a hydrometer.

Applicable standards

IS EN ISO 17892-4:2016 — Geotechnical investigation and testing — Laboratory testing of soil — Part 4: Determination of particle size distribution, Eurocode 7 — IS EN 1997-2:2007 (Irish National Annex) — Ground investigation and testing, IS EN ISO 14688-1:2018 — Identification and classification of soil — Part 1: Identification and description, BS 1377-2:1990 — Methods of test for soils for civil engineering purposes — Classification tests (referenced in Irish practice)

Reference parameters

Parameter	Typical value
Coarse fraction method	Mechanical sieving, 63 mm to 63 µm
Fine fraction method	Hydrometer sedimentation (Stoke's Law)
Hydrometer reading intervals	0.5 min to 1440 min (24 h)
Testing standard	IS EN ISO 17892-4:2016
Sample mass (coarse)	500 g to 5 kg depending on Dmax
Sample mass (fines)	50 g dry mass passing 63 µm
Dispersant	Sodium hexametaphosphate (40 g/L solution)
Reported parameters	D10, D30, D60, Cu, Cc, soil group symbol

Frequently asked questions

What does a combined sieve and hydrometer analysis cost for a Cork project?

For standard Cork jobs, the combined test runs between €90 and €190 per sample, depending on whether we are processing a single bulk bag or multiple depth intervals from a borehole. The price covers the full mechanical sieve stack, the 24-hour hydrometer sedimentation run with duplicate readings, the grading curve chart, and the soil classification per IS EN ISO 14688. If you need Atterberg limits on the same sample, we can bundle them at a reduced rate—just ask when you drop off the material at the lab.

How much sample do you need for a full grain size test in Cork's glacial soils?

It depends on the maximum particle size. For the typical sandy gravel till we see across Cork, a 5 kg bulk bag is enough to run the full sieve stack and still have sufficient fines for the hydrometer. If the material is mostly clay and silt—like the alluvium along the Lee—1 kg is plenty. The key is keeping the sample sealed from collection to lab so the natural moisture content does not shift; we can supply airtight containers if you are drilling on a wet Cork morning.

Why is the hydrometer step necessary when a sieve analysis already shows mostly sand?

Because even a 'clean sand' in Cork often carries 5–10% silt and clay that sieves cannot quantify properly. That small fraction controls permeability, frost heave potential, and how the material behaves under cyclic loading. We have seen Cork sands with less than 8% fines that still classify as silty sand under IS EN ISO 14688, and that changes the drainage design completely. The hydrometer also picks up the clay mineral tail—smectite or illite—that influences long-term settlement, something no sieve stack will ever tell you.

Grain size analysis (sieve + hydrometer) for geotechnical projects in Cork

Our service areas

Investigation

In-Situ Testing

Laboratory

Our approach and scope

Site-specific factors

Need a geotechnical assessment?

Applicable standards

Reference parameters

Frequently asked questions

Location and service area