HolmesCo: Site Investigation

Week 5 handout — Experimental design

HolmesCo Site Investigation Report

This is a fictional document for teaching purposes. Any resemblance to real consultancy reports is deliberate and cautionary.

GROUND INVESTIGATION REPORT

Prepared for: DNS Wind Developments Ltd

Project: Proposed Esh Spinning Wind Farm, County Durham

Report No.: HC-2026-0347

Date: February 2026

Classification: CONFIDENTIAL

1. Introduction

HolmesCo was appointed by DNS Wind Developments Ltd to undertake a ground investigation at the proposed Esh Spinning Wind Farm, a twelve-turbine development on the ridge running from Quebec through Esh to Hill Top, above the Deerness valley, County Durham (NGR NZ 190 440, approximate).

The purpose of the investigation was to determine whether the bedrock is suitable to support wind turbine foundations.

2. Methodology

Three boreholes were drilled to a depth of 15 metres each:

Borehole Location Elevation (m AOD)
BH-01 Verge of access road, Deerness valley floor 104
BH-02 Verge of access road, 200 m east of BH-01 108
BH-03 Lay-by on access road, lower valley side 116

Drilling was carried out by HolmesCo’s in-house rotary rig between 8–10 January 2026. Continuous core was recovered from all three boreholes. Standard penetration tests (SPTs) were conducted at 1.5 m intervals.

Borehole locations were selected alongside existing access roads to ensure safe and efficient access for the drill rig, which requires a level, firm platform and proximity to a road for support vehicles.

3. Results

All three boreholes encountered the following sequence:

  • 0–2 m: Glacial till (firm to stiff clay with gravel)
  • 2–15 m: Sandstone (Pennine Lower Coal Measures Formation), moderately strong to strong, thinly to medium bedded, occasional thin mudstone and siltstone interbeds

SPT N-values in the sandstone ranged from 35 to 50+ (refusal), indicating competent rock.

No groundwater was encountered during drilling. No significant discontinuities, voids or zones of weakness were observed in the recovered core.

4. Conclusions

Based on the results of this investigation, the bedrock at the Esh Spinning Wind Farm site comprises competent sandstone of the Pennine Lower Coal Measures throughout. The site is considered suitable for wind turbine foundations subject to detailed foundation design.

HolmesCo recommends proceeding to the detailed design phase with confidence.

This report has been prepared by HolmesCo for the exclusive use of DNS Wind Developments Ltd and should not be relied upon by third parties. HolmesCo accepts no liability for losses arising from use of this report by others. Standard terms and conditions apply.

Instructor notes

What students should identify:

  1. All boreholes are in the valley (104–116 m AOD). The turbines will stand on the Quebec–Esh–Hill Top ridge, at 190–235 m AOD — around 100 m higher. The investigation doesn’t test the ground where it matters.

  2. The valley is a different formation. HolmesCo’s own log names it: Pennine Lower Coal Measures (mudstone, siltstone and sandstone). The ridge is capped by Pennine Middle Coal Measures — a sandstone, plus mudstone, siltstone and sandstone members. “Throughout the site” extends a result from one formation to another. Anyone who checks the report against a BGS map can catch this in five minutes; the point for students is that the report contains the evidence against its own conclusion.

  3. Much of the ridge is made ground. This is coalfield country, and large parts of the ridgetop are made ground rather than bedrock. The roads sit on bedrock for historic and engineering reasons, so a road-side borehole is biased even on the ridge: access and good ground are correlated, and sampling by access selects the good ground. Good follow-up question: where would you expect shallow mine workings, and how would you find out? (Coal Authority records.)

  4. Three boreholes is a very small sample for a twelve-turbine site spanning several square kilometres with at least two formations and made ground. Three road-side points 200 m apart cannot characterize that range.

  5. Selection bias / convenience sampling. The report explicitly states that locations were “selected alongside existing access roads to ensure safe and efficient access.” This is convenience sampling dressed up as methodology.

  6. No control or comparison. What does “competent” mean without context? No reference to design loading, no comparison to similar sites, no indication of acceptable vs unacceptable SPT values for turbine foundations.

  7. Overconfident conclusion. “Throughout” implies the entire site has been characterized; in reality, three points in one small area have been tested. The recommendation to “proceed with confidence” is not supported by the evidence.

Discussion prompt: What would a proper investigation look like? (Desk study first: BGS geology, made-ground extent, Coal Authority mining records. Then boreholes on the ridge, at proposed turbine locations, including away from the roads; stratified across the formations and the made ground; enough of them to capture variability; reference to foundation design requirements.)