HolmesCo: Site Investigation

Selection bias — Week 5 handout with instructor notes

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: Weardale Wind Developments Ltd

Project: Proposed Middlehope Wind Farm, County Durham

Report No.: HC-2026-0347

Date: February 2026

Classification: CONFIDENTIAL

1. Introduction

HolmesCo was appointed by Weardale Wind Developments Ltd to undertake a ground investigation at the proposed Middlehope Wind Farm site, located on high ground between Weardale and Teesdale in County Durham (NGR NY 890 350, 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 Valley floor, Middlehope Burn 285
BH-02 Valley floor, 200m south of BH-01 288
BH-03 Low terrace, east bank of burn 292

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 1.5m intervals.

Borehole locations were selected to ensure safe and efficient access for the drill rig, which requires a level platform and proximity to a track 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 (Stainmore Formation), moderately strong to strong, thinly to medium bedded, occasional thin mudstone 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 or zones of weakness were observed in the recovered core.

4. Conclusions

Based on the results of this investigation, the bedrock at the Middlehope Wind Farm site comprises competent sandstone of the Stainmore Formation 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 Weardale 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 (285–292 m AOD). Wind turbines will be sited on the ridgetops (typically 500–600 m AOD in this area). The investigation doesn’t test the ground where it matters.

  2. Three boreholes is a very small sample for a site that may span several square kilometres with varying geology. The Stainmore Formation includes both sandstone and mudstone — three valley-floor boreholes can’t characterise the full range.

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

  4. 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.

  5. Overconfident conclusion. “Throughout” implies the entire site has been characterised; 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? (Boreholes on the ridgetops, at proposed turbine locations; enough boreholes to capture geological variability; a stratified sampling design; reference to foundation design requirements.)