When a major UK construction firm decided to examine the origins of their natural stone imports, they weren't expecting to find what they found. They were expecting perhaps some minor compliance gaps—maybe a missing certificate here, an outdated declaration there. What they uncovered was altogether more serious.

This is the story of how deep-tier auditing revealed forced labour in their supply chain, and what happened next.

The Starting Point

The firm had made public commitments around ethical sourcing. Their Modern Slavery Statement, published annually as required under the Modern Slavery Act 2015, declared robust due diligence processes. But if they were honest, their visibility beyond tier one suppliers was limited.

They knew who they bought stone from—reputable UK distributors with proper paperwork. But they had little insight into where those distributors sourced from, or what conditions existed at the quarries where stone was actually extracted.

The construction industry has known for years that stone supply chains can be problematic. Reports from human rights organisations had highlighted concerns about quarries in certain regions using bonded labour, child workers, and dangerous conditions. But knowledge and action are different things.

The catalyst was a client requirement. A major housing association, themselves under pressure from regulators, asked for evidence of ethical sourcing throughout the supply chain. Not just first-tier—all the way back to source. The firm couldn't provide it.

The Deep-Tier Investigation

The company used their supplier management platform to cascade a structured questionnaire beyond their direct suppliers. They asked their stone distributors to identify their sources, and then asked those sources about their practices.

The questionnaire was designed to probe specific issues: worker contracts, wage payment records, age verification, working hours, safety equipment provision. Not generic tick-boxes, but specific evidence requests.

Some suppliers responded fully and provided documentation. Others responded partially. And some—a handful of quarry operations in one particular region—either didn't respond at all or provided answers that raised more questions than they resolved.

One response stood out. A quarry that supplied a significant proportion of their decorite stone reported that workers were "family members" who "didn't require formal contracts." When pushed for wage records, they provided none. When asked about worker ages, they became evasive.

The On-Site Audit

The firm arranged an independent on-site audit of the suspect quarry. What the auditors found was deeply concerning.

Workers at the quarry had no formal employment contracts. Many had been recruited from impoverished rural areas with promises of good wages, but found themselves indebted to the quarry owners for housing, food, and tools. This debt bondage—a form of modern slavery—trapped them in effectively unpaid labour.

Several workers appeared to be minors, though the quarry management claimed they were simply "small for their age." Safety equipment was minimal. Working hours stretched to twelve-hour days with no rest periods.

The auditors documented their findings with photographs, interviews (conducted away from management), and financial records analysis. The report made for harrowing reading.

The Response

The construction firm faced a choice. They could quietly find alternative sources and say nothing—protecting their reputation but doing nothing to help the workers they'd discovered. Or they could take a different path.

They chose transparency and engagement. They didn't walk away from the supplier immediately. Instead, they worked with an ethical sourcing NGO to develop a remediation plan. The quarry owners were given a clear choice: implement proper employment practices, or lose the contract.

The remediation plan included formal employment contracts for all workers, debt cancellation for those trapped in bondage arrangements, age verification with third-party documentation, proper safety equipment and training, and regular independent monitoring.

The quarry owners, facing loss of their largest European customer, agreed. Over six months, working conditions were transformed. Workers received back pay for effective wage theft. Those who were underage were removed from hazardous work and connected with educational programmes.

The construction firm maintained the relationship, but with ongoing monitoring requirements built into their contract. Unannounced audits became a condition of supply.

The Broader Impact

The case had ripple effects beyond this single quarry. The construction firm published a case study—anonymised but detailed—about what they'd found and how they'd responded. It prompted other buyers from the same region to ask harder questions.

Within their own organisation, the experience transformed how they thought about supply chain due diligence. Tier one compliance wasn't enough. Real visibility required going deeper, asking specific questions, and being prepared to act on uncomfortable answers.

They implemented a structured deep-tier monitoring programme covering all high-risk material categories. Stone, timber, metals sourced from certain regions—all now subject to cascade questionnaires and risk-based auditing.

Lessons for Ethical Sourcing

This case study offers several lessons for organisations serious about tackling modern slavery in their supply chains.

First, visibility is the foundation. You cannot address what you cannot see. Cascade questionnaires, supplier transparency requirements, and willingness to look beyond tier one are essential.

Second, generic declarations are worthless. Asking suppliers to sign statements saying they don't use forced labour achieves nothing. Specific, evidence-based questions about contracts, wages, and working conditions reveal reality.

Third, engagement can be better than abandonment. Walking away from problem suppliers feels clean, but it changes nothing for affected workers. Remediation—where suppliers are willing—creates actual improvement.

Fourth, this is a business issue, not just an ethical one. The Modern Slavery Act creates legal exposure. Client requirements create commercial pressure. Reputational risk is real. Ethical sourcing is increasingly just good business practice.

The construction firm's stone now comes with documentation tracing it to specific quarries with verified working conditions. It costs slightly more than it did before—the premium for actually paying workers properly. That premium is a price worth paying.