Active and Passive Anchor Systems for Retaining Structures in Cambridge

Q: What is the difference between an active and a passive anchor?

An active anchor is pre-stressed with a hydraulic jack after grouting, so it applies a known force to the retaining wall before any movement occurs. A passive anchor is not pre-stressed; it develops resistance only when the wall starts to deflect and the tendon elongates. In Cambridge we use active anchors for permanent basement walls where movement must be kept below 10 mm, and passive anchors for short-term sheet-pile cofferdams where a few centimetres of displacement is acceptable.

Q: What load can a ground anchor carry in the soils found around Cambridge?

In the Gault Clay, a 12–15 metre active anchor with a 6-metre fixed length typically reaches a working load of 250–350 kN after proving. In the denser River Terrace Gravels we have achieved 400–500 kN where the gravel is at least 4 metres thick and well-graded. The limiting factor is usually the grout-to-ground bond, not the steel tendon capacity.

Q: How much does anchor design and testing cost for a Cambridge project?

For a typical scheme with 8–15 anchors and full design, suitability testing, and acceptance testing, the engineering fee ranges from £780 to £2,640 depending on the number of investigation anchors and the complexity of the corrosion protection class. The drilling, grouting, and stressing contractor costs are separate and depend on access and anchor depth.

Q: Do you need a trial anchor before installing production anchors?

BS 8081 requires investigation tests on at least three sacrificial anchors when the design relies on empirical bond values and the site has not been previously characterised for anchor performance. In Cambridge we almost always recommend at least one investigation test per soil unit because the Gault Clay stiffness varies significantly across the city, and a failed acceptance test on a production anchor costs far more in programme delay than the trial itself.

The flat fen-edge landscape around Cambridge conceals a layered geology that challenges even well-planned retaining structures. With the Cam river cutting through the city centre and groundwater perched in the River Terrace Deposits, excavation support here rarely follows a textbook case. We see tie-back anchors working in stiff Gault Clay at the Addenbrooke’s biomedical campus, while passive ground anchors stabilise sheet piles along the Chesterton sidings.
The difference between active and passive systems matters most when adjacent buildings are Grade II listed and only a few metres from the excavation line. Before committing to a shoring layout, we usually recommend a CPT test to map the clay-to-gravel transition and identify any sand lenses that could bleed grout during installation. When the borehole log shows highly plastic clay, the anchor bond length can double the figure that a textbook formula would suggest.

A 15-metre active anchor in Cambridge Gault Clay typically carries a working load of 250–350 kN after proving to 1.25 times that figure on site.

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How we work

Anchor installation in Cambridge starts with a Klemm 806 or similar compact drilling rig that can work within the tight access typical of college backlands and mews sites. For active anchors we use a double-corrosion-protection detail, threading 15.7 mm strands through a corrugated sheath and centralising the tendon with spacers every 2 metres. The fixed anchor length is pressure-grouted in stages: a primary grout fills the bore, and after 12 hours a secondary sleeve-grout injection lifts the bond stress above 400 kPa in the Gault Clay.
Passive anchors, by contrast, are typically solid bars resin-fixed into the soil mass without pre-stressing. They activate only when the wall starts to move, which suits temporary works where the client wants to avoid stressing jacks on a congested site. We tie the acceptance criteria back to the plate load test results taken on the same formation, giving us a direct correlation between anchor creep behaviour and the soil’s stiffness modulus under sustained load.

Active and Passive Anchor Systems for Retaining Structures in Cambridge — Technical reference — Cambridge

Local considerations

The Gault Clay beneath Cambridge has an average undrained shear strength between 60 and 120 kPa, but its high plasticity index — often above 30% — makes it prone to creep under sustained anchor loads. We design active anchors to a bond stress of 150–200 kPa in this material, well below the ultimate values reported in some databases, because long-term relaxation can transfer load to the wall at rates that surprise even experienced contractors.
Water-bearing sand lenses within the clay are the other hazard. Uncontrolled grout loss into a lens can reduce the fixed anchor length by half before anyone notices the pressure drop. We mitigate this with stage grouting and real-time flow monitoring on the pump, stopping injection the moment volume exceeds the theoretical bore capacity by 15%. The 2019 collapse of a temporary sheet-pile wall on Mill Road — where passive anchors pulled out of softened clay after a wet winter — reinforced how sensitive these systems are to seasonal groundwater rise.

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Regulatory framework

BS 8081:2015 — Code of practice for grouted anchors, BS EN 1997-1:2004 (Eurocode 7) with UK National Annex, BS EN ISO 22477-5:2018 — Anchor testing, CIRIA C760 — Guidance on embedded retaining wall design

Typical values

Parameter	Typical value
Tendon type (active)	7-wire strand, 15.7 mm, fpu 1,860 MPa
Tendon type (passive)	High-yield bar, 40 mm Ø, fy 500 MPa
Bore diameter	120–150 mm depending on overburden
Primary grout	Neat cement w/c 0.40–0.45, CEM I 52.5N
Secondary grout pressure	5–10 bar via tube-à-manchette
Proof load (active)	1.25 × working load per BS 8081
Creep criterion	<2 mm over 10-minute observation period
Corrosion protection	Double barrier (sheath + grout column)

Questions and answers

What is the difference between an active and a passive anchor?

An active anchor is pre-stressed with a hydraulic jack after grouting, so it applies a known force to the retaining wall before any movement occurs. A passive anchor is not pre-stressed; it develops resistance only when the wall starts to deflect and the tendon elongates. In Cambridge we use active anchors for permanent basement walls where movement must be kept below 10 mm, and passive anchors for short-term sheet-pile cofferdams where a few centimetres of displacement is acceptable.

What load can a ground anchor carry in the soils found around Cambridge?

In the Gault Clay, a 12–15 metre active anchor with a 6-metre fixed length typically reaches a working load of 250–350 kN after proving. In the denser River Terrace Gravels we have achieved 400–500 kN where the gravel is at least 4 metres thick and well-graded. The limiting factor is usually the grout-to-ground bond, not the steel tendon capacity.

How much does anchor design and testing cost for a Cambridge project?

For a typical scheme with 8–15 anchors and full design, suitability testing, and acceptance testing, the engineering fee ranges from £780 to £2,640 depending on the number of investigation anchors and the complexity of the corrosion protection class. The drilling, grouting, and stressing contractor costs are separate and depend on access and anchor depth.

Do you need a trial anchor before installing production anchors?

BS 8081 requires investigation tests on at least three sacrificial anchors when the design relies on empirical bond values and the site has not been previously characterised for anchor performance. In Cambridge we almost always recommend at least one investigation test per soil unit because the Gault Clay stiffness varies significantly across the city, and a failed acceptance test on a production anchor costs far more in programme delay than the trial itself.

Location and service area

We serve projects in Cambridge and surrounding areas.

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