We had a project on Kellogg Boulevard where the excavation hit an old buried stream channel nobody expected. The contractor’s shoring plan, which was a generic design, started showing distress within 48 hours. The saturated silts from the glacial Lake Agassiz deposits were pushing harder than the original soil report suggested. We redesigned the support system using a combination of passive rock anchors socketed into the St. Peter Sandstone and active tiebacks in the overburden. In St. Paul, working along the river corridor between the High Bridge and downtown means you’re constantly dealing with a layered stratigraphy of alluvium, decayed shale, and competent bedrock at varying depths. A standard anchor bond length simply won’t cut it, which is why we typically validate the load transfer through field pull-out tests before finalizing the production anchor lengths.
If you don't account for the slickensided surfaces in the Decorah Shale, your passive anchor design is just a guess.
Service characteristics in St. Paul
Risks and considerations in St. Paul
A common mistake we see is engineers specifying a uniform unbonded length for an entire anchored wall on Summit Avenue. The failure plane for a slope in the Decorah Shale is not a straight line, and the St. Paul topography means the backslope angle can change drastically. If your unbonded length doesn’t start at least 5 feet behind the critical failure surface, the anchor just doesn’t mobilize. We’ve had to remediate walls where the corrosion protection was compromised because the contractor used a centralizer that was too tight, scraping the epoxy coating off the bar as it was pushed through the casing. In the limestone layers, karst features are a real risk. You can lose your grout into a void and end up with a zero-bond zone. That’s why our design always includes a contingency for secondary grouting through a tube-a-manchette when the takes exceed a specific threshold.
Our services
We handle the design of both active and passive anchors for temporary and permanent applications in St. Paul. The distinction matters: active anchors lock off a specific load to control movement immediately, while passive anchors only engage when the ground moves. Our scope covers the load distribution and the performance testing protocol.
Tieback Anchor Design for Deep Excavations
Design of active tiebacks for soldier pile and lagging walls in downtown St. Paul. We calculate the horizontal subgrade reaction modulus based on the stiff glacial till, ensuring the design restricts lateral movement to less than half an inch to protect adjacent historic structures.
Passive Rock Bolt Systems
Design of fully grouted passive rock bolts for stabilizing the limestone and sandstone cuts along Shepard Road. We specify the bar diameter and spacing based on the Barton’s Q-system adjusted for the local joint conditions, ensuring the bond length penetrates beyond the weathered zone.
Common questions
What is the difference between active and passive anchors for a St. Paul excavation?
An active anchor is tensioned and locked off against the wall immediately after grouting, which prevents any movement before the next excavation lift. This is critical next to existing basements in Lowertown. A passive anchor is not tensioned; it only starts working when the soil mass begins to move and transfers the load to the bar. We use passive anchors in competent rock cuts where very small displacements are tolerable.
How much does an anchor design package typically cost?
For a standard anchored wall in St. Paul, a full design package including load calculations, corrosion protection details, and a testing specification generally runs between US$1.130 and US$3.220 depending on the number of anchor rows and the complexity of the soil-rock interface.
Do you handle the proof testing or just the design?
We primarily provide the design and the performance criteria, including the acceptance testing protocol. We specify the jacking loads, the alignment criteria, and the creep test duration per ASTM standards. While we don’t own the drilling rigs, our field engineers can oversee the verification tests to confirm the bond stress assumptions we made in the calculations. More info.