Secant Pile Walls in Seismic Zones: Lessons from Yerba Buena Tower

Joe Lazzaretti
Aug 12
7 min read

Updated: Sep 9

Secant pile walls are often used in areas with earthquakes. These walls use overlapping concrete piles, which are strong and can resist water. They also deal well with moving ground during quakes. The Yerba Buena Tower in San Francisco shows off how well they work, facing stuff like ground turning soft, high water in the soil, and side pressure during shakes.

Key things we learned from the Yerba Buena Tower job:

Design Points: The team made secant pile walls to stand up to quakes by looking at the pile shape, making them strong, and putting them deep. They planned for 70% of what you'd see in a big, rare quake.
Building Issues: In tight city spots with lots of water in the ground, they had to drill just right, use temp hold-ups, and watch close to keep things firm.
How It Went: Tests after they built it showed the walls did not move much side to side and pushed the quake forces down to stable earth.

This job shows it's key to check the site well, tailor the engineering, and keep strict building methods to stay safe and strong in places where quakes may hit.

Secant Pile Drilling at BDPL3&4 Seismic Upgrade

Work Site and Ground Issues

The work site is in a busy city area inside a zone with quake risks, giving hard tasks for both design and build. A deep check of the site found big quake dangers and ground issues, setting the stage for the engineering plan. Next, we'll look at the quake risks, ground types, and needs for the retaining walls.

Site Place and Quake Danger

Close to a big fault line, like the West Valley Fault, the site needed designs tough enough to deal with shaking ground and possible shifts. To handle these dangers, engineers made walls of secant piles that could bear the extra forces from quakes, keeping it stable while building.

Ground Types and Engineering Impact

A study of the ground showed layers below, with a layer of added fill on top of stronger soils. This meant the wall system had to go deep into the stable soils for support. Also, a high water level in the ground was found, needing careful water control to keep the dig safe and stop the ground from getting weak.

Wall Needs for Doing Well

The wall system had to meet many key needs. It had to limit side moves to keep close buildings safe, fight both still and moving earth forces, block water during digs, and support heavy loads later on. These needs helped shape the engineering and building methods for the project.

Design Plans for Earthquake Effects

The plan for the secant pile wall thought about both still and shaking needs, making sure the build could take the hits from quakes. Let's see the basic methods used to hit these key aims.

Secant Wall Design Parts

The plan checked main parts like pile shape, fit, toughening, and deep set, all made to last still and shake forces.

Pile Shape: The size and gap between the piles were set to take the biggest shakes from quakes. This shape leads the wall's strong core.
Fit: Next-by piles were set with enough fit to form a full block. This part helps share shake hits all over the wall, stopping too much stress at one spot.
Toughening: Tough bits were put where stress would hit hardest, reaching into firm ground to hold the wall and fight side push.
Deep Set: Piles went down deep into hard earth, way below where we dig, to keep moves small when shakes hit.

These parts made up a base to handle dynamic quake hits in the build.

Adding Quake Loads

Shake hits were smartly added into full load plans to keep the wall strong in quakes.

Engineers thought about shake needs by using a low quake case, taking 70% of the Peak Ground Shake (PGA) of a 475-year quake plan [1]. For this work, the set PGA was 0.28g [1], which feels more like a 225-year risk than the full 475-year norm, which is often used for builds meant to last.

A side shake bit of 0.14g was used, with shape move limits: up ways moves cut at 34.0 mm and side moves kept to 0.3% of the wall's tall [1]. These rules made sure the design hit shake needs while keeping the build whole.

Looking at Systems: Secant Piles vs Other Ways

Secant pile walls are a good and strong way for city build where shake action is key. By using top number models, like the Finite Detail Way (FEM) in PLAXIS 2D 2018 [1], engineers made a mock-up of wall acts during and after digging. The study used the Hard Soil Small‐strain (HSS) plan, which changes ground hard based on stress spots and follows the Mohr-Coulomb give rule. This allowed for sharp signs of wall acts under both still and shake loads [1].

The results showed that secant pile walls not only meet shake needs but also make it easy to put in with small tools, causing less bother to near spots - an needed win in full city places.

Building Ways and Steps

Making walls of secant piles in cities at risk of quakes asks for good plans and true doing. The work takes on hard tech tasks like making a wall that stands whole and dealing with tight space in full cities. Here, we dive into the ways to drill, hold up systems, and steps of order used on-site.

How to Drill and Pile Links

A secant pile wall works best when piles close to each other, touch just right. At the site, they kept piles about 3.94 feet apart, set 7.87 inches off from each other, to make sure they touch right[1]. This way makes a full wall and lets you move building gear well.

In places with shakes, the earth changes call for tight drill steps. Here, they first put down temp piles with a hard status of 725 psi after 28 days. Then came piles with steel, much stronger at about 5075 psi[1]. This mix lets drills break through the first piles without hurting the wall's strong build.

During drilling, watching the work well is key. Small wrong moves can make holes in the wall, making it weak in holding earth and staying up during shakes.

Holding Things Up While Digging

After drilling, they put up holds to keep the pile wall stable while they dig more. Temp holds come in bits, each set up before digging goes deeper.

Brace systems that sit in many levels often hold things inside by linking walls across. These are great in box digs with good spans, helping to take loads well and make everything more firm.

Tieback systems, though, lock the wall back to earth or rock past the dig. Even though they need deep checks of anchor load, they help by not adding more stress to close builds.

Measures at the site saw wall moves of 1.77 inches, more than the max 1.51 inches allowed[1]. Though engineers said to make holds better after these big moves, the build kept going. This led to harm in builds nearby, like broken floor looks and walls pushed out of line[1].

Comparing Hold Systems: Braces vs. Tiebacks

Here we look at things that help pick between braces in and tiebacks:

Factor	Internal Bracing	Tieback Systems
Space Use	Fills some dig room	Cuts dig room use
How Hard to Set	Less hard, sets and leaves fast	Harder with drilling and filling
City Rules	Small hit on close sites	Must work with area lines
How Much Weight	Holds a lot, strong links	Hinges on dirt and how it's built
Work Speed	Up quick in dig steps	Slow, waits for hardening and filling
Cost	Less for stuff and workers	More, needs skilled steps

Groundwater not deep - about 2.13 to 2.62 feet down - can make putting in tiebacks hard. When this happens, drying out the area is key to keep things running right.

Earthquake Tests and Wall Hold

Keeping walls strong in quakes is key - not just for the wall but also for the safety of other close buildings. For this task, the team used known earth fix plans to help wall hold up under quake shakes, adding to the build help plans said before.

As part of this work, pros put in the PROPEX Armormax set. This set has Engineered Earth Anchors and a High-Performance Turf Reinforcement Mat (HPTRM). Both are made to up safe levels under both still and shake states. Also, the secant pile wall was put deep into firm soil spots, giving strong block against quake moves. These plans worked as one to keep the wall fixed during shake hits.

Project Work and Easy Lessons

From the design and build ways we used before, the project results and what we learned show good plans for working in areas with earthquakes.

Wall Works Well

After the work was done, checks at the Yerba Buena Tower showed that the secant pile wall, made with care, cut down side shifts and moved forces well to strong dirt layers. This shows why we must think about earthquakes a lot when we make plans.

Tips for Engineers

For engineers working with earthquake issues, full checks on the ground are needed. This means looking at how dirt acts both still and when moved to really get how dirt and buildings react when an earthquake hits. Also, don’t miss out on detailed earthquake checks, even for short-term holds, as these must last through earthquake shakes.

Good Ways to Work in Earthquake Areas

Main lessons from this work for places at high risk of earthquakes include:

Doing deep earthquake checks to know about stability and shape-change risks.
Making hold structures that can bend and have extra ways to take on moving loads well.
Sticking to build order and keeping tight on quality to meet earthquake needs.

These steps help make secant pile wall systems that stay strong during earthquakes, giving good tips for the next works like this.

FAQs

How do secant pile walls make things safer when the ground shakes in quake zones?

Secant pile walls are key in making sure buildings stay safe in places where quakes happen often. Made of overlapping piles of concrete, these walls form a solid barrier. They can take on the force from the earth and shake from quakes. By taking in and spreading out these quake shakes, they help with issues like ground turning to liquid and spreading, making sure the earth stays firm and nearby buildings are kept safe.

The way these walls overlap, and with the help of strong concrete, adds to the bend and give of secant pile walls. This lets them soak up and let go of quake energy better than stiff choices, showing they are a good pick for keeping things steady in places with big quake risks.