[Leaning Tower of the Bay Area]

Millennium Tower located in the South District of downtown San Francisco, it is a "very famous" super high-rise building.

The tallest building in San Francisco: Millennium Tower

However, the Millennium Building is not famous because it is the tallest building in San Francisco, but because it has been sinking below the ground since its completion. Since its completion in 2009, the building has been sinking at a certain rate every year. As of May 2021, the building had sunk as much as 18 inches (46cm).

On 10th May 2021 Building settlement monitoring data

What's more fatal is that the settlement of each part of the Millennium Building is not uniform. In the northwest corner, the settlement of the foundation is more serious than other areas, causing the entire building to tilt toward the northwest corner, and the top of the building has an offset of up to 24 inches (61cm).

The building tilts towards the northwest corner

Uneven settlement caused a large number of cracks to appear in the basement of the building, making the residents in the building worry about their safety every day.

Several cracks appeared in the basement

The main reason for the settlement of the Millennium Building is that the piles under the foundation are too short. The end of the foundation pile is only located in the sand layer less than 30m deep underground, and does not extend down to the bedrock layer. The huge weight of the building is applied to the old clay in the Bay Area below the sand layer, causing the moisture in the clay to be squeezed out and compressed.

Foundation piles do not extend to bedrock

Another reason is that the foundation is affected by the excavation of the foundation pit of the nearby transportation hub.

The main focus of this article is that during the strengthening of the foundation of the Millennium Tower, a new round of foundation settlement was made.

[Better late than never]

As early as 2018, the serious settlement problem of the Millennium Tower had attracted close attention from all parties. At the end of 2018, in order to solve the safety problems caused by the continuous settlement of the foundation, especially whether the excessive tilt of the building would affect the seismic performance, engineers designed a repair plan to add 52 reinforced piles located in the rock layer on the west and north sides where the settlement was most serious to suppress the occurrence of uneven settlement (Perimeter Pile Upgrade).

Diagrammatic sketch of the foundation repair plan

These reinforcement piles are arranged very densely, with a distance of only 6 feet (about 1.8m) between piles, close to the edges of the west and north sides of the building.

Enhanced pile layout

After the reinforcement piles are constructed, a slab extension will be installed at the basement floor of the building. The slab extension is connected to the original basement floor through a wedge. There is a movable gap between the slab extension and the pile top, and the slab and the pile can move freely up and down.

Basement extension slab

Then, a hydraulic device will be installed on the upper part of the extension plate to apply pressure on the top of the pile through the jack, so  the basement floor will be lifted and restored to its position before the settlement. At the same time, part of the weight of the building will be transferred to the reinforcement pile.

Lift the basement floor with a jack

Eventually, the northwest corner of the building will be lifted as a whole, with the aim of minimizing the uneven settlement of the building and "straightening" the building.

The entire building was straightened

In this process, there is a key problem that needs to be solved: how to ensure that the weight of the building will be transmitted to the bedrock along the pile?

The depth of the reinforced pile is close to 75m, and the pile end needs to pass through a soil layer several tens of meters thick before reaching the bedrock. If the conventional cast-in-place pile process is used, when the pile top begins to apply pressure, the pressure is likely not to be transmitted to the rock layer, but to be absorbed by the friction of the soil layer around the pile.

The pressure on the pile top may be absorbed by the friction of the soil layer

To this end, different construction plans were developed along the depth of the stratum to reduce the impact of soil friction on the force transmission of the piles.

First, a 36-inch (91.5 cm) diameter steel casing was constructed on the ground to about 90 feet (27.5 m) underground, just passing through the bottom of the sand layer. This steel pipe is a temporary casing, the main purpose of which is to avoid disturbing the building foundation piles and foundation soil during the subsequent reinforcement pile construction.

The 36-inch steel casing was constructed first

After the 36-inch steel casing is constructed to the specified depth, a smaller diameter 24-inch (61cm) permanent casing is constructed inside the casing, extending to the bedrock layer, with a depth of approximately 250 feet (76.2m). This steel pipe will remain underground to participate in the load-bearing of the pile. A special coating will be applied to the surface of the 24-inch steel pipe, which can greatly reduce the friction between the steel pipe and the soil layer.

24-inch steel pipe construction to bedrock surface

Subsequently, the 36-inch temporary steel casing will be pulled out. At the same time, due to the limited space on site, the gap between the two steel pipes will be back-filled with controlled low-strength material (CLSM). The strength of CLSM is much lower than that of concrete. Due to its fluidity, it is usually used for back-filling in narrow spaces.

CLSM

After back-filling is completed, a 20-inch (51 cm) diameter hole will be drilled 30 feet (9.1 m) into the bedrock in the 24-inch steel pipe, and finally steel bars and concrete will be placed to form a reinforced pile with the pile end in the bedrock layer.

Final reinforcement pile composition

It can be seen that the entire reinforced pile construction process is very complicated, and the purpose is to find ways to avoid further disturbance of the building foundation during construction.

After the design plan was approved by the government, on 12th May 2021 the first 36-inch temporary steel casing officially started construction .

However, to everyone's surprise, as the construction progressed, the settlement of the Millennium Building, which had been convergent, developed rapidly almost at the same time.

[Emergency stop]

The reinforcement work started from the west side of the building, with 36-inch temporary steel casing being constructed from south to north.

36-inch steel casing construction sequence

On-site construction photo, the 36-inch steel casing that has not yet been pressed in can be seen on the right

From the photos, we can see that the excavation inside the 36-inch  casing hole should be done by the rotary drilling process used in conjunction with the drilling bucket.

On-site construction photos

Since the start of construction, monitoring data shows that the settlement rate on the west side of the building has begun to increase. By 22nd June 2021, after the construction of 24 poles 36-inch steel casing pipes on the west side, the settlement has increased by a maximum of 0.3 inches (7.6mm), which even exceeds the settlement rate of the entire year before the construction of the building (0.24 inches/year).

On 23rd June 2021 Uneven settlement monitoring data of the building

 After the construction of the 36-inch steel casing on the north side was stopped on 2nd August 2021, the construction of the 24-inch permanent steel pipe on the west side did not stop, but continued until August 22.

Construction of 24-inch permanent steel pipe (Grey exterior) with friction reducing coating

As can be seen from the figure below, the settlement of the building did not ease during the construction of the 24-inch permanent steel pipe, and the development speed was even greater than that during the construction of the 36-inch steel casing. 

Building settlement development trend

Because the building settlement rate was beyond expectations, and the 24-inch steel pipes were also stopped on 22nd August 2021, the project was completely shut down.

[Less is More]

There are still many different opinions on the real reason for the increase in the building's settlement. However, there is no doubt that the development of settlement is closely related to the construction of reinforced piles, because after the reinforcement project was stopped, the settlement was suppressed almost at the same time, and the growth rate returned to the state before the construction started in May 2021.

On 13th October 2021 building settlement monitoring data

According to the statement of the designers of the building reinforcement project at a hearing last year, they did not deny that the settlement was caused by the construction of reinforced piles. The designers attributed the development of the settlement to two main reasons:

(1) During the construction of the steel casing, there was over-excavation in the pile hole, which caused deformation and shrinkage of the soil layer under the casing;

(2) The vibration of the drilling rig during hole drilling affected the compaction of the sand layer, thereby disturbing the foundation of the building.

The reasons for settlement summarized by the designer

On 10th January 2022 building settlement monitoring data

In August 2022 San Francisco building officials agreed to a scaled-down plan to allow engineers to do the work needed to support one corner of the building. The revised plan relies on just 18 piles, instead of the original 52, to anchor the high-rise to bedrock on the two sides where it leans and tilts the most, on Mission and Fremont streets. With the scaled-down plan, the fix was completed in June 2023.

By the end of 2022, the skyscraper had also sunk 28 inches, as measured from the roof.

February 22, 2024, the engineer of the so-called fix of the troubled Millennium Tower in San Francisco recently acknowledged there’s been less than expected tilt improvement over the first six months of the $120 million project’s completion.

While the Millennium Tower is no longer sinking anymore at the corner, and has reversed some settlement there, data shows there’s only been about an inch of tilt reversal, a quarter of what the model had predicted.

Contractors in charge of the fix had made the more optimistic prediction under the theory that once supported on the north and west, the tower’s newly shifted weight would cause it to settle more on its east and south sides.

But two outside engineers say the data suggests instead of sinking at the opposite sides, the tower appears to be sinking more in the center.

What can we do about foundation subsidence in high-rise buildings?

If you’re facing issues with settlement or subsidence, its critical to select a competent, experienced partner to deliver a suitable design solution, that is fit for purpose.

Digital Analogue Applications

We simulate the real settlement and tilting situation of the building according to the specific geology of the building, and on this basis, we simulate the expected lifting effect after reinforcement and tilting rectification, which is used as a reference basis to design the most safe, reasonable and high-quality construction plan.

How Do We Resolve High-Rise Buildings Subsidence in 15-30 Days?

• Our experts drill holes in the affected area

At just 6-10mm, the holes are smaller than a dime.

•  We inject special inorganic materials into the holes

Our technicians expertly inject a special inorganic materials that will harden to support and lift your property.

•  The inorganic materials instantly hardens like concrete

In just 1-30 seconds, the inorganic materials are 90% cured and has hardened to fill any voids and stabilize the ground.

•  The property is raised and foundations stabilised

You’ll see the results instantly. In most cases, cracks will disappear and doors will unstick as we gently raise the property.

Projects We Have Completed

32 Storey 113 Meter Residential Building Foundation Settlement Reinforcement Lifting Rectification Project in Qinghai Xining, China

27 Storey 110 Meter Building Foundation Settlement Reinforcement Lifting Rectification Project in Guangxi Wuzhou Mengshan Yuyuan, China

26 Storey 103 Meter International Building Foundation Settlement Reinforcement Lifting Rectification Project in Guangxi Cenxi City Xinjian, China

Cooperation and Development

Hengxiang Hongye has the latest world-leading building subsidence and lifting technology, with a number of international patents, such as those of the United States, Germany, China, etc., and has successfully solved a number of high-rise building subsidence problems, while we have been following the development of the Millennium Tower's subsidence situation, and are seeking to further explore and co-operate with it, in order to provide assistance for the safety of the whole mankind's residence.