The National Large-Scale Seismic Engineering Simulation Research Facility project, constructed by China Construction Eighth Engineering Division, is a major national science and technology infrastructure project under China's 13th Five-Year Plan. It includes an Experiment Center and a Simulation Center. The Experiment Center's foundation pit covers an area of 47,500 square meters, with a general excavation depth of 14.7 meters and a maximum depth of 18.4 meters.

Experiment Center foundation pit

To meet the process requirements of seismic simulation research, the structural integrity of the foundation pit must be ensured, and temporary components must not penetrate the platform structure. Conventional support and retaining systems cannot meet these requirements. Furthermore, the project is located in the soft soil area of Tianjin, where conventional pile-anchor systems are highly prone to deformation, posing safety hazards.

Schematic diagram of the foundation pit

Under stringent construction conditions, the project overcame the challenge of a multi-level support system without internal bracing. It uses "one vertical and one inclined" rectangular piles as the primary support and a jet-grouting pile-anchor system as the secondary support. This is the first and deepest multi-level support system without internal bracing in a soft soil area nationwide.

"One vertical and one inclined" rectangular support piles

Jet-grouting pile anchors

Super-large and ultra-deep foundation pit support system without internal bracing

1. Selection of support system without internal bracing

Through comprehensive comparative analysis and evaluation of multiple schemes, a self-stabilizing support system without internal bracing was selected: "slope cutting + alternating vertical and inclined rectangular piles + jet-grouting pile anchors." While ensuring structural safety and stability, this support system does not affect structural continuity, has the smallest structural design width, and offers significant advantages in environmental protection, schedule, and cost.

Slope cutting + alternating vertical and inclined piles + jet-grouting pile anchors

Selection of self-stabilizing vertical and inclined piles

By comparing parameters such as the ultimate retaining depth, foundation pit deformation and environmental impact, pile deformation, and pile bending moment of various cantilever support systems, an important basis was provided for selecting a highly self-stabilizing support system without internal bracing. Compared with cantilever support structures, the self-stabilizing support system without internal bracing reduces the maximum displacement by 2 to 5 times and can increase the maximum ultimate retaining depth by 50%.

Selection of inclined pile angle

Combinations of 0°+10° and 0°+20° vertical and inclined piles were compared. Simulation experiment results show that under the same horizontal displacement at the pile top, the 0°+20° combination has stronger support capacity and meets the requirements for deeper foundation pit excavation. Compared with all-vertical and all-inclined support forms, the support effect of alternating inclined and vertical piles is significantly improved.

Selection of combined support parameters

Based on comparison of experimental data for different parameters, a combination with a higher safety factor was selected. While the impact on the safety factor is relatively small, economic efficiency was fully considered. The final parameters were determined as a depth combination of 8/6.7 m, a platform width of 7 m, and a secondary support of single-row piles + anchor cables.

2. Optimization of deformation control capability of multi-level support

Through multi-channel modeling and numerical comparative analysis, the deformation control capability under iterative coupling of multi-level support was optimized, solving the design challenge of a deep and large foundation pit without internal bracing in a soft soil area. Overall analysis showed a maximum deformation value of 27.83 mm at the interface of multi-level support. Rechecking with Qimstar software confirmed that the foundation pit is safe and controllable.

Numerical simulation calculation

Recheck calculation

3. Installation of thickened reinforced cushion

After excavation, an 8 m wide, 20 cm thick reinforced cushion was installed, anchored to the engineering piles and braced against the surrounding support piles. This achieved advance support, reduced the exposure time of the pit bottom without bracing, and effectively reduced foundation pit deformation.

Actual view of reinforced cushion

Plan view of reinforced cushion

Section view of reinforced cushion

Self-stabilizing support with alternating vertical and inclined precast hollow rectangular piles

1. Determination of optimal combination angle for vertical and inclined piles

Alternating vertical and inclined precast hollow rectangular piles, with a pile type of 375 mm × 500 mm and a pile length of 12 m, were tested through force analysis and comparison. The optimal combination angle for alternating vertical and inclined rectangular piles is 0°+20°. This system has self-stabilizing characteristics of "inclined bracing, rigid frame, gravity, and settlement reduction," with a maximum horizontal displacement of 41 mm (warning value of 50 mm), keeping the foundation pit in a safe state.

2. Multi-angle hydraulic pile driving for vertical and inclined piles

During pile driving, the YZY800XJ inclined mixing pile machine was innovatively adopted. An adjustable-angle pile clamp fixes the precast pile and adjusts the pile pressing angle, and the hydraulic piston rod of the pile pressing cylinder precisely drives the pile, achieving multi-angle pile driving for vertical and inclined piles. The soil between inclined piles is protected by hanging mesh and shotcrete to ensure the overall support effect.

Working schematic of YZY800XJ inclined mixing pile machine

Setting out of vertical and inclined piles

Actual pile driving effect of vertical and inclined piles

3. Prefabricated assembly installation of precast crown beams

The vertical and inclined piles innovatively use precast crown beams, with segment lengths of 3.6 m, prefabricated in factory molds, and hoisted on site by lifting machinery. Segments of the crown beam are quickly connected with reinforcing steel bars using split sleeves. The connection area between the pile heads of vertical and inclined piles and the precast crown beams is cast with early-strength concrete of the same grade. The pile bodies and precast crown beams are assembled into a stable system, enabling green and rapid construction of foundation pit support.

Assembled system of vertical and inclined piles with precast crown beams

Jet-grouting pile anchors with enlarged-head anchoring and alternating long and short stress-dispersing type

1. Enlarged-head anchored jet-grouting pile anchor cables

Innovatively adopted enlarged-head anchored jet-grouting pile anchor cables. The enlarged head is equipped with three anchor plates of Φ100 mm × 10 mm. Jet grouting and two-spray-two-mix soil reinforcement are used, with the jet-grouting pile length being about 2/3, increasing the load-bearing strength of the anchorage section.

Structural schematic of enlarged-head anchored stress-dispersing anchor cable

Actual view of enlarged-head anchored stress-dispersing anchor cable structure

2. Alternating long and short stress-dispersing pile anchors

The equal-length pile-anchor system (length 16 m) was optimized to an alternating long and short stress-dispersing pile-anchor system (long anchors 20 m, short anchors 12 m), reducing the risk of progressive collapse of the pile-anchor system caused by iterative coupling of multi-level support. The maximum deformation value is 21 mm (warning value of 40 mm), and the foundation pit is safe and controllable.

Innovative monitoring technology for precast rectangular hollow support piles

Innovatively adopted construction technology for inclinometer tubes in rectangular hollow support piles of foundation pits, as well as fixing structures and installation technology for stress meters in rectangular hollow support piles, solving the challenges of inclination and stress monitoring for precast hollow rectangular vertical and inclined piles. Without increasing the pile's own strength to the maximum extent, precise deformation monitoring of alternating vertical and inclined rectangular support piles was achieved.

Rebar stress meter

Deformation monitoring of alternating vertical and inclined rectangular piles