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Foundation Pit Support Construction Process: A Comprehensive Guide

Introduction

The foundation pit utilizes a combined support system of grouting piles + internal bracing (two layers) + anchor cables. This article takes you through the construction site, detailing the project management team’s approach to ensuring quality control in temporary support works by following structural engineering standards. We will explore four key construction processes:

  1. Foundation Pit Soil Nailing Wall Construction

  2. Retaining Pile Construction

  3. Steel Lattice Column Construction

  4. Concrete Support Beam Construction

1. Foundation Pit Soil Nailing Wall Construction

1.1 Slope Shaping

  • Large excavators dig the slope profile based on the pit’s upper and lower boundaries.

  • Manual work and small excavators refine the slope while slope gauges ensure compliance with design requirements.

  • The finished slope should be smooth, straight, and free of noticeable irregularities.

1.2 Soil Nail Drilling

  • After positioning, horizontal drilling machines create holes at a 15°–20° angle using dry drilling methods.

1.3 Soil Nail Installation & Grouting

  • Reinforced soil nails are inserted immediately after drilling.

  • Secondary cement grouting ensures structural integrity.

1.4 Reinforced Mesh Binding

  • HPB300 rebar (φ6@250×250) is used with a 30mm protective layer thickness.

  • Joints are secured with binding connections.

1.5 Shotcreting

  • C20 fine aggregate concrete is applied using air compressors, working from bottom to top.

  • Target thickness: 80mm with minimal rebound and high strength.

1.6 Final Inspection

  • The slope surface should be smooth, with clean joints and well-defined edges.

2. Retaining Pile Construction

2.1 Pile Hole Inspection

  • Post-drilling, hole diameter and depth are checked using measuring tapes and ropes.

  • Each hole is secured with safety barriers.

2.2 Reinforced Cage Fabrication

  • Cages are built per design, ensuring strong main rebar welds and proper spiral reinforcement spacing.

2.3 Cage Installation

  • Cages are lowered slowly to maintain verticality and elevation accuracy.

  • Suspension bars are welded to the top for precise positioning.

2.4 Concrete Pouring

  • C30 ready-mix concrete is poured via tremie pipes.

  • An extra 50mm height is added above the design level to ensure top strength.

2.5 Pile Integrity Testing

  • Tests verify material density and continuity.

2.6 Pile Head Trimming

  • Pneumatic breakers remove excess concrete to expose solid material.

2.7 Prestressed Anchor Cable Installation

  • Holes are drilled at specified locations, and Φ15.24-2 1860-grade steel strands are installed with double grouting.

2.8 Capping Beam Rebar Binding

  • Pile head rebars are straightened before beam reinforcement is placed.

  • Lap joints receive extra stirrups.

2.9 Capping Beam Formwork

  • 15mm film-coated plywood + timber + tie bolts form the support system.

  • Strict checks ensure dimensional accuracy and leak-proof joints.

2.10 Capping Beam Completion & Anchor Tensioning

  • After curing, anchor cables are tensioned, and bearing plates are fixed.

3. Steel Lattice Column Construction

3.1 Survey & Positioning

  • Excavators level the ground, and total stations mark column locations.

3.2 Drilling

  • Drilling rigs align with marked points after verifying verticality.

3.3 Reinforced Cage Placement

  • 1000mm cages are lowered for base anchorage.

3.4 Steel Lattice Column Fabrication & Installation

  • 700×700mm columns (angle steel + welded plates) are hoisted into place.

3.5 Cage-Column Connection

  • Columns are welded to pile cages and lowered to the design elevation.

3.6 Backfilling

  • After concrete pouring, soil backfill seals the hole for safety.

4. Concrete Support Beam Construction

4.1 Survey & Layout

  • Excavators and manual labor prepare the base, followed by precise marking.

4.2 Base Slab Pouring

  • 100 mm-thick C15 concrete is laid and finished with wood floats.

4.3 Isolation Layer

  • After curing (1.2N/mm² strength), varnish-coated felt is laid for easy slab removal later.

4.4 Beam Rebar Binding

  • Key intersections are pre-marked to guide reinforcement placement.

4.5 Beam Formwork

  • Similar to capping beams, 15mm plywood + timber + tie bolts ensure precision.

4.6 Concrete Pouring

  • Truck-mounted pumps deliver concrete.

  • Construction joints are treated: weak layers are removed, and steel mesh + rebar reinforce new joints.

4.7 Demolding & Curing

  • Forms are stripped post-setting, and fabric covers + regular watering maintain moisture.

Final Results: Foundation Pit Support System

  • Uniformly spaced piles with consistent shotcrete finishes.

  • Horizontal safety nets protect workers above/below supports.

  • Dust suppression sprays minimize airborne particles.

This systematic approach ensures structural stability, safety, and efficiency in foundation pit support construction.

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