Foundation Treatment and Foundation Pit Support Engineering (Revised Edition)

Author: Huang Shenggen / et al.
Editor-in-Chief: Wu Qiaosheng
Publisher:
Publish Date: 1999-09-01
Features: This book consists of three relatively independent parts: Part 1: Foundation Treatment, which provides detailed information on various foundation treatment methods widely used in engineering practice, including reinforcement principles, design calculation methods, construction techniques, and quality inspection; Part 2: Foundation Pit Support Engineering, which systematically introduces methods for calculating earth pressure, design principles and calculation methods for foundation pit support structures, soil nail anchoring technology, cement-soil retaining wall support technology, and soil nail support technology; Part 3: Dewatering Engineering, which elaborates on the design of drainage projects, the basic principles of dewatering projects, and the design and construction techniques of various wellpoint dewatering methods. The book is rich in content, balancing theory and practice, and emphasizes the unity of scientific rigor, advanced technology, and practicality. It can serve as a textbook or reference for students in geological engineering, civil engineering, and engineering geology at universities and colleges, as well as for technical personnel and site managers in construction, water conservancy, transportation, railway, geology, and metallurgy.
Excerpt: Plain soil, lime soil, and fly ash-lime soil cushion layers are collectively referred to as soil cushion layers, suitable for treating soft soil layers with a thickness of 1 to 4 meters. The volume ratio of lime to soil in lime soil cushion layers is generally optimal at 2:8 or 3:7. The strength of the cushion layer increases with the amount of lime, but beyond a certain threshold, the strength increase becomes negligible. A fly ash-lime cushion layer is prepared by mixing lime and fly ash in a 2:8 or 3:7 volume ratio with an appropriate amount of water and compacting them in layers. Its strength is significantly higher than that of lime soil cushion layers and is often used to treat the collapsibility of loess.
(1) Thickness Determination
The determination of soil cushion layer thickness on soft soil foundations is the same as for sand cushion layers. For cushion layers on non-self-weight collapsible loess foundations, the thickness should ensure that the pressure exerted on the natural loess layer is less than its initial collapsibility pressure. Based on experimental results, when the thickness of a rectangular foundation cushion layer is 0.8 to 1.0 times the width of the base, and the thickness of a strip foundation cushion layer is 1.0 to 1.5 times the width of the base, some to most of the collapsibility of the non-self-weight collapsible loess foundation can be eliminated. When the cushion layer thickness is 1.0 to 1.5 times the width of the column base or 1.5 to 2.0 times the width of the strip foundation base, the collapsibility of the non-self-weight collapsible loess foundation can be largely eliminated. On self-weight collapsible loess foundations, the cushion layer thickness should be greater than that on non-self-weight collapsible loess foundations, or the remaining collapsibility should be controlled to no more than 20 cm to achieve good results.
(2) Width Determination
The width of lime soil cushion layers can be taken as b' = b + 2.5z, while the width of plain soil cushion layers can be determined by one of the following methods:
(1) When the cushion layer thickness is less than 2 meters, the width can be taken as b' = b + 1.5z, and b' ≥ b + 0.6 (m); when the thickness is greater than 2 meters, the width should be adjusted accordingly, and b' ≥ b + 1.4 (m).
(2) Add 0.2b to 0.3b on each side, but not less than 30 cm and not more than 70 cm.
(3) Calculate using b' = b + 2z·tanθ, with θ = 22° for plain soil and θ = 30° for lime soil.
(3) Planar Treatment Range
Plain soil or lime soil cushion layers can be classified as local or full-area cushion layers. The width of a full-area plain soil cushion layer can be taken as b' ≥ b + 3 (m), and it can be further adjusted when z > 2m. In collapsible loess areas, if only the collapsibility of the treated soil layer under the foundation needs to be eliminated, a local or full-area plain soil cushion layer should be used; if improving soil bearing capacity or water stability is also required, a local or full-area lime soil cushion layer should be used. The planar treatment range of a local cushion layer can be calculated using the formula:
b' = b + 2z·tanθ + c (b' ≥ 2/z) (2.1-5)
where:
c – additional width due to construction equipment, preferably 200 mm.
The planar treatment range of a full-area cushion layer should extend beyond the outer edge of the building's exterior foundation by at least the thickness of the cushion layer and not less than 2 meters.
### III. Fly Ash Cushion Layer
The chemical composition of fly ash and natural soil is highly similar, with the main components being oxides of silicon, aluminum, iron, etc., with the total content of silicon and aluminum oxides exceeding 70%. Research has confirmed that fly ash exhibits pozzolanic properties, exhibits hardening properties under moist conditions, and, under alkaline substance activation, undergoes hydration reactions with SiO?, Al?O?, and other substances to form hydration products, which bind and solidify the densely compacted fly ash particles into a block structure, thereby increasing strength, reducing compressive deformation, and enhancing impermeability and water stability. Fly ash possesses excellent physical and chemical properties, making it an ideal material for replacement filling. Its compaction curve is similar to that of cohesive soil, with a relatively wide optimal moisture content range, meaning its dry density is less sensitive to moisture content compared to cohesive soil. Therefore, the optimal moisture content corresponding to the maximum dry density in fly ash replacement filling construction is easier to control.
When a fly ash compaction cushion layer comes into contact with water, its strength decreases by 20% to 30%, and its compressive deformation increases by approximately 10%. The internal friction angle, cohesion, compressive modulus, and permeability coefficient of fly ash cushion layers vary with the material and compaction density of the fly ash and should be determined through laboratory soil tests. The design of fly ash cushion layers can refer to sand cushion layer design methods and relevant technical requirements. When using fly ash cushion layers without sufficient data or engineering experience, physical, chemical, and mechanical property tests of the material should be conducted to provide design data and technical parameters.
When determining the thickness of a fly ash cushion layer, the pressure diffusion angle can be taken as 22°, with calculation methods similar to those for sand cushion layers. The bearing capacity of a fly ash cushion layer is generally determined through field tests. When no test data is available, the following data can be referenced:
① For artificially compacted (rammed) fly ash cushion layers, when the compaction coefficient is controlled at 0.90 and the dry density is 0.9ρdmax (t/m3), the bearing capacity can reach 120 to 150 kPa.
② When the compaction coefficient is controlled at 0.95 and the dry density is 0.95ρdmax (t/m3), the bearing capacity can reach 300 kPa, but the strength of the underlying layer should be verified.
### Section 2: Compaction Effects of Soil
#### I. Mechanism of Soil Compaction
Practice has shown that to achieve the best compaction effect, the moisture content of the soil must be appropriate. Compacting overly wet soil (or tamping, vibrating) results in "rubber soil," which cannot increase density. Compacting very dry soil (or tamping, vibrating) also fails to fully compact the soil. In engineering practice, the inspection of cushion layer compaction quality requires achieving the maximum dry density ρdmax of the fill, which can be determined through laboratory compaction tests.
Based on laboratory standard compaction tests, a relationship curve between the dry density ρd and the prepared moisture content w can be plotted, as shown in Figure 2.2-1. The peak value of ρd on the ρd-w curve represents ρdmax, and the corresponding moisture content is the optimal moisture content wop.