1. Introduction

The semi top-down is a construction method for the underground portion of high-rise buildings, utilizing a top-down approach. In this method, the diaphragm wall, floor slab, and kingpost system play a crucial role in supporting the structure. Before starting the construction of the basement, the diaphragm wall (which will later serve as the enclosure for the entire basement) and bored piles are installed first. The basement is then excavated from top to bottom, with floors being supported by temporary steel columns (kingposts) fixed into the top of the bored piles. Excavation and material transportation are done through openings in the floors, which are sealed only after the final basement floor is completed, following a bottom-up sequence.

Advantages of the semi top-down method:

• Space efficiency: It requires less excavation area, making it ideal for urban areas or sites surrounded by existing buildings, where space is limited.
• Safety and stability: The use of floor slab and the structure itself to support the diaphragm wall ensures high stability, reduces risks, and increases safety.
• Cost efficiency: It saves on formwork and scaffolding costs and, as basement levels are constructed once the ground floor is completed, it minimizes the impact of weather conditions.
• Faster project completion: The method accelerates construction progress, reduces costs related to temporary bracing systems, and minimizes groundwater management risks, ensuring safety for neighboring structures.

However, there are some limitations to the semi top-down method:

• Complex design: The connections between the diaphragm wall, floors, and beams require precise calculation and positioning, which can complicate the construction process.
• Load-bearing requirements: The temporary steel columns (kingposts) must be capable of supporting the weight of the floors above, adding complexity to the design.
• Challenges in mechanization: Working in a confined basement space makes mechanical work more difficult, requiring proper ventilation and artificial lighting to ensure workers' health and safety.
• High technical and cost demands: The method requires skilled engineers and more advanced technical expertise, leading to higher construction costs compared to other methods.

2. Scope of application

• The semi top-down method is effectively applied to projects with deep basements (greater than 3 basement levels).

3. Construction sequence

The construction sequence of semi top-down for a 3-basement project is shown in the Fig. 1.