--- title: "Formwork for excavation, timbering, RCC formwork" subject: "Building Technology" chapter: 8 key: "semester_6/building_technology:formwork-timbering-062p" source: https://www.bctengineer.com/q/semester_6/building_technology/formwork-timbering-062p site: BCT Engineering --- # Formwork for excavation, timbering, RCC formwork Building Technology · Chapter 8: Temporary construction - Asked in: 2062 Poush (old course) - Video: [Explain four method of formwork for Excavation in Trenches with figure | Building Construction |](https://www.youtube.com/watch?v=CbNpIwzhioo) — Prashant YT - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/building_technology/formwork-timbering-062p - Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-8 ## Question Describe briefly on formwork for excavation and different methods used in timbering of trenches for firm and loose soil. Define formwork for reinforced concrete construction and its principal requirements. Illustrate sketches wherever necessary. ## Short answer Formwork for excavation is timbering to support trench sides, using stay bracing (spaced boards and struts) in firm soil and vertical sheeting, box sheeting, runners or sheet piling in loose soil. Formwork for RCC is a temporary mould that must be strong, rigid, leak-proof, smooth, oiled, economical, easily stripped, and removed at proper times. ## Answer #### Formwork for excavation (timbering) It is the temporary support of the sides of a trench or pit, to prevent the earth from falling in and to protect the workers. It is used when the depth exceeds about 1.5 m. **For firm soil:** - *Stay bracing:* vertical poling boards in pairs at 2-3 m intervals, held apart by cross struts, for depths up to about 2 m. **For loose soil:** - *Box sheeting:* closely placed vertical (or horizontal) boards held by walings and struts, forming a box, for depths up to about 4 m. - *Vertical sheeting:* for deep trenches (up to about 10 m) in soft soil; done in stages of 2-3 m with an offset at each stage, each stage having its own sheeting, walings and struts. - *Runners:* long boards driven ahead of the excavation in very soft, running soil. - *Sheet piling:* timber or steel sheets driven to depth for wide or deep excavations in soft, wet soil. ```text |poling| strut |poling| |board |==========|board | | | waling | | ``` #### Formwork for RCC construction **Formwork** is a temporary mould of timber, plywood or steel into which concrete is cast, and which is removed after the concrete has hardened. **Principal requirements:** - Strong and stable to carry the loads of wet concrete, workers and equipment without sagging. - Rigid, and correct in shape, line and level. - Tight joints, with no leakage of cement grout. - Smooth surfaces, oiled for easy stripping. - Quick and easy to erect and strip without damaging the concrete. - Economical and re-usable. - Supported on a firm base with sole plates and wedges. - Struck at the proper time as per IS 456. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Timbering for excavation is generally used when depth exceeds: A. 0.5 m B. 1.5 m C. 5 m D. 10 m Answer: B. Depths beyond 1.5 m need support. (Key point: Timbering depth) 2. Which pair is correct? A. Firm: runners; loose: stay bracing B. Firm soil: stay bracing; loose soil: vertical sheeting or runners C. Both: stay bracing D. Both: no support Answer: B. Stay bracing and box sheeting suit firm soil; vertical sheeting, runners and sheet piling suit loose soil. (Key point: Soil type method) 3. Formwork for RCC is: A. A type of aggregate B. Permanent cladding C. A reinforcement bar D. A mould removed after concrete hardens Answer: D. It is a temporary mould removed after hardening. (Key point: Formwork definition) 4. Why are formwork surfaces oiled? A. To weaken concrete B. For easy stripping C. To colour it D. To speed hardening Answer: B. Oil prevents sticking so forms strip without damaging the concrete. (Key point: Requirements) ## Related in Building Technology - [Formwork and timbering for trench excavation](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Atrench-formwork) — Building Technology, chapter 8, asked 3× - [Earthwork excavation in soft soil and hard soil](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aexcavation-soft-hard-compare) — Building Technology, chapter 2 - [Earthwork excavation in soft soil, hard rock, wet soil](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aexcavation-soft-hard-wet) — Building Technology, chapter 2 - [Temporary construction, types and requirements](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Atemp-construction-types-requirements) — Building Technology, chapter 8 - [Excavating foundation trenches in water-logged ground](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aexcavation-wet-methods) — Building Technology, chapter 2, asked 2× - [Factors in construction stage of trench excavation](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Atrench-excavation-factors) — Building Technology, chapter 2 ## Related in other subjects - [Aqueduct: excavation, concrete, brick, RCC](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Aaqueduct-v-four-items) — Estimating and Costing, chapter 6, asked 2× - [Excavation equipment and sketches](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Aexcavation-equipment-sketch) — Construction Management, chapter 4, asked 6× - [Earthwork, concrete, brick, RCC from drawing](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Adrawing-four-items-s) — Estimating and Costing, chapter 6, asked 2× - [RCC column with footing: concrete and steel](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Arcc-column-footing-h) — Estimating and Costing, chapter 6