Design rationale for shore pile and steel bracing system for deep excavation

Abstract

Deep excavation is a very important initial phase work for the construction of buildings with basements. It requires a suitable shore protection system to avoid any unexpected hazard during construction. Pile is a very popular shore protecting element which is subjected to the lateral pressure coming from the surrounding soil, structures and other surcharge conditions. As a result, these piles tend to deflect as well as overturn which is prevented by a suitable steel bracing system. At present, a huge number of building construction projects are going on in Dhaka City. For the safe and economic construction of basements in these projects, a rational study of the shore pile and bracing system is necessary. Such a study will help to get a direction about optimum strut location, depth of embedment and strut spacing for typical Dhaka city soil. Here, a rational study has been performed on the shore pile and steel bracing system for two types of typical Dhaka city soil: ‘clay underlain by sand’ (BUET Soil) and ‘deep deposit of clay’ (Uttara Soil).

A typical 60ft x 80ft. excavation area and a specific depth of excavation (H) as 25ft. (considering possible 3 basements) have been selected for the study. Three different depths of embedment of shore pile below the dredge line have been analyzed in this study. For each depth of embedment, four strut locations are studied individually. The depths of embedment were 13.0 ft., 15.0 ft. and 18.0 ft. and strut locations were 4.0 ft., 5.0 ft., 6.0 ft. and 8.0 ft. from ground level. A shore pile model was developed in ETABS software using the soil data from the soil reports. Active pressures were applied on the pile model while the passive pressures were replaced by Winkler soil springs whose spring constants were similar to the bilinear approximation of ‘p-y’ curve. After obtaining the results, design of bracing system was performed by ETABS software as per AISC ASD01 (2001) method.

From the study, optimum strut location for both type of Dhaka city soil is found to be between 5.0 feet to 6.0 feet. Optimum depth of embedment is found to be 15.0 feet for ‘BUET Soil’ and 13.0 feet for ‘Uttara Soil’. For 15.0 feet depth of embedment and 6.0 feet strut location, the findings are: (i) strut reaction for ‘Uttara Soil’ is 1.85 times of that in ‘BUET Soil’, (ii) maximum shear force of shore pile in ‘BUET Soil’ is 58.5% of that in ‘Uttara Soil’, (iii) maximum bending moment for ‘BUET Soil’ is 52.5% of that for ‘Uttara Soil’, (iv) ‘Uttara Soil’ requires 27% more area of strut and 46% more area of wale than ‘BUET Soil’, (v) total weight of strut and wale in case of ‘BUET Soil’ is nearly 57% of that in case of ‘Uttara Soil’, (vi) maximum deflection of shore pile in ‘Uttara Soil’ is 2.4 times of that in ‘BUET Soil’.

Cross sectional area and section modulus of wale and strut increased with deeper strut location from GL. If strut location is changed from 6.0 feet depth to 8.0 feet depth (depth of embedment=15.0 feet), cross-sectional area of wale is increased by about 10.5% for ‘BUET Soil’ and 13% for Uttara Soil. For the same variation in strut location, steel strut area remains unchanged for ‘BUET Soil’ while strut area increases by 21.3% for ‘Uttara Soil’.

Again, if strut spacing is increased by around 30%, total weight of the bracing members is increased by 37% for ‘BUET Soil’ and 27% for ‘Uttara Soil’ (Figure 6.11). Hence, Cost for the bracing system will be increased proportionally. Considering all the findings, spacing between 15.0 feet to 16.0 feet may be considered optimum for the particular two types of Dhaka city soil for the particular case considered in this study.

The design graphs prepared in this study may be helpful in getting idea about the rational design of bracing system and shore pile for these two types of Dhaka soils.