Design of foundations to resist shear during earthquakes

Abstract

At present, there is no uniform approach to designing structures to resist base shear. Designers usually assume that most of the base shear will be resisted by passive resistance of soil acting against vertical surfaces such as beam sides and basement walls. This is a convenient assumption because such passive pressures calculated using traditional Rankine earth pressure theory are generally very large. The actual mechanisms of resisting base shear are not well understood.

This study has investigated the behaviour of three typical foundation systems under lateral loading: simple slab-on-grade, slab-on-grade with two parallel foundation beams, and a slab and beam foundation interacting with a pile. Details typical of New Zealand construction practice were followed as closely as possible.

The base sliding characteristics for simple slab-on-grade construction were determined by building large (2m wide x 3 m long) slabs and pushing them back and forth with a hydraulic actuator. One and two layers of dampcourse were used and some slabs were weighted with ballast. Peak friction angles ranged from 28 degrees for a single layer of dampcourse with 6.6 KN/m2 contact pressure to 12 degrees for a double layer of dampcourse with 3.1 KN/m2. Mean friction angles were all 2 degrees less than for the peak friction angles.

Three shallow foundations each 4.25m wide x 4.6m long consisting of a 100mm thick slab “on-grade” with two foundation beams 600 mm wide embedded 450 mm were constructed in coarse granular material. Each was tests by shoving back-and-forth by a powerful hydraulic actuator with several cycles of quasi-static lateral loading.

Lateral loading of the slab and beam foundations caused a wedge type of failure mechanism with significant passive soil pressures acting against the vertical faces of the foundations beams. The passive soil wedge developing against the trailing beam lifted one side of the structure vertically leaving hollow space beneath the floor slab. For the somewhat narrow structures tested, significant rotations of the structure occurred.

A simple method of analysis was developed and found to give good predictions for the experimental results while accounting for all of the main parameters. The analysis predicts that lateral load capacity is highly sensitive to the eccentricity (height above ground) of the applied lateral load.

Pile interaction with a slab and beam foundation was investigated by casting a single steel pipe pile (168mm x 4.05m long) into a slab and beam foundation and then subjecting the composite foundation to various cycles of lateral loading. The presence of the pile was found to increase the lateral load resistance of the foundation in two ways: by direct contribution from the lateral resistance of the pile reacting against the soil, and from uplift resistance of the pile adding to the “weight” of the foundation during the tilting and uplift associated with generation of the soil passive wedges.

A general approach for design and analysis of combined beam-slab-pile foundations using standard methodologies is given.