Sites & Foundations - Summary

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Settlement

All ground settles in the first 5 years.

Some structures settle 20-30+ years later.

Amount of settlement depends on type of soil & imposed loads.

Types of Settlement

Uniform Settlement

Differential Settlement

Soil Types & Problems

Clay: heave & shrink

Sandy Soils: frost heave, washed away by water

Organic Soils: compressible & hold water

Fills: unknown problems can occur...

Problems with Vegetation

Extract water form soils to cause shrinkage & subsidence.

Felled trees can cause heave.

Particular problem in southeast where soil is mostly clay.

Buildings with foundations in the 'root zone' can be affected.

Minimum distances between trees & buildings are required.

Preventing Settlement

Desk Study (Always)

Walkover Survey (Always)

Soil Analysis (If Concerns Found From Above)

"Pay for a Site Investigation if You Have One or Not"

Types of Foundations

Strip

Piles

Raft

Reasons for Inadequate Foundations

No/missing foundations

Design faults

Incorrect spec. of concrete reinforcement

Alteration of building loads causing localised weaknesses

Reasons for Foundation Problems

Settlement - occurs when soil consolidates under load & removes excess water (if uniform, cracking should not occur)

Subsidence

Heave

Mining & vibrations through earthquakes.

Reasons for Failure of Pile Foundations

Lack of adequate boring.

Inaccurate soil classification.

Insufficient penetration.

Soft strata under tip of pile.

Inadequate data.

Misinterpretation of loads.

Damaged, buckled or broken piles.

Vibrations causing movement.

Flowing strata.

Tension failure of pile due to lack of reinforcement.

Eccentricity due to bowing.

Decay due to lower ground water level.

Insect and marine borer attack & corrosion.

Poor quality concrete or reactive aggregate.

Collapse of thin shell of the piles.

Overweight due to earth fill.

Methods of Investigating Foundation Movement

Trial Pits

Desk study

Boreholes

Soil & Root Sampling

Drain testing

Crack monitoring

Methods to Prevent Pile Foundation Failure

Early repair (encasement or replacement)

Removal of partial load

Underpinning

Other Reasons for Cracking

Sulphate attack

Wall tie failure

Thermal/moisture movement

Lateral pressure from poorly tied roofs

Defective arches and lintels

Crack Identification

The following suggests foundation movement:

Wide cracks running along wall.

Cracks extending down to ground level.

Tapered cracks.

Distorted window & door openings.

Tilting external walls.

Sloping floors.

Crack Classification (BRE Digest 251 - Table 1)

Category 1 - hairline cracks of less than 0.1mm - classified as negligible - No Action Required.

Category 2 - fine cracks with widths up to 1mm which can be treated easily with decoration - damage generally restricted to internal wall finishes & barely visible on external brickwork.

Category 3 - cracks with widths up to 5mm that are easily filled & not necessarily visible externally with some re-pointing possibly required.

Category 4 - cracks with widths between 5-15mm which required opening up & patching by a mason. Re-pointing required, bricks replaced, water-tightness impaired and possibly fracturing of service pipes.

Category 5 - extensive damage with crack widths of 15-25mm requiring breaking out and replacing of sections of walls. Doors & windows are distorted, floor sloping, loss of bearing in beams and services pipes disrupted.

Category 6 - structural damage caused by cracks with widths exceeding 25mm which requires a major repair job involving partial or complete rebuilding. Beams lose bearing, walls lean & require shoring, windows broken with distortion & danger of instability.

Remedial Measures for Cracking

Depends on severity of cracking:

Decoration

Repointing and alteration

Replacement of components

Removal of structure (partial or total) and rebuilding

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