5.3: Earth materials

5.3 Book

Houben, H, Guillaud H (1994). Earth construction : a comprehensive guide, IT Pubs, (TH 1421.H6)

5.3 Earth

5.3 Earth materials

• Kiln dried

• Sun dried– Stabilised– Unstabilised

5.3 Earth materials – soil

5.3 Earth materials – soil – particle size

• Pebbles – 200mm – 20mm

• Gravel – 20mm – 2mm

• Course sand – 2mm – 0.2mm

• Fine sand – 0.2mm – 0.02mm

• Silt – 0.02mm – 0.002mm

• Clay - > 0.002mm

5.3.1 Earth materials – Burned bricks

• Bricks moulded and then energy (heat) added at 850-1000oC

• Clays vitrify (form glasses)

• Quality varies with– Soil type (mainly clay content)– Firing

5.3.1 Earth materials – Burned bricks –brick bonds

5.3.1 Earth materials – Burned bricks–Advantages and Disadvantages

Advantages Disadvantages

• Good compressive strength

• Slightly porous permitting moisture to “move”

• Good thermal capacity

• Good fire resistance

• Weather (rain) resistant

• Poor quality bricks can be used for other purposes –no waste

• Labour intensive

• High fuel consumption (5 MJ kg-1 – deforestation

• Simple kilns produce bricks of varying quality

• Lime blowing

• Efflorescence

5.3.1 Earth materials – Burned bricks –Clamp kiln

5.3.1 Earth materials – Burned bricks –Clamp kiln

5.3.1 Earth materials – Burned bricks–Clamp kiln - Advantages and Disadvantages

Advantages Disadvantages

• They are cheap and straightforward to build

– No permanent structure to install and maintain

• Can be built next to the supply of clay and fuel

– Low transport costs

• Can be left once lit

• Good scalability

• Can be fired continuously,

• Clamps can be fired with a large variety of fuels

• The least energy efficient method of firing bricks

– Fuel consumption of 2.8 to 3.5 MJ/kg fired brick

• Labour intensive

• Poor quality control a very – Up to 20% of the bricks

over or under fired

• Slow to fire, – several days to heat up

and cool down

• Susceptible to the prevailing weather conditions

5.3.1 Earth materials – Burned bricks –Bull’s trench kiln

5.3.1 Earth materials – Burned bricks –Bull’s trench kiln

5.3.1 Earth materials – Burned bricks–Bull’s trench kiln - Advantages and Disadvantages

Advantages Disadvantages

• More fuel efficiency compared to periodic kilns

– 2.5-2.8 MJ.kg

• Low initial investment.

• High capacity

• Demands good organization of the brick production,

• Production cannot easily be adjusted to fluctuations in the brick market.

• High skill

• Moveable chimneys have a short working life.

• Exhaust temperature is high, causing a less than optimum firing condition and fuel economy.

5.3.1 Earth materials – Burned bricks –Vertical shaft brick kiln

5.3.1 Earth materials – Burned bricks –Vertical shaft brick kiln

5.3.1 Earth materials – Burned bricks–Vertical shaft brick kiln - Advantages and DisadvantagesAdvantages Disadvantages

• Very energy efficient– ~1 – 1.3 MJ/kg– 2-3x bulls trench

• Low cost

• High quality

• The kiln is cheap and straightforward to build,

• Very compact

• Not effected by variations in the weather,

• Reduced local air pollution

• More than one shaft allows for firing flexibility

• Labour requirements are low

• Novelty

• Requires good quality green bricks,

– Have to be able to withstand being stacked 5 meters high in the firing shaft.

5.3.2 Earth materials – Soil tests – Initial tests

• Visual examination

• Smell tests

• Nibble test

• Touch test

• Washing test

5.3.2 Earth materials – Soil tests – Lustre test

• Cut a slightly moist ball of soil with a knife– Shiny surface – clayey– Dull surface - silty

5.3.2 Earth materials – Soil tests –Adhesion test

• Cut into a slightly moist ball of soil with a knife– Knife penetrates easily – low in clay– Soil resists penetration - clayey

5.3.2 Earth materials – Soil tests – Water retention test

• A moist ball squeezed into a egg-shaped ball with just enough water to hold it together

• Press the ball into one hand and tap with the other until water forms at the surface– 5-10 taps fine sand or course silt– 20-30 taps plastic silt or silty clay– No reaction - clayey

5.3.2 Earth materials – Soil tests –Consistency test

• Roll an olive-sized ball of moist soil into a thread about Ø3 mm– If it breaks before 3mm – too dry – raise water

content

• Reroll the thread into a ball and squeeze– Hard to crush – high clay content– Cracking or crumbling – low clay content– Breaks before ball is formed – high silt or sand

content– Soft and spongy – high organics - reject

5.3.2 Earth materials – Soil tests –Cohesion test

• Squeeze a moist ball of soil into a strip (~12-15mm thickness)

• Carefully dangle loose end from fingers

• Measure how long it is when it breaks– 25-30 cm – clayey– 5-10 cm low clay content

5.3.2 Earth materials – Soil tests – Dry strength test

• Flatten egg sized ball to ~1cm thickness

• Allow to dry

• Break piece between thumb and forefinger– Breaks with difficulty and doesn’t crumble – clay– Can be easily crushed to powder – silty or

sandy clay– Pulverised without effort – silt or sand with low

clay content

5.3.2 Earth materials – Soil tests – Settling test

• Fill jar about 1/3 full with soil and compact slightly

• Add water to about 2/3 full. Add a pinch of salt (honest!)

• Shake and leave to settle. Shake again after an hour– Gravel and sand settle after 1 min– Silt settles after 30 mins– Clay will take ~ 24 hours

5.3.2 Earth materials – Soil tests – Settling test

5.3.2 Earth materials – Soil tests – Sieve analysis

5.3.2 Earth materials – Soil tests – Sieve analysis chart

5.3.2 Earth materials – Soil tests –Shrinkage test

• Grease inside of box

• Pack in soil

• Measure length of dried pieces after 3 days in sun or 7 in shade– High shrinkage is bad (aim for <<10%)

5.3.2 Earth materials – Soil tests – Optimal Water content

5.3.2 Earth materials – Soil tests – Optimal Water content test

5.3.2 Earth materials – Soil tests –Atterberg limits – Liquid limit test –Casagrande apparatus

• Dry soil and weigh

• Add some water until soil is a thick paste

• Place in apparatus

• Make a groove with grooving tool

• Turn handle until groove is breached by 13mm– When breach is 13mm after 25 turns, soil is at the liquid limit

• Its actually more complex – see BS1377

5.3.2 Earth materials – Soil tests –Atterberg limits – Liquid limit test

• Pack soil in bowel till it is about 8mm deep in the centre

• Make a groove with grooving tool

• Tap with hand– Soil is at the liquid limit when the gap is

breached by 13mm after 10 taps

5.3.2 Earth materials – Soil tests –Atterberg limits – Plastic limit test

• Add water to dry soil

• Roll about 5g into a ball until it dries and cracks

• Divide the sample into two parts and make each into a sausage about Ø6mm

• Roll each part to a thread of Ø3mm– If it breaks before 3mm – dryer than plastic limit– Remains whole after 3mm – wetter than plastic limit– Should break into lengths of about 10mm

• Check moisture content by weighing

PI is the Plasticity index

LL is the liquid limit

PL is the plastic limit

5.3.2 Earth materials – Soil tests –Atterberg limits – Plasticity index

PI LL PL= −

5.3.2 Earth materials – Soil tests –Atterberg limits chart

5.3.2 Earth materials – Soil tests –Cohesion test

5.3.2 Earth materials – Soil tests –Cohesion test chart

5.3.2 Earth materials – Soil tests –Moisture/density chart

5.3.3 Earth materials – Construction methods

5.3.3 Earth materials – Construction methods – Cob

5.3.3 Earth materials – Construction methods – Wattle and daub

5.3.3 Earth materials – Construction methods - Adobe

5.3.3 Earth materials – Construction methods - Adobe

5.3.3 Earth materials – Construction methods – Extruded adobe

5.3.3 Earth materials – Construction methods – Soil suitability for adobe

5.3.1 Earth materials – Construction methods - Adobe - Advantages and Disadvantages

Advantages Disadvantages

• Simple and cheap

• Easily worked

• Very low energy input

• Fire resistant

• Good climatic performance

• Unlimited reusability

• Environmentally benign

• Needs constant maintenance

• Water absorption can cause cracks and even complete fluidisation

• Low tensile strength –bad in earthquakes

• Low acceptability for many “poor mans material”

• Low institutional support

5.3.3 Earth materials – Construction methods – Rammed earth

5.3.3 Earth materials – Construction methods – Rammed earth –Mechanical rammers

5.3.3 Earth materials – Construction methods – Rammed earth – Site features

5.3.3 Earth materials – Construction methods – Rammed earth charts

5.3.3 Earth materials – Construction methods – Rammed earth -Advantages and Disadvantages

Advantages Disadvantages

• Good compressive strength

• Good surface finish – no further treatment is usually required

• Well understood in many parts of the world

• Very low energy input

• Needs a suitable soil wit good grain distribution

• Can suffer in rain and rising damp so needs a good foundation (good pair of boots and a hat)

• (Hard) labour intensive

• Formwork can be costly

• Corners can be problematic

• Must be made on-site

5.3.3 Earth materials – Construction methods – Compressed blocks

5.3.3 Earth materials – Construction methods – compressed block charts

5.3.3 Earth materials – Construction methods – Compressed blocks -Advantages and Disadvantages

Advantages Disadvantages

• High compressive strength

• Small drying and storage requirement

• Easy transportation

• Good surface finish

• Good thermal capacity

• Good fire resistance

• Often lower cost than burned bricks

• Can be made to any shape (interlocking is popular)

• Labour intensive

• Needs a suitable soil wit good grain distribution

• Binders are often necessary and mix can be critical

• Non-traditional, therefore suspicion and a high cost of failure

• Labour intensive

5.3.4 Earth materials – Stabilisation – why stabilise

• Clay expansion on wetting– De-lamination of renders– Separation of elements such as doors and

windows

• Saturation– Unstabilised soil has much reduced

(sometimes zero) cohesion when wet– Alternative to “good hat and boots”

5.3.4 Earth materials – Stabilisation

Stabiliser Nature Method Mode Principle

Without stabiliser Mechanical

minerals

Densification

Create a dense medium blocking pores and capillary

inert

fibres

Physical

Reinforcement

Create an anisotropic network, limiting movement

Cementation

Create an inert matrix opposing movement

linkage Create stable chemical bonds between clay crystals

Imperviousness

Surround earth particle with a waterproof film

Water proofing

Eliminate absorption and adsorption

Water proofers

binderschemical chemical

With stabiliser

5.3.4 Earth materials – Stabilisation