1 BEHAVIOUR OF PEAT REACTION WHEN LOADED – THE GOOD AND THE BAD

Peat can react in two ways when load is applied to its surface:

A. slowly, with a steady settlement and volume change as water is forced out of the peat mass. This is the desired method for the construction of a floating road and permits the peat to gradually compress and consolidate allowing time for it to gain in strength and take up the new load. For this to happen the loading phases need to be carefully controlled in order to keep the stresses induced in the peat below the strength of the peat at the time. This is a key consideration for the construction of a stable floating road.

B. rapidly, accompanied by sudden spread and shear of the peat causing failure. This rapid failure scenario has to be avoided in floating road construction by carefully managing the loading phases of the road. It can however be used as an effective engineering technique (“displacement”) where it is intended that the road should be founded on the hard strata below. This practice is out with the remit of this report however.

It is therefore vitally important that the Designer should have an appreciation of how construction loading rates can affect the consolidation and settlement behaviour of peat in order to avoid a failure on site. Modern site investigation and analysis techniques can quantify such risks so that appropriate measures can be put in place to ensure that the works can be constructed safely.

Peat should be loaded slowly to allow the underlying peat to respond to the increasing load and be given sufficient time to consolidate and gain strength rather than shear. If a floated road is placed too quickly so as to approach, or exceed, the in situ strength of the underlying peat then failure can follow. If peat is loaded too quickly, without allowing time for water pressures to be released, the in situ peat will effectively have the shear strength of its water, i.e. zero. This has to be avoided at all costs. Modern design methodologies and risk management strategies can help prevent this but designers should be aware that serious shear stresses can be induced in peat, even by moderate fills, if loadings are not sufficiently controlled.

1.1 1.7. Effects of water & preconsolidation

Peat is a relatively recent material compared to other soils, having formed over the years in the Northern

Periphery since the retreat of the glaciers around 10,000 years ago. Unlike most northern soils, peat has not

been preconsolidated by the weight of a glacier. It can however sometimes be preconsolidated where the water

level in the peat have been lowered during its formation, eg by a drought, drainage or through dewatering by

vegetation such as forests.

 

All changes in water level within a peat deposit, whether natural or man-made, have an effect on the way the

insitu peat deals with stress. Lowering the water table inside a bog will reduce the pore water pressure within the

peat matrix and cause an increase in effective pressure triggering consolidation, (ie total stress minus reduced

pore water pressure = increased effective stress).

Stress in soil (a).

Stress in soil (b) Showing how lowering the water table can produce an increase in effective stress.

The effect is not so common in the case of raised bogs but commonly happens in the blanket bogs of Iceland. 

For this reason it is important that the hydrology of a peat deposit is preserved during roadworks, and afterwards, as an unintended change in water management can have unexpected results. This is particularly the case with the excavation of new or deepened drains close to a floating road on peat after it has been constructed. New drainage can result in significant unexpected consolidation in the peat that can damage otherwise good construction.

 

Diagrams showing the effect of digging new ditches close to an existing floating road on peat.

 

Drainage ahead of construction can however sometimes be beneficial, especially as in Iceland where a stable water regime can be established for the finished road in the long term. Such work will invariably require the consent of the local environmental authority.

 

2 GIROUD AND NOIRA Y METHOD

2.1 General Giroud and Noiray (1981) use the geometric model shown in Fig. 10. 1 for a wheel load pressure of pec on an area (B x L), which dissipates through h0 thickness of aggregate base without geotextile and 'h' thickness of aggregate base with geotextile.

Fig. 10. 1 Load distribution through sub base (After Giroud and Noiray, 1981)

The geometry indicated results in a stress of p0 (without geotextile) and p (with geotextile) on the soil subgrade as follows:

(10.1)

where,

P = Axle load,

γ= Unit weight of aggregate,

α= load dispersion angle for unreinforced case, and

α0= load dispersion angle for reinforced case.

shallow foundation theory of geotechnical engineering can be utilized. Assuming that the soil subgrade consists of fine-grained silt and clay in saturated condition, shear strength is taken as the undrained cohesion.

Without geotextile, it is again assumed that the maximum pressure that can be maintained corresponds to the elastic limit of the soil, that is,

p0 = p∙C + γ∙h0 (10.1)

and that with geotextile the limiting pressure can be increased to the ultimate bearing capacity of the soil, that is,

p* = (π+2) C + γ h∙ ∙ 0

3 Unpaved Road without Geotextile by Quasi-Static Analysis (h0 vs CBR)

For the case of no geotextile reinforcement, equation (10.1) and (10.3) can be solved, resulting in equation (10.5), which yields the desired aggregate thickness response curve without the use of a geotextile:

where,

C = cohesion,

P = axle load,

Pc = tyre inflation pressure,

h0= aggregate thickness, and

ao=load dispersion angle (assumed as 26 de.blfccs).

The cohesion and CBR (%)of the subgrade soil is related empirically using the equation (IRC: 37-2001) as

CBR = 30 c ∙

where, c is the cohesion in kPa.

Thus, graphs can be plotted connecting aggregate thickness (ho) and CBR of the subgrade soil for different values of axle load and tyre pressure. Typical one for a wheel load of 81. 7kN with a tyre pressure of 480kPa is shown in Fig.10.2

or the case where geotextile reinforcement is used, p* in equation (lOA) is replaced by (p-pg), where pg is a function of tension in the geotextile; hence its elongation is significant. On the basis of the probable deflected shape of the geotextile- soil system pg is expressed as,

where,

Es = Modulus of coir geotextile,

ε= Strain,

a = geometric property (Fig. 0.1), and

S = Settlement under the wheel (rut depth).

Combining equations 10.2, 10.4 and 10.7 and using p* = P-Pg, it gives equation 10.8