Preliminary sizing of naturalistic engineering works is one of the most important operations to be carried out before the detailed design of the works, certainly preceding technical checks and subsequent final sizing. Having quick and efficient tools at your disposal to perform initial checks and assess whether the chosen techniques can solve the issues has always been a goal we aspired to achieve.
Living support piling
- Single wall
- Double wall
Consolidating unstable slopes can be done using larch, chestnut, or pine wood stakes with a diameter of 20÷30 cm, placed alternately in the longitudinal direction (L = 3÷4 m) and in the transverse direction (L = 1.80÷2.50 m) to form a wooden structure.
They are fastened together with nails or reinforced steel rods with a diameter of 12÷14 mm (even with improved adhesion) and a length slightly less than that of the two overlapping trunks. The stake will be buried with a 10°÷15° upstream inclination, and the frontal face will have a final inclination of 60° to ensure optimal plant growth.
A series of beams can subsequently reinforce the stake at the base to counteract sliding and overturning forces. The entire structure will be filled with inert materials obtained from excavation, and between the horizontal rods, woody cuttings of suitable species for vegetative reproduction, as well as rooted plants of pioneer shrub species that can withstand crowding, will be placed. Branches and plants must protrude outward by 10÷25 cm from the stake (minimum 3÷5 buds) and reach the ground behind.
The stake cannot have a height greater than 2÷2.5 m because the consolidating capacity of the plants is limited to a depth of 2÷3 m (the B/H ratio must be prudently kept around one). For fixing, it is recommended to partially drill the two trunks to be attached, ensuring a solid grip without the risk of causing wood ruptures or cracks. The stakes should be positioned offset from the lower plane to allow for a correct distribution of weights and pressures (grid structure) in favor of stability. The intervention period corresponds to the vegetative rest.
The consolidating effect of the wooden structure, once established, will be gradually replaced over time by the development of the root system. In the case of using wood with a thick bark, it must be removed
- For the single wall: a single external horizontal line of trunks, and shorter elements perpendicular to the slope are sharpened and inserted into the slope. The height of this type of stake is generally modest (1÷1.5 m)
- For the double wall: a line of longitudinal trunks both on the exterior and the interior. The stake can be constructed for individual sections no higher than 1.5÷2 m, as the consolidating capacity of the plants is limited.
The size of the wood and correct positioning
- Utilization of trunks with a diameter not less than 20 cm is recommended to avoid the breaking of wood at joint areas (holes and fixings – in this regard, refer to the chapter dedicated to joints) and not greater than 30÷35 cm, due to possible handling difficulties and legal limits imposed for manual handling of loads.
- For each layer, both longitudinal and transverse, it is necessary to use trunks with approximately equal diameters, ensuring a uniform result in the end. The use of conical trunks causes issues and requires reductions or the insertion of wedges; the opportunity to eliminate the highly conical material or cut the respective part must always be evaluated
- In the case of thick-barked wood, it is appropriate for the wood to be peeled to avoid the formation of gaps and non-contact points between the wood at contact points, due to bark loss through decomposition.
- External and internal cords must be installed in an offset manner, similar to how the cords must be offset on different planes; the stakes must also be installed in an offset manner on different planes to achieve an optimal distribution of weights and potential pressures.
- The following figure illustrates two works characterized by the same height (3.30 m), width (7.50 m), and distance between longitudinal trunks (1.50 m), but with a different arrangement: the one on the left is defined as an alternate arrangement, while the other arrangement is continuous.
- The alternate arrangement of the transverse stakes, with the same applied load, allows for horizontal and vertical deformation, on average, over 35% greater than the continuous arrangement. Given the greater ability to deform in the case of possible ground movements, this type of structure is recommended in all situations characterized by less stable slopes.
- The work will always be completed with a row of beams to contain the overlying soil as effectively as possible.
- In the case of the living spondal pillar with a frontal stake, it is advisable to add additional stakes placed in an offset manner..
- To efficiently fulfill its stabilizing function, it is possible that transverse trunks (stakes) be driven into the slope. This operation is particularly useful in the case of living spondal pillars with a single or double wall.
- At the completion of the work, the transverse trunks that protrude from the facade during construction must be cut following the inclination line
- If the work is used as the foundation for the road, it is recommended that the last row of stakes have a density (spacing) such that it does not exceed four times the diameter of the stake to ensure the bearing capacity of the work itself.
Assessment of the overall stability of the slope
In practical application of preliminary sizing, the following general rule is applied, which must be verified in each case:
The calculation of soil pressure in the case of support works involves the assessment of the following elements:
Loads acting on the structure.
- Self-weight of the structure: This is easily determined if the volume of the construction and the unit weight of the material are known.
- Active soil pressure: This depends on the height of the living support pillar and the characteristics of the soil. The active pressure is the force exerted by the soil against the structure and is crucial for stability considerations.
- Passive soil pressure: This represents the resistance (stabilizing force) of the soil to the pressure exerted by the structure, preventing overturning and sliding along the plane of the structure’s foundation. It is usually modest compared to other loading actions, and for safety reasons, it is often neglected in calculations.
- Any additional load behind the structure: This is considered uniformly distributed.
- Seismic loads
For determining active pressure,the method of limit equilibrium can be adopted, which assumes that the failure surface generally has a cylindrical shape, that the entire shear resistance of the soil mobilizes on it, and that only rigid displacements are considered for the equilibrium of the retaining structure.
The design philosophy of a structure in a seismic zone according to current Italian standards (NTC), expanding on what is provided in Eurocode 8, conventionally chooses four Limit States:
- Two Ultimate Limit States,considering seismic events with a low probability of occurrence (and, implicitly, with a high recurrence period): a Life Safety Limit State (LSLS), for the event with a probability of occurrence over the structure’s reference life (RL) of 10%, or a Collapse Limit State (CLS), with a probability of occurrence of 5%. For these events, it is accepted that the structure may suffer severe damage, including from a structural perspective, while still maintaining the ability to support vertical loads without collapsing. In the case of LSLS, the structure is required to have a residual capacity to resist horizontal actions, i.e., seismic aftershocks of lower intensity, while in the case of CLS, only the capacity to support vertical loads in the post-seismic phase is required, without additional reserves.
- Two Serviceability Limit States, hence seismic events with a high probability of occurrence (low recurrence period): a Damage Limit State (DLS), for the event with a probability of occurrence over the structure’s reference life (RL) of 63%, or an Operability Limit State (OLS), with a probability of occurrence of 81%. In the case of DLS, even if limited damage occurs, the structure must remain usable after the earthquake, and this is conventionally verified by limiting the relative displacements of the plane (in certain cases, for structures of exceptional importance, the strength of the elements may also need to be verified), while in the case of OLS, the structure must remain fully operational, including regarding facilities and equipment (verification is carried out by controlling the displacements and accelerations suffered by the equipment).
In performing the seismic check, for static pressure calculation, as described earlier, two additional pressure contributions must be added (Mononobe-Okabe method).
Consolidated and Reinforced Soils – MRE
“MRE” stands for the software module for Slope Stability, used for the design and verification of CONSOLIDATED SOILS with: metal elements, geogrids, geosynthetics, double-twisted gabions, and wooden pillars under static and seismic conditions. The program facilitates easy data input through dedicated tools, such as the automatic generation of reinforcement positions, the creation of the consolidated terrain profile with the user’s option to choose between a profile with a constant slope or with steps.
In the Slope Stability software, naturalistic engineering works have been implemented in MRE: double-row piling works.