Timely Guidance on Soil Handling for Green Infrastructure Creation
Lockhart Garratt (LG) were brought to design and implement a 6ha community woodland creation as mitigation for a 30ha logistics development near Coalville, in the National Forest.
The wood designed had to service multiple purposes, including:
- Three balancing lagoon voids in final land form to take surface water from the warehouse development;
- Wet woodland designed into the voids to create a habitat and landscape feature; and
- Public access to be included in the scheme with 3m surfaced paths contouring the high ground above the ponds.
At an early meeting with WLL and the earthworks contractor in March 2016, LG had the opportunity to see the progress of soil movement in the final land form. At this meeting LG supervised the contractor in excavating trial holes to assess the top soils and their suitability for the woodland establishment. The soils were silty loams and clay loams, and movement timing had crossed over with wet and sub-optimal conditions during the spring, resulting in some water-logging and compaction. These issues needed to be addressed to allow good rooting of the woodland plants.
Overcoming the Challenges
LG looked at all the options for remediating the wet soils. Dozer ripping was not possible because repeat tracking of the already wet site would further compact the surface. Weather reports for the coming 3 weeks were for continued rain. First and foremost LG recommended a halt to soil placement and handling, until drier weather had been experienced for 2 weeks.
At that point a further inspection and trial pits were undertaken which confirmed the ground was now accessible. LG trialled double digging the soils with a landscape rake but the clayey texture meant they did not break up as hoped. Instead LG identified that the deeper the machinery could rip, the greater the drying effect and recommended that a single ripper tooth was used on the arm of a 40 tonne excavator, to focus the impact of the tool.
The wide tracked machine worked the site backwards and ripped to 1m depth within 4 days. Within a further 3 weeks, the top 600mm of the soil was significantly drier and able to support a tracked dozer to rip to 500mm depth in two directions across the site. This combined approach broke up the structure of the soils very effectively.
By way of the 2 stage remediation process the newly formed site was accessible in May 2016 by wheeled agricultural tractors using low ground pressure tyres, for seeding. A low maintenance amenity grass mix established well during the summer to create the walking routes within the woodland area and supress broadleaves weeds within the planting blocks.
What Worked Best
Transparent communication with the earthworks contractors and the client identified the constraints which a compacted soil medium would present against delivering a high profile woodland park. LG were able to provide strong technical justification to all parties and work closely with the machine operators on the ground, to practically deliver the right result.
Survival of the 60-90cm transplants and 3m standards is 90% at the end of the first growing season, and growth extension is 30-50cm. Above all, the plants have the ability now to root deeply into the placed soils for good anchorage and good growth as the wood matures.
LG are continuing to work with WLL and the developer Mountpark on other sites in the National Forest and are now applying their knowledge of soil handling to Soils Utilisation / Resource Survey as early as possible prior to earthworks. This advice ensures the soil resources are focused to the best establishment and growth of the chosen mitigation habitats.