Monday, 3 May 2021

Is there a shortfall of material to restore Straitgate to BMV land and prop up the A30?

In the application to quarry Straitgate Farm, Aggregate Industries’ Supporting Statement claims:
3.3.5 The total overburden volume is estimated at 510,000m3.
4.9.3 All topsoil and the two subsoil (upper and lower) units will be stripped and stored separately to enable later reuse as part of final restoration. Backfilling of the floor with a minimum of 1.65m of overburden with 300mm of topsoil, over 250mm of loamy subsoil which will provide 2.2m in total.
However, the statement also claims:
3.8.2 The restoration landform allows for accommodation of the 720,000m3 of restoration materials by means of the creation of gently graded slopes down to the restored base of the quarry. The base of the extraction area will be restored by placement of at least 2 metres combined thickness of topsoil, subsoil and overburden over the quarry floor to replicate current ground conditions. For areas of the site that are to be developed as species rich grassland, then topsoil will not be used in the restoration soil profile and the subsoil would be placed more thinly on the overburden to create the less fertile growing conditions required for establishing this type of plant community. 
The new resource statement, subject of the recent consultation, puts the overburden at 600,000m3. 

But given that the "restoration landform allows for accommodation of the 720,000m3 of restoration materials" is there a shortfall of material to restore Straitgate, a shortfall of 120,000m3, some 228,000 tonnes, some 8000 truck loads? 

It’s worth asking. Not only because Aggregate Industries is miraculously claiming "no loss of best and most versatile land" – which means handling, storing, restoring exactly the same profile of soils and subsoils that were removed – but also because a large but unquantified amount of overburden would be needed to buttress the A30.

Buttressing of extraction slopes shall be undertaken in line with the submitted plans with an agreed slope profile as shown on drawing SF HWYS/1
So the question is, if all the soils and overburden removed are to go back to restore BMV agricultural land, where is the material to buttress the A30 coming from? 

It’s an important question. Aggregate Industries has suffered quarry face slippages at a sand and gravel quarry before. The MOD was involved. Aggregate Industries ended up proposing a 20m standoff and a 1 in 3 slope. Whereas, at Straitgate, the company has proposed a mere 10m standoff for the A30 and a slope of as much as 1 in 0.5, before buttressing to 1 in 6 to 1 in 8 at some later date. 

In its previous application to quarry Straitgate, Aggregate Industries pointed to the geological properties of the Chester Pebble Beds to back its claim that there would be no land stability problems: 
2.6 ...the mineral at Straitgate comprises the Chester Pebble Beds (formerly the Budleigh Salterton Pebble Beds) underlain by the Aylesbere Mudstone (silts and clays) both of Triassic Age. 2.10 Aggregate Industries has a number of operations which extract this deposit and with this evidence the assumed properties for the Chester Pebble Beds are: • cohesion (c) 25kPa • internal friction angle (φ) 35º • unit weight 20kNm³... etc 2.15 ...[with] a 10m standoff to the site boundary there is no probability of the excavation affecting the A30 carriageway. Additionally the planned buttressing adds further to the stability of the faces. 
Why does no such detail back up the current application? Is the company still sure there is "no probability of the excavation affecting the A30"? We don't know. The company doesn't say.

Perhaps the same sort of conclusion was put forward before Berry Hill Quarry in Mansfield was permitted, a now worked-out quarry and subject of a recent landslide, that is:
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