Glass Mountain

Glass Mountain Pumice™ Lightweight Backfill

We are able to provide our customers with a natural lightweight aggregate mined and produced from a volcanic deposit near Tulelake, California. The high porosity and low density of this material combines to make an ideal base or backfill material to replace normal weight aggregate bases and soils where a reduction in ground loading and dead weight is required.

At the present time Cal Trans is generally requiring two types of material: a coarser gradation usually specified as Lightweight Fill or Structure Backfill, and a somewhat finer gradation described as Lightweight Aggregate Base. We are able to provide satisfactory materials that would fit any typical grading requirement for these applications. Beyond grading, additional specifications usually deal with Unit Weight, R-value, Compaction, Durability and occasionally Specific Gravity, Absorption, Friction Angle, and Resistivity. Specifications are not uniform and may vary from one job to another depending on engineering demands.

Bulk Loose Unit Weight

  • 45 - 60 pcf ( 720 - 960 Kg/m3 ) at +23% moisture content, dependent upon grading
  • 35 - 50 pcf ( 560 - 800 Kg/m3 ) equivalent dry loose unit weight.


  • High silica content ( >73% SiO2 ) yields a durable and compactable material.

Chemical Stability

  • Totally inert with sulfates and chlorides less than 10%.


  • Free draining at ambient moisture levels.

Rail Loading and Distribution Facilities

Our backfill material can be delivered FOB via truck or railcar either at our Tionesta, CA plant or our McClellan Park, CA (near Sacramento, CA) loading facility.

Conditions of Sale

Our backfill is sold by volume, with billing on the basis of tons, FOB trucks or railcars. Our material is provided in a natural 'loose' state, containing natural water contents of 22 - 26% by wt.

Our volume determinations for billing are based on converting weight, as determined by weighbridge truck scales, to tons by using 'natural' (ie not-dry) Loose Unit Wt. determined by Caltrans Test 212 for Unit Wt., Procedure C ( Loose by shovelling procedure ). We carry out this test very regularly on loads despatched from the Tionesta plant and adjust our calculated volumes for billing on a truck-by-truck or railcar by railcar basis. Such testing is done by our Quality Control laboratory at the Tionesta Plant.

Volume determinations for purchasing

All material despatched from our plant is weighed by the truckload on our certified weighscale at Tionesta. When we are loading railcars, truck weights are tallied to give railcar weights. In order to convert this to volumes for billing, trucks and railcars are regularly sampled during loading , and Unit Wts are determined at our on-site Quality Assurance laboratory using Caltrans test method 212, procedure (c) - by shovelling. Unit Wt values for trucks and railcars are averaged to provide a conversion factor from mass to volume for billing purposes. If we are loading railcars, three samples will be taken per railcar, and the average loose unit wt determined on site by Caltrans CT 212 method, procedure (c) - by shovelling. This factor is then used to calculate the volume of material in each railcar.

  • cu ft wt ( by CT 212 shovel method ) X 27 = cu yd wt
  • cu yds per truck = Wt of truck / cu yd wt

Normally we would seek to reconcile our tallies with those of our customers, through an arrangement put in place before the fill is moved.

Volume estimates based on Jigging Procedures for Unit Wt

We should point out that in many cases current specifications set for Caltrans bids and contracts specify that material quantities are to be calculated / estimated on the basis of a Loose Unit Wt. (CT 212) determined by Procedure B (Compacted by jigging procedure) for As Dry materials. For bidding estimates or contracting purposes, we must leave the responsibility with the purchaser to make the adjustment from Loose Unit Wt by shovel procedure (as delivered - with moisture) to Unit Wt by jigging procedure (dry wt).

To assist our customers, we have included a regularly updated comparison between the two methods for determining Unit Wt for our fill. This is not a compacted or 'in-place' Unit Wt, but simply an 'as delivered' Unit Wt, containing moisture.

We regret that because we are not a registered constructor or engineering company, we cannot assume any liability for quoting this test comparison for your estimations.

Glass Mtn Pumice ¾" Lightweight Backfill
Loose Unit Wt by CTM 212, values are indicative only

  Method B ( jigging )

Method C (shovel )

As received (contains moisture)

63.1 pcf (1.001 Kg/m3 ) ( 25% moisture )

56.3 pcf ( 0.901 Kg/m3 )#
(contains moisture) (31% moisture )


Dry Dry 49.7 pcf ( 0.795 Kg/m3 ) 44.9 pcf ( 0.718 Kg/m3 )

# Factor used for determination of volumes by Glass Mtn. Lightweight Aggregate
Test carried out by Kleinfelder, Reno, 13 & 22 March, 2002.
Conversion factor to metric Unit Wt is .... (pcf) x 0.016 = Kg/m3

The material 'as delivered' is warranted to meet material specifications set by the purchaser, and agreed to by GMP. For mass and volume conversions needed for estimating and bidding we advise the purchaser to have these tests carried out themselves to provide current values.

We do not warranty on the basis of engineering specifications

We should point out to our buyers that our 'as delivered' or FOB specifications are distinct from engineering specifications for 'in-place' material (eg compacted unit wts and volumes, shrinkage), and Glass Mtn. Lightweight Aggregate provides no liability for emplacement, compaction, or shrinkage of backfill material. Responsibility for shrinkage estimates or determinations of the volumes of materials to be compacted rests solely with the buyer.

Please click here for our data sheet.